[PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

This fixes virtio on Xen guests as well as on any other platform
that uses virtio_pci on which physical addresses don't match bus
addresses.

This can be tested with:

    virtme-run --xen xen --kimg arch/x86/boot/bzImage --console

using virtme from here:

    https://git.kernel.org/cgit/utils/kernel/virtme/virtme.git

Without these patches, the guest hangs forever.  With these patches,
everything works.

This should be safe on all platforms that I'm aware of.  That
doesn't mean that there isn't anything that I missed.

Thanks to everyone for putting up with the development of this
series.  Hopefully it'll be the end of DMA issues in virtio. :)

Changes from v3:
 - virtio_pci only asks virtio_ring to use the DMA_API if
   !PCI_DMA_BUS_IS_PHYS.
 - Reduce tools/virtio breakage.  It's now merely as broken as before
   instead of being even more broken.
 - Drop the sg_next changes -- Rusty's version is better.

Changes from v2:
 - Reordered patches.
 - Fixed a virtio_net OOPS.

Changes from v1:
 - Using the DMA API is optional now.  It would be nice to improve the
   DMA API to the point that it could be used unconditionally, but s390
   proves that we're not there yet.
 - Includes patch 4, which fixes DMA debugging warnings from virtio_net.

Andy Lutomirski (4):
  virtio_ring: Support DMA APIs if requested
  virtio_pci: Use the DMA API for virtqueues
  virtio_net: Don't set the end flag on reusable sg entries
  virtio_net: Stop doing DMA from the stack

 drivers/lguest/lguest_device.c         |   3 +-
 drivers/misc/mic/card/mic_virtio.c     |   2 +-
 drivers/net/virtio_net.c               |  59 +++++++----
 drivers/remoteproc/remoteproc_virtio.c |   4 +-
 drivers/s390/kvm/kvm_virtio.c          |   2 +-
 drivers/s390/kvm/virtio_ccw.c          |   4 +-
 drivers/virtio/virtio_mmio.c           |   5 +-
 drivers/virtio/virtio_pci.c            |  41 ++++++--
 drivers/virtio/virtio_ring.c           | 187 +++++++++++++++++++++++++++++----
 include/linux/virtio_ring.h            |   1 +
 tools/virtio/linux/dma-mapping.h       |  17 +++
 tools/virtio/linux/virtio.h            |   1 +
 tools/virtio/virtio_test.c             |   2 +-
 tools/virtio/vringh_test.c             |   3 +-
 14 files changed, 268 insertions(+), 63 deletions(-)
 create mode 100644 tools/virtio/linux/dma-mapping.h

-- 
1.9.3

[PATCH v4 1/4] virtio_ring: Support DMA APIs if requested

[Andy Lutomirski]

virtio_ring currently sends the device (usually a hypervisor)
physical addresses of its I/O buffers.  This is okay when DMA
addresses and physical addresses are the same thing, but this isn't
always the case.  For example, this never works on Xen guests, and
it is likely to fail if a physical "virtio" device ever ends up
behind an IOMMU or swiotlb.

The immediate use case for me is to enable virtio on Xen guests.
For that to work, we need vring to support DMA address translation
as well as a corresponding change to virtio_pci or to another
driver.

With this patch, if enabled, virtfs survives kmemleak and
CONFIG_DMA_API_DEBUG.  virtio-net warns (correctly) about DMA from
the stack in virtnet_set_rx_mode.

This explicitly supports !CONFIG_HAS_DMA.  If vring is asked to use
the DMA API and CONFIG_HAS_DMA is not set, then vring will refuse to
create the virtqueue.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
 drivers/lguest/lguest_device.c         |   3 +-
 drivers/misc/mic/card/mic_virtio.c     |   2 +-
 drivers/remoteproc/remoteproc_virtio.c |   4 +-
 drivers/s390/kvm/kvm_virtio.c          |   2 +-
 drivers/s390/kvm/virtio_ccw.c          |   4 +-
 drivers/virtio/virtio_mmio.c           |   5 +-
 drivers/virtio/virtio_pci.c            |   3 +-
 drivers/virtio/virtio_ring.c           | 187 +++++++++++++++++++++++++++++----
 include/linux/virtio_ring.h            |   1 +
 tools/virtio/linux/dma-mapping.h       |  17 +++
 tools/virtio/linux/virtio.h            |   1 +
 tools/virtio/virtio_test.c             |   2 +-
 tools/virtio/vringh_test.c             |   3 +-
 13 files changed, 199 insertions(+), 35 deletions(-)
 create mode 100644 tools/virtio/linux/dma-mapping.h

diff --git a/drivers/lguest/lguest_device.c b/drivers/lguest/lguest_device.c
index d0a1d8a45c81..f0eafbe82ed4 100644
--- a/drivers/lguest/lguest_device.c
+++ b/drivers/lguest/lguest_device.c
@@ -301,7 +301,8 @@ static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
 	 * barriers.
 	 */
 	vq = vring_new_virtqueue(index, lvq->config.num, LGUEST_VRING_ALIGN, vdev,
-				 true, lvq->pages, lg_notify, callback, name);
+				 true, false, lvq->pages,
+				 lg_notify, callback, name);
 	if (!vq) {
 		err = -ENOMEM;
 		goto unmap;
diff --git a/drivers/misc/mic/card/mic_virtio.c b/drivers/misc/mic/card/mic_virtio.c
index f14b60080c21..d633964417b1 100644
--- a/drivers/misc/mic/card/mic_virtio.c
+++ b/drivers/misc/mic/card/mic_virtio.c
@@ -256,7 +256,7 @@ static struct virtqueue *mic_find_vq(struct virtio_device *vdev,
 	mvdev->vr[index] = va;
 	memset_io(va, 0x0, _vr_size);
 	vq = vring_new_virtqueue(index, le16_to_cpu(config.num),
-				 MIC_VIRTIO_RING_ALIGN, vdev, false,
+				 MIC_VIRTIO_RING_ALIGN, vdev, false, false,
 				 (void __force *)va, mic_notify, callback,
 				 name);
 	if (!vq) {
diff --git a/drivers/remoteproc/remoteproc_virtio.c b/drivers/remoteproc/remoteproc_virtio.c
index a34b50690b4e..e31f2fefa76e 100644
--- a/drivers/remoteproc/remoteproc_virtio.c
+++ b/drivers/remoteproc/remoteproc_virtio.c
@@ -107,8 +107,8 @@ static struct virtqueue *rp_find_vq(struct virtio_device *vdev,
 	 * Create the new vq, and tell virtio we're not interested in
 	 * the 'weak' smp barriers, since we're talking with a real device.
 	 */
-	vq = vring_new_virtqueue(id, len, rvring->align, vdev, false, addr,
-					rproc_virtio_notify, callback, name);
+	vq = vring_new_virtqueue(id, len, rvring->align, vdev, false, false,
+				 addr, rproc_virtio_notify, callback, name);
 	if (!vq) {
 		dev_err(dev, "vring_new_virtqueue %s failed\n", name);
 		rproc_free_vring(rvring);
diff --git a/drivers/s390/kvm/kvm_virtio.c b/drivers/s390/kvm/kvm_virtio.c
index a1349653c6d9..91abcdc196d0 100644
--- a/drivers/s390/kvm/kvm_virtio.c
+++ b/drivers/s390/kvm/kvm_virtio.c
@@ -206,7 +206,7 @@ static struct virtqueue *kvm_find_vq(struct virtio_device *vdev,
 		goto out;
 
 	vq = vring_new_virtqueue(index, config->num, KVM_S390_VIRTIO_RING_ALIGN,
-				 vdev, true, (void *) config->address,
+				 vdev, true, false, (void *) config->address,
 				 kvm_notify, callback, name);
 	if (!vq) {
 		err = -ENOMEM;
diff --git a/drivers/s390/kvm/virtio_ccw.c b/drivers/s390/kvm/virtio_ccw.c
index d2c0b442bce5..2462a443358a 100644
--- a/drivers/s390/kvm/virtio_ccw.c
+++ b/drivers/s390/kvm/virtio_ccw.c
@@ -478,8 +478,8 @@ static struct virtqueue *virtio_ccw_setup_vq(struct virtio_device *vdev,
 	}
 
 	vq = vring_new_virtqueue(i, info->num, KVM_VIRTIO_CCW_RING_ALIGN, vdev,
-				 true, info->queue, virtio_ccw_kvm_notify,
-				 callback, name);
+				 true, false, info->queue,
+				 virtio_ccw_kvm_notify, callback, name);
 	if (!vq) {
 		/* For now, we fail if we can't get the requested size. */
 		dev_warn(&vcdev->cdev->dev, "no vq\n");
diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
index c600ccfd6922..693254e52a5d 100644
--- a/drivers/virtio/virtio_mmio.c
+++ b/drivers/virtio/virtio_mmio.c
@@ -366,8 +366,9 @@ static struct virtqueue *vm_setup_vq(struct virtio_device *vdev, unsigned index,
 			vm_dev->base + VIRTIO_MMIO_QUEUE_PFN);
 
 	/* Create the vring */
-	vq = vring_new_virtqueue(index, info->num, VIRTIO_MMIO_VRING_ALIGN, vdev,
-				 true, info->queue, vm_notify, callback, name);
+	vq = vring_new_virtqueue(index, info->num, VIRTIO_MMIO_VRING_ALIGN,
+				 vdev, true, false, info->queue,
+				 vm_notify, callback, name);
 	if (!vq) {
 		err = -ENOMEM;
 		goto error_new_virtqueue;
diff --git a/drivers/virtio/virtio_pci.c b/drivers/virtio/virtio_pci.c
index 3d1463c6b120..a1f299fa4626 100644
--- a/drivers/virtio/virtio_pci.c
+++ b/drivers/virtio/virtio_pci.c
@@ -430,7 +430,8 @@ static struct virtqueue *setup_vq(struct virtio_device *vdev, unsigned index,
 
 	/* create the vring */
 	vq = vring_new_virtqueue(index, info->num, VIRTIO_PCI_VRING_ALIGN, vdev,
-				 true, info->queue, vp_notify, callback, name);
+				 true, false, info->queue,
+				 vp_notify, callback, name);
 	if (!vq) {
 		err = -ENOMEM;
 		goto out_activate_queue;
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 4d08f45a9c29..7e10770edd0f 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -24,6 +24,7 @@
 #include <linux/module.h>
 #include <linux/hrtimer.h>
 #include <linux/kmemleak.h>
+#include <linux/dma-mapping.h>
 
 #ifdef DEBUG
 /* For development, we want to crash whenever the ring is screwed. */
@@ -54,6 +55,12 @@
 #define END_USE(vq)
 #endif
 
+struct vring_desc_state
+{
+	void *data;			/* Data for callback. */
+	struct vring_desc *indir_desc;	/* Indirect descriptor, if any. */
+};
+
 struct vring_virtqueue
 {
 	struct virtqueue vq;
@@ -64,6 +71,9 @@ struct vring_virtqueue
 	/* Can we use weak barriers? */
 	bool weak_barriers;
 
+	/* Should we use the DMA API? */
+	bool use_dma_api;
+
 	/* Other side has made a mess, don't try any more. */
 	bool broken;
 
@@ -93,8 +103,8 @@ struct vring_virtqueue
 	ktime_t last_add_time;
 #endif
 
-	/* Tokens for callbacks. */
-	void *data[];
+	/* Per-descriptor state. */
+	struct vring_desc_state desc_state[];
 };
 
 #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
@@ -113,6 +123,83 @@ static inline struct scatterlist *sg_next_arr(struct scatterlist *sg,
 	return sg + 1;
 }
 
+/* Map one sg entry. */
+static dma_addr_t vring_map_one_sg(const struct vring_virtqueue *vq,
+				   struct scatterlist *sg,
+				   enum dma_data_direction direction)
+{
+#ifdef CONFIG_HAS_DMA
+	/*
+	 * We can't use dma_map_sg, because we don't use scatterlists in
+	 * the way it expects (we sometimes use unterminated
+	 * scatterlists, and we don't guarantee that the scatterlist
+	 * will exist for the lifetime of the mapping.
+	 */
+	if (vq->use_dma_api)
+		return dma_map_page(vq->vq.vdev->dev.parent,
+				    sg_page(sg), sg->offset, sg->length,
+				    direction);
+#endif
+
+	return sg_phys(sg);
+}
+
+static dma_addr_t vring_map_single(const struct vring_virtqueue *vq,
+				   void *cpu_addr, size_t size,
+				   enum dma_data_direction direction)
+{
+#ifdef CONFIG_HAS_DMA
+	if (vq->use_dma_api)
+		return dma_map_single(vq->vq.vdev->dev.parent,
+				      cpu_addr, size,
+				      direction);
+#endif
+
+	return virt_to_phys(cpu_addr);
+}
+
+static void vring_unmap_one(const struct vring_virtqueue *vq,
+			    struct vring_desc *desc)
+{
+#ifdef CONFIG_HAS_DMA
+	if (!vq->use_dma_api)
+		return;		/* Nothing to do. */
+
+	if (desc->flags & VRING_DESC_F_INDIRECT) {
+		dma_unmap_single(vq->vq.vdev->dev.parent,
+				 desc->addr, desc->len,
+				 (desc->flags & VRING_DESC_F_WRITE) ?
+				 DMA_FROM_DEVICE : DMA_TO_DEVICE);
+	} else {
+		dma_unmap_page(vq->vq.vdev->dev.parent,
+			       desc->addr, desc->len,
+			       (desc->flags & VRING_DESC_F_WRITE) ?
+			       DMA_FROM_DEVICE : DMA_TO_DEVICE);
+	}
+#endif
+}
+
+static void vring_unmap_indirect(const struct vring_virtqueue *vq,
+				 struct vring_desc *desc, int total)
+{
+	int i;
+
+	if (vq->use_dma_api)
+		for (i = 0; i < total; i++)
+			vring_unmap_one(vq, &desc[i]);
+}
+
+static int vring_mapping_error(const struct vring_virtqueue *vq,
+			       dma_addr_t addr)
+{
+#ifdef CONFIG_HAS_DMA
+	return vq->use_dma_api &&
+		dma_mapping_error(vq->vq.vdev->dev.parent, addr);
+#else
+	return 0;
+#endif
+}
+
 /* Set up an indirect table of descriptors and add it to the queue. */
 static inline int vring_add_indirect(struct vring_virtqueue *vq,
 				     struct scatterlist *sgs[],
@@ -146,7 +233,10 @@ static inline int vring_add_indirect(struct vring_virtqueue *vq,
 	for (n = 0; n < out_sgs; n++) {
 		for (sg = sgs[n]; sg; sg = next(sg, &total_out)) {
 			desc[i].flags = VRING_DESC_F_NEXT;
-			desc[i].addr = sg_phys(sg);
+			desc[i].addr =
+				vring_map_one_sg(vq, sg, DMA_TO_DEVICE);
+			if (vring_mapping_error(vq, desc[i].addr))
+				goto unmap_free;
 			desc[i].len = sg->length;
 			desc[i].next = i+1;
 			i++;
@@ -155,7 +245,10 @@ static inline int vring_add_indirect(struct vring_virtqueue *vq,
 	for (; n < (out_sgs + in_sgs); n++) {
 		for (sg = sgs[n]; sg; sg = next(sg, &total_in)) {
 			desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
-			desc[i].addr = sg_phys(sg);
+			desc[i].addr =
+				vring_map_one_sg(vq, sg, DMA_FROM_DEVICE);
+			if (vring_mapping_error(vq, desc[i].addr))
+				goto unmap_free;
 			desc[i].len = sg->length;
 			desc[i].next = i+1;
 			i++;
@@ -173,15 +266,26 @@ static inline int vring_add_indirect(struct vring_virtqueue *vq,
 	/* Use a single buffer which doesn't continue */
 	head = vq->free_head;
 	vq->vring.desc[head].flags = VRING_DESC_F_INDIRECT;
-	vq->vring.desc[head].addr = virt_to_phys(desc);
-	/* kmemleak gives a false positive, as it's hidden by virt_to_phys */
-	kmemleak_ignore(desc);
+	vq->vring.desc[head].addr =
+		vring_map_single(vq,
+				 desc, i * sizeof(struct vring_desc),
+				 DMA_TO_DEVICE);
+	if (vring_mapping_error(vq, vq->vring.desc[head].addr))
+		goto unmap_free;
 	vq->vring.desc[head].len = i * sizeof(struct vring_desc);
 
 	/* Update free pointer */
 	vq->free_head = vq->vring.desc[head].next;
 
+	/* Save the indirect block */
+	vq->desc_state[head].indir_desc = desc;
+
 	return head;
+
+unmap_free:
+	vring_unmap_indirect(vq, desc, i);
+	kfree(desc);
+	return -ENOMEM;
 }
 
 static inline int virtqueue_add(struct virtqueue *_vq,
@@ -197,7 +301,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 	struct scatterlist *sg;
-	unsigned int i, n, avail, uninitialized_var(prev), total_sg;
+	unsigned int i, n, avail, uninitialized_var(prev), total_sg, err_idx;
 	int head;
 
 	START_USE(vq);
@@ -256,7 +360,10 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 	for (n = 0; n < out_sgs; n++) {
 		for (sg = sgs[n]; sg; sg = next(sg, &total_out)) {
 			vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
-			vq->vring.desc[i].addr = sg_phys(sg);
+			vq->vring.desc[i].addr =
+				vring_map_one_sg(vq, sg, DMA_TO_DEVICE);
+			if (vring_mapping_error(vq, vq->vring.desc[i].addr))
+				goto unmap_release;
 			vq->vring.desc[i].len = sg->length;
 			prev = i;
 			i = vq->vring.desc[i].next;
@@ -265,7 +372,10 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 	for (; n < (out_sgs + in_sgs); n++) {
 		for (sg = sgs[n]; sg; sg = next(sg, &total_in)) {
 			vq->vring.desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
-			vq->vring.desc[i].addr = sg_phys(sg);
+			vq->vring.desc[i].addr =
+				vring_map_one_sg(vq, sg, DMA_FROM_DEVICE);
+			if (vring_mapping_error(vq, vq->vring.desc[i].addr))
+				goto unmap_release;
 			vq->vring.desc[i].len = sg->length;
 			prev = i;
 			i = vq->vring.desc[i].next;
@@ -279,7 +389,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 
 add_head:
 	/* Set token. */
-	vq->data[head] = data;
+	vq->desc_state[head].data = data;
 
 	/* Put entry in available array (but don't update avail->idx until they
 	 * do sync). */
@@ -301,6 +411,20 @@ add_head:
 	END_USE(vq);
 
 	return 0;
+
+unmap_release:
+	err_idx = i;
+	i = head;
+
+	for (n = 0; n < total_sg; n++) {
+		if (i == err_idx)
+			break;
+		vring_unmap_one(vq, &vq->vring.desc[i]);
+		i = vq->vring.desc[i].next;
+	}
+
+	vq->vq.num_free += total_sg;
+	return -EIO;
 }
 
 /**
@@ -480,22 +604,33 @@ static void detach_buf(struct vring_virtqueue *vq, unsigned int head)
 	unsigned int i;
 
 	/* Clear data ptr. */
-	vq->data[head] = NULL;
+	vq->desc_state[head].data = NULL;
 
 	/* Put back on free list: find end */
 	i = head;
 
 	/* Free the indirect table */
-	if (vq->vring.desc[i].flags & VRING_DESC_F_INDIRECT)
-		kfree(phys_to_virt(vq->vring.desc[i].addr));
+	if (vq->desc_state[head].indir_desc) {
+		u32 len = vq->vring.desc[i].len;
+
+		BUG_ON(!(vq->vring.desc[i].flags & VRING_DESC_F_INDIRECT));
+		BUG_ON(len == 0 || len % sizeof(struct vring_desc));
+		vring_unmap_indirect(vq, vq->desc_state[head].indir_desc,
+				     len / sizeof(struct vring_desc));
+		kfree(vq->desc_state[head].indir_desc);
+		vq->desc_state[head].indir_desc = NULL;
+	}
 
 	while (vq->vring.desc[i].flags & VRING_DESC_F_NEXT) {
+		vring_unmap_one(vq, &vq->vring.desc[i]);
 		i = vq->vring.desc[i].next;
 		vq->vq.num_free++;
 	}
 
+	vring_unmap_one(vq, &vq->vring.desc[i]);
 	vq->vring.desc[i].next = vq->free_head;
 	vq->free_head = head;
+
 	/* Plus final descriptor */
 	vq->vq.num_free++;
 }
@@ -552,13 +687,13 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
 		BAD_RING(vq, "id %u out of range\n", i);
 		return NULL;
 	}
-	if (unlikely(!vq->data[i])) {
+	if (unlikely(!vq->desc_state[i].data)) {
 		BAD_RING(vq, "id %u is not a head!\n", i);
 		return NULL;
 	}
 
 	/* detach_buf clears data, so grab it now. */
-	ret = vq->data[i];
+	ret = vq->desc_state[i].data;
 	detach_buf(vq, i);
 	vq->last_used_idx++;
 	/* If we expect an interrupt for the next entry, tell host
@@ -719,10 +854,10 @@ void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
 	START_USE(vq);
 
 	for (i = 0; i < vq->vring.num; i++) {
-		if (!vq->data[i])
+		if (!vq->desc_state[i].data)
 			continue;
 		/* detach_buf clears data, so grab it now. */
-		buf = vq->data[i];
+		buf = vq->desc_state[i].data;
 		detach_buf(vq, i);
 		vq->vring.avail->idx--;
 		END_USE(vq);
@@ -761,6 +896,7 @@ struct virtqueue *vring_new_virtqueue(unsigned int index,
 				      unsigned int vring_align,
 				      struct virtio_device *vdev,
 				      bool weak_barriers,
+				      bool use_dma_api,
 				      void *pages,
 				      bool (*notify)(struct virtqueue *),
 				      void (*callback)(struct virtqueue *),
@@ -775,7 +911,13 @@ struct virtqueue *vring_new_virtqueue(unsigned int index,
 		return NULL;
 	}
 
-	vq = kmalloc(sizeof(*vq) + sizeof(void *)*num, GFP_KERNEL);
+#ifndef CONFIG_HAS_DMA
+	if (use_dma_api)
+		return NULL;
+#endif
+
+	vq = kmalloc(sizeof(*vq) + num * sizeof(struct vring_desc_state),
+		     GFP_KERNEL);
 	if (!vq)
 		return NULL;
 
@@ -787,6 +929,7 @@ struct virtqueue *vring_new_virtqueue(unsigned int index,
 	vq->vq.index = index;
 	vq->notify = notify;
 	vq->weak_barriers = weak_barriers;
+	vq->use_dma_api = use_dma_api;
 	vq->broken = false;
 	vq->last_used_idx = 0;
 	vq->num_added = 0;
@@ -805,11 +948,9 @@ struct virtqueue *vring_new_virtqueue(unsigned int index,
 
 	/* Put everything in free lists. */
 	vq->free_head = 0;
-	for (i = 0; i < num-1; i++) {
+	for (i = 0; i < num-1; i++)
 		vq->vring.desc[i].next = i+1;
-		vq->data[i] = NULL;
-	}
-	vq->data[i] = NULL;
+	memset(vq->desc_state, 0, num * sizeof(struct vring_desc_state));
 
 	return &vq->vq;
 }
diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index 67e06fe18c03..60f761a38a09 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -70,6 +70,7 @@ struct virtqueue *vring_new_virtqueue(unsigned int index,
 				      unsigned int vring_align,
 				      struct virtio_device *vdev,
 				      bool weak_barriers,
+				      bool use_dma_api,
 				      void *pages,
 				      bool (*notify)(struct virtqueue *vq),
 				      void (*callback)(struct virtqueue *vq),
diff --git a/tools/virtio/linux/dma-mapping.h b/tools/virtio/linux/dma-mapping.h
new file mode 100644
index 000000000000..4f93af89ae16
--- /dev/null
+++ b/tools/virtio/linux/dma-mapping.h
@@ -0,0 +1,17 @@
+#ifndef _LINUX_DMA_MAPPING_H
+#define _LINUX_DMA_MAPPING_H
+
+#ifdef CONFIG_HAS_DMA
+# error Virtio userspace code does not support CONFIG_HAS_DMA
+#endif
+
+#define PCI_DMA_BUS_IS_PHYS 1
+
+enum dma_data_direction {
+	DMA_BIDIRECTIONAL = 0,
+	DMA_TO_DEVICE = 1,
+	DMA_FROM_DEVICE = 2,
+	DMA_NONE = 3,
+};
+
+#endif
diff --git a/tools/virtio/linux/virtio.h b/tools/virtio/linux/virtio.h
index 5a2d1f0f6bc7..5d42dc6a6201 100644
--- a/tools/virtio/linux/virtio.h
+++ b/tools/virtio/linux/virtio.h
@@ -78,6 +78,7 @@ struct virtqueue *vring_new_virtqueue(unsigned int index,
 				      unsigned int vring_align,
 				      struct virtio_device *vdev,
 				      bool weak_barriers,
+				      bool use_dma_api,
 				      void *pages,
 				      bool (*notify)(struct virtqueue *vq),
 				      void (*callback)(struct virtqueue *vq),
diff --git a/tools/virtio/virtio_test.c b/tools/virtio/virtio_test.c
index 00ea679b3826..860cc89900a7 100644
--- a/tools/virtio/virtio_test.c
+++ b/tools/virtio/virtio_test.c
@@ -99,7 +99,7 @@ static void vq_info_add(struct vdev_info *dev, int num)
 	vring_init(&info->vring, num, info->ring, 4096);
 	info->vq = vring_new_virtqueue(info->idx,
 				       info->vring.num, 4096, &dev->vdev,
-				       true, info->ring,
+				       true, false, info->ring,
 				       vq_notify, vq_callback, "test");
 	assert(info->vq);
 	info->vq->priv = info;
diff --git a/tools/virtio/vringh_test.c b/tools/virtio/vringh_test.c
index 14a4f4cab5b9..67d3c3a1ba88 100644
--- a/tools/virtio/vringh_test.c
+++ b/tools/virtio/vringh_test.c
@@ -312,7 +312,8 @@ static int parallel_test(unsigned long features,
 		if (sched_setaffinity(getpid(), sizeof(cpu_set), &cpu_set))
 			err(1, "Could not set affinity to cpu %u", first_cpu);
 
-		vq = vring_new_virtqueue(0, RINGSIZE, ALIGN, &gvdev.vdev, true,
+		vq = vring_new_virtqueue(0, RINGSIZE, ALIGN, &gvdev.vdev,
+					 true, false,
 					 guest_map, fast_vringh ? no_notify_host
 					 : parallel_notify_host,
 					 never_callback_guest, "guest vq");
-- 
1.9.3

[PATCH v4 2/4] virtio_pci: Use the DMA API for virtqueues

[Andy Lutomirski]

A virtqueue is a coherent DMA mapping.  Use the DMA API for it.
This fixes virtio_pci on Xen.

As an optimization, this only enables asks virtio_ring to use the
DMA API if !PCI_DMA_BUS_IS_PHYS.  Eventually, once the DMA API is
known to be efficient on all relevant architectures, this
optimization can be removed.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
 drivers/virtio/virtio_pci.c | 40 +++++++++++++++++++++++++++++++---------
 1 file changed, 31 insertions(+), 9 deletions(-)

diff --git a/drivers/virtio/virtio_pci.c b/drivers/virtio/virtio_pci.c
index a1f299fa4626..226b46b08727 100644
--- a/drivers/virtio/virtio_pci.c
+++ b/drivers/virtio/virtio_pci.c
@@ -80,8 +80,9 @@ struct virtio_pci_vq_info
 	/* the number of entries in the queue */
 	int num;
 
-	/* the virtual address of the ring queue */
-	void *queue;
+	/* the ring queue */
+	void *queue;			/* virtual address */
+	dma_addr_t queue_dma_addr;	/* bus address */
 
 	/* the list node for the virtqueues list */
 	struct list_head node;
@@ -417,20 +418,32 @@ static struct virtqueue *setup_vq(struct virtio_device *vdev, unsigned index,
 	info->num = num;
 	info->msix_vector = msix_vec;
 
-	size = PAGE_ALIGN(vring_size(num, VIRTIO_PCI_VRING_ALIGN));
-	info->queue = alloc_pages_exact(size, GFP_KERNEL|__GFP_ZERO);
+	size = vring_size(num, VIRTIO_PCI_VRING_ALIGN);
+	info->queue = dma_zalloc_coherent(vdev->dev.parent, size,
+					  &info->queue_dma_addr, GFP_KERNEL);
 	if (info->queue == NULL) {
 		err = -ENOMEM;
 		goto out_info;
 	}
 
 	/* activate the queue */
-	iowrite32(virt_to_phys(info->queue) >> VIRTIO_PCI_QUEUE_ADDR_SHIFT,
+	iowrite32(info->queue_dma_addr >> VIRTIO_PCI_QUEUE_ADDR_SHIFT,
 		  vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN);
 
-	/* create the vring */
+	/*
+	 * Create the vring.  If there is an IOMMU of any sort, including
+	 * Xen paravirt's ersatz IOMMU, use it.  If the host wants physical
+	 * addresses instead of bus addresses, the host shouldn't expose
+	 * an IOMMU.
+	 *
+	 * As an optimization, if the platform promises to have physical
+	 * PCI DMA, we turn off DMA mapping in virtio_ring.  If the
+	 * platform's DMA API implementation is well optimized, this
+	 * should have almost no effect, but that's a dangerous thing to
+	 * rely on.
+	 */
 	vq = vring_new_virtqueue(index, info->num, VIRTIO_PCI_VRING_ALIGN, vdev,
-				 true, false, info->queue,
+				 true, !PCI_DMA_BUS_IS_PHYS, info->queue,
 				 vp_notify, callback, name);
 	if (!vq) {
 		err = -ENOMEM;
@@ -463,7 +476,8 @@ out_assign:
 	vring_del_virtqueue(vq);
 out_activate_queue:
 	iowrite32(0, vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN);
-	free_pages_exact(info->queue, size);
+	dma_free_coherent(vdev->dev.parent, size,
+			  info->queue, info->queue_dma_addr);
 out_info:
 	kfree(info);
 	return ERR_PTR(err);
@@ -494,7 +508,8 @@ static void vp_del_vq(struct virtqueue *vq)
 	iowrite32(0, vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN);
 
 	size = PAGE_ALIGN(vring_size(info->num, VIRTIO_PCI_VRING_ALIGN));
-	free_pages_exact(info->queue, size);
+	dma_free_coherent(vq->vdev->dev.parent, size,
+			  info->queue, info->queue_dma_addr);
 	kfree(info);
 }
 
@@ -713,6 +728,13 @@ static int virtio_pci_probe(struct pci_dev *pci_dev,
 	if (err)
 		goto out;
 
+	err = dma_set_mask_and_coherent(&pci_dev->dev, DMA_BIT_MASK(64));
+	if (err)
+		err = dma_set_mask_and_coherent(&pci_dev->dev,
+						DMA_BIT_MASK(32));
+	if (err)
+		dev_warn(&pci_dev->dev, "Failed to enable 64-bit or 32-bit DMA.  Trying to continue, but this might not work.\n");
+
 	err = pci_request_regions(pci_dev, "virtio-pci");
 	if (err)
 		goto out_enable_device;
-- 
1.9.3

[PATCH v4 3/4] virtio_net: Don't set the end flag on reusable sg entries

[Andy Lutomirski]

Every time virtio_net calls skb_to_sgvec, an end flag gets set
somewhere on the queue's scatterlist and never gets cleared.  As
soon as a larger request happens, virtio_net sends the virtqueue a
scatterlist with an end mark set in the middle.  Once the vring code
starts using for_each_sg, this will blow up.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
 drivers/net/virtio_net.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 59caa06f34a6..c90466a4fab0 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -548,7 +548,7 @@ static int add_recvbuf_small(struct receive_queue *rq, gfp_t gfp)
 	hdr = skb_vnet_hdr(skb);
 	sg_set_buf(rq->sg, &hdr->hdr, sizeof hdr->hdr);
 
-	skb_to_sgvec(skb, rq->sg + 1, 0, skb->len);
+	skb_to_sgvec_nomark(skb, rq->sg + 1, 0, skb->len);
 
 	err = virtqueue_add_inbuf(rq->vq, rq->sg, 2, skb, gfp);
 	if (err < 0)
@@ -901,12 +901,12 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
 
 	if (can_push) {
 		__skb_push(skb, hdr_len);
-		num_sg = skb_to_sgvec(skb, sq->sg, 0, skb->len);
+		num_sg = skb_to_sgvec_nomark(skb, sq->sg, 0, skb->len);
 		/* Pull header back to avoid skew in tx bytes calculations. */
 		__skb_pull(skb, hdr_len);
 	} else {
 		sg_set_buf(sq->sg, hdr, hdr_len);
-		num_sg = skb_to_sgvec(skb, sq->sg + 1, 0, skb->len) + 1;
+		num_sg = skb_to_sgvec_nomark(skb, sq->sg + 1, 0, skb->len) + 1;
 	}
 	return virtqueue_add_outbuf(sq->vq, sq->sg, num_sg, skb, GFP_ATOMIC);
 }
-- 
1.9.3

[PATCH v4 4/4] virtio_net: Stop doing DMA from the stack

[Andy Lutomirski]

Now that virtio supports real DMA, drivers should play by the rules.
For virtio_net, that means that DMA should be done to and from
dynamically-allocated memory, not the kernel stack.

This should have no effect on any performance-critical code paths.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
 drivers/net/virtio_net.c | 53 ++++++++++++++++++++++++++++++++----------------
 1 file changed, 36 insertions(+), 17 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index c90466a4fab0..25703fd2df28 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -966,31 +966,43 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
 				 struct scatterlist *out)
 {
 	struct scatterlist *sgs[4], hdr, stat;
-	struct virtio_net_ctrl_hdr ctrl;
-	virtio_net_ctrl_ack status = ~0;
+
+	struct {
+		struct virtio_net_ctrl_hdr ctrl;
+		virtio_net_ctrl_ack status;
+	} *buf;
+
 	unsigned out_num = 0, tmp;
+	bool ret;
 
 	/* Caller should know better */
 	BUG_ON(!virtio_has_feature(vi->vdev, VIRTIO_NET_F_CTRL_VQ));
 
-	ctrl.class = class;
-	ctrl.cmd = cmd;
+	buf = kmalloc(sizeof(*buf), GFP_ATOMIC);
+	if (!buf)
+		return false;
+	buf->status = ~0;
+
+	buf->ctrl.class = class;
+	buf->ctrl.cmd = cmd;
 	/* Add header */
-	sg_init_one(&hdr, &ctrl, sizeof(ctrl));
+	sg_init_one(&hdr, &buf->ctrl, sizeof(buf->ctrl));
 	sgs[out_num++] = &hdr;
 
 	if (out)
 		sgs[out_num++] = out;
 
 	/* Add return status. */
-	sg_init_one(&stat, &status, sizeof(status));
+	sg_init_one(&stat, &buf->status, sizeof(buf->status));
 	sgs[out_num] = &stat;
 
 	BUG_ON(out_num + 1 > ARRAY_SIZE(sgs));
 	virtqueue_add_sgs(vi->cvq, sgs, out_num, 1, vi, GFP_ATOMIC);
 
-	if (unlikely(!virtqueue_kick(vi->cvq)))
-		return status == VIRTIO_NET_OK;
+	if (unlikely(!virtqueue_kick(vi->cvq))) {
+		ret = (buf->status == VIRTIO_NET_OK);
+		goto out;
+	}
 
 	/* Spin for a response, the kick causes an ioport write, trapping
 	 * into the hypervisor, so the request should be handled immediately.
@@ -999,7 +1011,11 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
 	       !virtqueue_is_broken(vi->cvq))
 		cpu_relax();
 
-	return status == VIRTIO_NET_OK;
+	ret = (buf->status == VIRTIO_NET_OK);
+
+out:
+	kfree(buf);
+	return ret;
 }
 
 static int virtnet_set_mac_address(struct net_device *dev, void *p)
@@ -1140,7 +1156,7 @@ static void virtnet_set_rx_mode(struct net_device *dev)
 {
 	struct virtnet_info *vi = netdev_priv(dev);
 	struct scatterlist sg[2];
-	u8 promisc, allmulti;
+	u8 *cmdbyte;
 	struct virtio_net_ctrl_mac *mac_data;
 	struct netdev_hw_addr *ha;
 	int uc_count;
@@ -1152,22 +1168,25 @@ static void virtnet_set_rx_mode(struct net_device *dev)
 	if (!virtio_has_feature(vi->vdev, VIRTIO_NET_F_CTRL_RX))
 		return;
 
-	promisc = ((dev->flags & IFF_PROMISC) != 0);
-	allmulti = ((dev->flags & IFF_ALLMULTI) != 0);
+	cmdbyte = kmalloc(sizeof(*cmdbyte), GFP_ATOMIC);
+	if (!cmdbyte)
+		return;
 
-	sg_init_one(sg, &promisc, sizeof(promisc));
+	sg_init_one(sg, cmdbyte, sizeof(*cmdbyte));
 
+	*cmdbyte = ((dev->flags & IFF_PROMISC) != 0);
 	if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_RX,
 				  VIRTIO_NET_CTRL_RX_PROMISC, sg))
 		dev_warn(&dev->dev, "Failed to %sable promisc mode.\n",
-			 promisc ? "en" : "dis");
-
-	sg_init_one(sg, &allmulti, sizeof(allmulti));
+			 *cmdbyte ? "en" : "dis");
 
+	*cmdbyte = ((dev->flags & IFF_ALLMULTI) != 0);
 	if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_RX,
 				  VIRTIO_NET_CTRL_RX_ALLMULTI, sg))
 		dev_warn(&dev->dev, "Failed to %sable allmulti mode.\n",
-			 allmulti ? "en" : "dis");
+			 *cmdbyte ? "en" : "dis");
+
+	kfree(cmdbyte);
 
 	uc_count = netdev_uc_count(dev);
 	mc_count = netdev_mc_count(dev);
-- 
1.9.3

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Benjamin Herrenschmidt]

On Mon, 2014-09-01 at 10:39 -0700, Andy Lutomirski wrote:
> Changes from v1:
>  - Using the DMA API is optional now.  It would be nice to improve the
>    DMA API to the point that it could be used unconditionally, but s390
>    proves that we're not there yet.
>  - Includes patch 4, which fixes DMA debugging warnings from virtio_net.

I'm not sure if you saw my reply on the other thread but I have a few
comments based on the above "it would be nice if ..."

So here we have both a yes and a no :-)

It would be nice to avoid those if () games all over and indeed just
use the DMA API, *however* we most certainly don't want to actually
create IOMMU mappings for the KVM virio case. This would be a massive
loss in performances on several platforms and generally doesn't make
much sense.

However, we can still use the API without that on any architecture
where the dma mapping API ends up calling the generic dma_map_ops,
it becomes just a matter of virtio setting up some special "nop" ops
when needed.

The difficulty here resides in the fact that we have never completely
made the dma_map_ops generic. The ops themselves are defined generically
as are the dma_map_* interfaces based on them, but the location of the
ops pointer is still more/less arch specific and some architectures
still chose not to use that indirection at all I believe.

Cheers,
Ben.

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

On Mon, Sep 1, 2014 at 3:16 PM, Benjamin Herrenschmidt
<benh@kernel.crashing.org> wrote:
> On Mon, 2014-09-01 at 10:39 -0700, Andy Lutomirski wrote:
>> Changes from v1:
>>  - Using the DMA API is optional now.  It would be nice to improve the
>>    DMA API to the point that it could be used unconditionally, but s390
>>    proves that we're not there yet.
>>  - Includes patch 4, which fixes DMA debugging warnings from virtio_net.
>
> I'm not sure if you saw my reply on the other thread but I have a few
> comments based on the above "it would be nice if ..."
>

Yeah, sorry, I sort of thought I responded, but I didn't do a very good job.

> So here we have both a yes and a no :-)
>
> It would be nice to avoid those if () games all over and indeed just
> use the DMA API, *however* we most certainly don't want to actually
> create IOMMU mappings for the KVM virio case. This would be a massive
> loss in performances on several platforms and generally doesn't make
> much sense.
>
> However, we can still use the API without that on any architecture
> where the dma mapping API ends up calling the generic dma_map_ops,
> it becomes just a matter of virtio setting up some special "nop" ops
> when needed.

I'm not quite convinced that this is a good idea.  I think that there
are three relevant categories of virtio devices:

a) Any virtio device where the normal DMA ops are nops.  This includes
x86 without an IOMMU (e.g. in a QEMU/KVM guest), 32-bit ARM, and
probably many other architectures.  In this case, what we do only
matters for performance, not for correctness.  Ideally the arch DMA
ops are fast.

b) Virtio devices that use physical addressing on systems where DMA
ops either don't exist at all (most s390) or do something nontrivial.
In this case, we must either override the DMA ops or just not use
them.

c) Virtio devices that use bus addressing.  This includes everything
on Xen (because the "physical" addresses are nonsense) and any actual
physical PCI device that speaks virtio on a system with an IOMMU.  In
this case, we must use the DMA ops.

The issue is that, on systems with DMA ops that do something, we need
to make sure that we know whether we're in case (b) or (c).  In these
patches, I've made the assumption that, if the virtio devices lives on
the PCI bus, then it uses the same type of addressing that any other
device on that PCI bus would use.

On x86, at least, I doubt that we'll ever see a physically addressed
PCI virtio device for which ACPI advertises an IOMMU, since any sane
hypervisor will just not advertise an IOMMU for the virtio device.
But are there arm64 or PPC guests that use virtio_pci, that have
IOMMUs, and that will malfunction if the virtio_pci driver ends up
using the IOMMU?  I certainly hope not, since these systems might be
very hard-pressed to work right if someone plugged in a physical
virtio-speaking PCI device.

>
> The difficulty here resides in the fact that we have never completely
> made the dma_map_ops generic. The ops themselves are defined generically
> as are the dma_map_* interfaces based on them, but the location of the
> ops pointer is still more/less arch specific and some architectures
> still chose not to use that indirection at all I believe.
>

I'd be happy to update the patches if someone does this, but I don't
really want to attack the DMA API on all architectures right now.  In
the mean time, at least s390 requires that we be able to compile out
the DMA API calls.  I'd rather see s390 provide working no-op dma ops
for all of the struct devices that provide virtio interfaces.

On a related note, shouldn't virtio be doing something to provide dma
ops to the virtio device and any of its children?  I don't know how it
would even try to do this, given how architecture-dependent this code
currently is.  Calling dma_map_single on the virtio device (as opposed
to its parent) is currently likely to crash on x86.  Fortunately,
nothing does this.

--Andy

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Benjamin Herrenschmidt]

On Mon, 2014-09-01 at 22:55 -0700, Andy Lutomirski wrote:
> 
> On x86, at least, I doubt that we'll ever see a physically addressed
> PCI virtio device for which ACPI advertises an IOMMU, since any sane
> hypervisor will just not advertise an IOMMU for the virtio device.
> But are there arm64 or PPC guests that use virtio_pci, that have
> IOMMUs, and that will malfunction if the virtio_pci driver ends up
> using the IOMMU?  I certainly hope not, since these systems might be
> very hard-pressed to work right if someone plugged in a physical
> virtio-speaking PCI device.

It will definitely not work on ppc64. We always have IOMMUs on pseries,
all PCI busses do, and because it's a paravirtualized environment,
napping/unmapping pages means hypercalls -> expensive.

But our virtio implementation bypasses it in qemu, so if virtio-pci
starts using the DMA mapping API without changing the DMA ops under the
hood, it will break for us.

Cheers,
Ben.

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Konrad Rzeszutek Wilk]

On Wed, Sep 03, 2014 at 06:53:33AM +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2014-09-01 at 22:55 -0700, Andy Lutomirski wrote:
> > 
> > On x86, at least, I doubt that we'll ever see a physically addressed
> > PCI virtio device for which ACPI advertises an IOMMU, since any sane
> > hypervisor will just not advertise an IOMMU for the virtio device.
> > But are there arm64 or PPC guests that use virtio_pci, that have
> > IOMMUs, and that will malfunction if the virtio_pci driver ends up
> > using the IOMMU?  I certainly hope not, since these systems might be
> > very hard-pressed to work right if someone plugged in a physical
> > virtio-speaking PCI device.
> 
> It will definitely not work on ppc64. We always have IOMMUs on pseries,
> all PCI busses do, and because it's a paravirtualized environment,
> napping/unmapping pages means hypercalls -> expensive.
> 
> But our virtio implementation bypasses it in qemu, so if virtio-pci
> starts using the DMA mapping API without changing the DMA ops under the
> hood, it will break for us.

What is the default dma_ops that the Linux guests start with as
guests under ppc64?

Thanks!
> 
> Cheers,
> Ben.
> 
> 

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Benjamin Herrenschmidt]

On Tue, 2014-09-02 at 16:56 -0400, Konrad Rzeszutek Wilk wrote:
> On Wed, Sep 03, 2014 at 06:53:33AM +1000, Benjamin Herrenschmidt wrote:
> > On Mon, 2014-09-01 at 22:55 -0700, Andy Lutomirski wrote:
> > > 
> > > On x86, at least, I doubt that we'll ever see a physically addressed
> > > PCI virtio device for which ACPI advertises an IOMMU, since any sane
> > > hypervisor will just not advertise an IOMMU for the virtio device.
> > > But are there arm64 or PPC guests that use virtio_pci, that have
> > > IOMMUs, and that will malfunction if the virtio_pci driver ends up
> > > using the IOMMU?  I certainly hope not, since these systems might be
> > > very hard-pressed to work right if someone plugged in a physical
> > > virtio-speaking PCI device.
> > 
> > It will definitely not work on ppc64. We always have IOMMUs on pseries,
> > all PCI busses do, and because it's a paravirtualized environment,
> > napping/unmapping pages means hypercalls -> expensive.
> > 
> > But our virtio implementation bypasses it in qemu, so if virtio-pci
> > starts using the DMA mapping API without changing the DMA ops under the
> > hood, it will break for us.
> 
> What is the default dma_ops that the Linux guests start with as
> guests under ppc64?

On pseries (which is what we care the most about nowadays) it's
dma_iommu_ops() which in turn call into the "TCE" code for populating
the IOMMU entries which calls the hypervisor.

Cheers,
Ben.

> Thanks!
> > 
> > Cheers,
> > Ben.
> > 
> > 

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Michael S. Tsirkin]

On Mon, Sep 01, 2014 at 10:55:29PM -0700, Andy Lutomirski wrote:
> On Mon, Sep 1, 2014 at 3:16 PM, Benjamin Herrenschmidt
> <benh@kernel.crashing.org> wrote:
> > On Mon, 2014-09-01 at 10:39 -0700, Andy Lutomirski wrote:
> >> Changes from v1:
> >>  - Using the DMA API is optional now.  It would be nice to improve the
> >>    DMA API to the point that it could be used unconditionally, but s390
> >>    proves that we're not there yet.
> >>  - Includes patch 4, which fixes DMA debugging warnings from virtio_net.
> >
> > I'm not sure if you saw my reply on the other thread but I have a few
> > comments based on the above "it would be nice if ..."
> >
> 
> Yeah, sorry, I sort of thought I responded, but I didn't do a very good job.
> 
> > So here we have both a yes and a no :-)
> >
> > It would be nice to avoid those if () games all over and indeed just
> > use the DMA API, *however* we most certainly don't want to actually
> > create IOMMU mappings for the KVM virio case. This would be a massive
> > loss in performances on several platforms and generally doesn't make
> > much sense.
> >
> > However, we can still use the API without that on any architecture
> > where the dma mapping API ends up calling the generic dma_map_ops,
> > it becomes just a matter of virtio setting up some special "nop" ops
> > when needed.
> 
> I'm not quite convinced that this is a good idea.  I think that there
> are three relevant categories of virtio devices:
> 
> a) Any virtio device where the normal DMA ops are nops.  This includes
> x86 without an IOMMU (e.g. in a QEMU/KVM guest), 32-bit ARM, and
> probably many other architectures.  In this case, what we do only
> matters for performance, not for correctness.  Ideally the arch DMA
> ops are fast.
> 
> b) Virtio devices that use physical addressing on systems where DMA
> ops either don't exist at all (most s390) or do something nontrivial.
> In this case, we must either override the DMA ops or just not use
> them.
> 
> c) Virtio devices that use bus addressing.  This includes everything
> on Xen (because the "physical" addresses are nonsense) and any actual
> physical PCI device that speaks virtio on a system with an IOMMU.  In
> this case, we must use the DMA ops.
> 
> The issue is that, on systems with DMA ops that do something, we need
> to make sure that we know whether we're in case (b) or (c).  In these
> patches, I've made the assumption that, if the virtio devices lives on
> the PCI bus, then it uses the same type of addressing that any other
> device on that PCI bus would use.
> 
> On x86, at least, I doubt that we'll ever see a physically addressed
> PCI virtio device for which ACPI advertises an IOMMU, since any sane
> hypervisor will just not advertise an IOMMU for the virtio device.

How exactly does one not advertise an IOMMU for a specific
device? Could you please clarify?

> But are there arm64 or PPC guests that use virtio_pci, that have
> IOMMUs, and that will malfunction if the virtio_pci driver ends up
> using the IOMMU?  I certainly hope not, since these systems might be
> very hard-pressed to work right if someone plugged in a physical
> virtio-speaking PCI device.

One simple fix is to defer this all until virtio 1.0.
virtio 1.0 has an alternative set of IDs for virtio pci,
that can be used if you are making an incompatible change.
We can use that if there's an iommu.


> >
> > The difficulty here resides in the fact that we have never completely
> > made the dma_map_ops generic. The ops themselves are defined generically
> > as are the dma_map_* interfaces based on them, but the location of the
> > ops pointer is still more/less arch specific and some architectures
> > still chose not to use that indirection at all I believe.
> >
> 
> I'd be happy to update the patches if someone does this, but I don't
> really want to attack the DMA API on all architectures right now.  In
> the mean time, at least s390 requires that we be able to compile out
> the DMA API calls.  I'd rather see s390 provide working no-op dma ops
> for all of the struct devices that provide virtio interfaces.
> 
> On a related note, shouldn't virtio be doing something to provide dma
> ops to the virtio device and any of its children?  I don't know how it
> would even try to do this, given how architecture-dependent this code
> currently is.  Calling dma_map_single on the virtio device (as opposed
> to its parent) is currently likely to crash on x86.  Fortunately,
> nothing does this.
> 
> --Andy

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

On Tue, Sep 2, 2014 at 1:53 PM, Benjamin Herrenschmidt
<benh@kernel.crashing.org> wrote:
> On Mon, 2014-09-01 at 22:55 -0700, Andy Lutomirski wrote:
>>
>> On x86, at least, I doubt that we'll ever see a physically addressed
>> PCI virtio device for which ACPI advertises an IOMMU, since any sane
>> hypervisor will just not advertise an IOMMU for the virtio device.
>> But are there arm64 or PPC guests that use virtio_pci, that have
>> IOMMUs, and that will malfunction if the virtio_pci driver ends up
>> using the IOMMU?  I certainly hope not, since these systems might be
>> very hard-pressed to work right if someone plugged in a physical
>> virtio-speaking PCI device.
>
> It will definitely not work on ppc64. We always have IOMMUs on pseries,
> all PCI busses do, and because it's a paravirtualized environment,
> napping/unmapping pages means hypercalls -> expensive.
>
> But our virtio implementation bypasses it in qemu, so if virtio-pci
> starts using the DMA mapping API without changing the DMA ops under the
> hood, it will break for us.
>

Let's take a step back from from the implementation.  What is a driver
for a virtio PCI device (i.e. a PCI device with vendor 0x1af4)
supposed to do on ppc64?

It can send the device physical addresses and ignore the normal PCI
DMA semantics, which is what the current virtio_pci driver does.  This
seems like a layering violation, and this won't work if the device is
a real PCI device.  Alternatively, it can treat the device like any
other PCI device and use the IOMMU.  This is a bit slower, and it is
also incompatible with current hypervisors.

There really are virtio devices that are pieces of silicon and not
figments of a hypervisor's imagination [1].  We could teach virtio_pci
to use physical addressing on ppc64, but that seems like a pretty
awful hack, and it'll start needing quirks as soon as someone tries to
plug a virtio-speaking PCI card into a ppc64 machine.

Ideas?  x86 and arm seem to be safe here, since AFAIK there is no such
thing as a physically addressed virtio "PCI" device on a bus with an
IOMMU on x86, arm, or arm64.

[1] https://lwn.net/Articles/580186/

> Cheers,
> Ben.
>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

On Tue, Sep 2, 2014 at 2:10 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Mon, Sep 01, 2014 at 10:55:29PM -0700, Andy Lutomirski wrote:
>> On Mon, Sep 1, 2014 at 3:16 PM, Benjamin Herrenschmidt
>> <benh@kernel.crashing.org> wrote:
>> > On Mon, 2014-09-01 at 10:39 -0700, Andy Lutomirski wrote:
>> >> Changes from v1:
>> >>  - Using the DMA API is optional now.  It would be nice to improve the
>> >>    DMA API to the point that it could be used unconditionally, but s390
>> >>    proves that we're not there yet.
>> >>  - Includes patch 4, which fixes DMA debugging warnings from virtio_net.
>> >
>> > I'm not sure if you saw my reply on the other thread but I have a few
>> > comments based on the above "it would be nice if ..."
>> >
>>
>> Yeah, sorry, I sort of thought I responded, but I didn't do a very good job.
>>
>> > So here we have both a yes and a no :-)
>> >
>> > It would be nice to avoid those if () games all over and indeed just
>> > use the DMA API, *however* we most certainly don't want to actually
>> > create IOMMU mappings for the KVM virio case. This would be a massive
>> > loss in performances on several platforms and generally doesn't make
>> > much sense.
>> >
>> > However, we can still use the API without that on any architecture
>> > where the dma mapping API ends up calling the generic dma_map_ops,
>> > it becomes just a matter of virtio setting up some special "nop" ops
>> > when needed.
>>
>> I'm not quite convinced that this is a good idea.  I think that there
>> are three relevant categories of virtio devices:
>>
>> a) Any virtio device where the normal DMA ops are nops.  This includes
>> x86 without an IOMMU (e.g. in a QEMU/KVM guest), 32-bit ARM, and
>> probably many other architectures.  In this case, what we do only
>> matters for performance, not for correctness.  Ideally the arch DMA
>> ops are fast.
>>
>> b) Virtio devices that use physical addressing on systems where DMA
>> ops either don't exist at all (most s390) or do something nontrivial.
>> In this case, we must either override the DMA ops or just not use
>> them.
>>
>> c) Virtio devices that use bus addressing.  This includes everything
>> on Xen (because the "physical" addresses are nonsense) and any actual
>> physical PCI device that speaks virtio on a system with an IOMMU.  In
>> this case, we must use the DMA ops.
>>
>> The issue is that, on systems with DMA ops that do something, we need
>> to make sure that we know whether we're in case (b) or (c).  In these
>> patches, I've made the assumption that, if the virtio devices lives on
>> the PCI bus, then it uses the same type of addressing that any other
>> device on that PCI bus would use.
>>
>> On x86, at least, I doubt that we'll ever see a physically addressed
>> PCI virtio device for which ACPI advertises an IOMMU, since any sane
>> hypervisor will just not advertise an IOMMU for the virtio device.
>
> How exactly does one not advertise an IOMMU for a specific
> device? Could you please clarify?

See https://software.intel.com/en-us/blogs/2009/09/11/decoding-the-dmar-tables-in-acpiiommu-part-2

I think that all that needs to happen is for ACPI to not list the
device in the scope of any drhd unit.  I don't know whether this works
correctly, but it looks like the iommu_dummy and the
init_no_remapping_devices code in intel-iommu.c exists for almost
exactly this purpose.

>
>> But are there arm64 or PPC guests that use virtio_pci, that have
>> IOMMUs, and that will malfunction if the virtio_pci driver ends up
>> using the IOMMU?  I certainly hope not, since these systems might be
>> very hard-pressed to work right if someone plugged in a physical
>> virtio-speaking PCI device.
>
> One simple fix is to defer this all until virtio 1.0.
> virtio 1.0 has an alternative set of IDs for virtio pci,
> that can be used if you are making an incompatible change.
> We can use that if there's an iommu.

How?  If someone builds a physical device compliant with the virtio
1.0 specification, how do can that device know whether it's behind an
IOMMU?  The IOMMU is part of the host (or Xen, sort of), not the PCI
device.  I suppose that virtio 1.0 could add a bit indicating that the
virtio device is a physical piece of hardware (presumably this should
be PCI-specific).

--Andy

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Benjamin Herrenschmidt]

On Tue, 2014-09-02 at 14:37 -0700, Andy Lutomirski wrote:

> Let's take a step back from from the implementation.  What is a driver
> for a virtio PCI device (i.e. a PCI device with vendor 0x1af4)
> supposed to do on ppc64?

Today, it's supposed to send guest physical addresses. We can make that
optional via some nego or capabilities to support more esoteric setups
but for backward compatibility, this must remain the default behaviour.

> It can send the device physical addresses and ignore the normal PCI
> DMA semantics, which is what the current virtio_pci driver does.  This
> seems like a layering violation, and this won't work if the device is
> a real PCI device.

Correct, it's an original virtio implementation choice for maximum
performances.

>   Alternatively, it can treat the device like any
> other PCI device and use the IOMMU.  This is a bit slower, and it is
> also incompatible with current hypervisors.

This is a potentially a LOT slower and is backward incompatible with
current qemu/KVM and kvmtool yes.

The slowness can be alleviated using various techniques, for example on
ppc64 we can create a DMA window that contains a permanent mapping of
the entire guest space, so we could use such a thing for virtio.

Another think we could do potentially is advertize via the device-tree
that such a bus uses a direct mapping and have the guest use appropriate
"direct map" dma_ops.

But we need to keep backward compatibility with existing
guest/hypervisors so the default must remain as it is.

> There really are virtio devices that are pieces of silicon and not
> figments of a hypervisor's imagination [1].

I am aware of that. There are also attempts at using virtio to make two
machines communicate via a PCIe link (either with one as endpoint of the
other or via a non-transparent switch).

Which is why I'm not objecting to what you are trying to do ;-)

My suggestion was that it might be a cleaner approach to do that by
having the individual virtio drivers always use the dma_map_* API, and
limiting the kludgery to a combination of virtio_pci "core" and arch
code by selecting an appropriate set of dma_map_ops, defaulting with a
"transparent" (or direct) one as our current default case (and thus
overriding the iommu ones provided by the arch).

>   We could teach virtio_pci
> to use physical addressing on ppc64, but that seems like a pretty
> awful hack, and it'll start needing quirks as soon as someone tries to
> plug a virtio-speaking PCI card into a ppc64 machine.

But x86_64 is the same no ? The day it starts growing an iommu emulation
in qemu (and I've heard it's happening) it will still want to do direct
bypass for virtio for performance.

> Ideas?  x86 and arm seem to be safe here, since AFAIK there is no such
> thing as a physically addressed virtio "PCI" device on a bus with an
> IOMMU on x86, arm, or arm64.

Today .... I wouldn't bet on it to remain that way. The qemu
implementation of virtio is physically addressed and you don't
necessarily have a choice of which device gets an iommu and which not.

Cheers,
Ben.

> [1] https://lwn.net/Articles/580186/
> 
> > Cheers,
> > Ben.
> >
> >
> 
> 
> 

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

On Tue, Sep 2, 2014 at 3:10 PM, Benjamin Herrenschmidt
<benh@kernel.crashing.org> wrote:
> On Tue, 2014-09-02 at 14:37 -0700, Andy Lutomirski wrote:
>
>> Let's take a step back from from the implementation.  What is a driver
>> for a virtio PCI device (i.e. a PCI device with vendor 0x1af4)
>> supposed to do on ppc64?
>
> Today, it's supposed to send guest physical addresses. We can make that
> optional via some nego or capabilities to support more esoteric setups
> but for backward compatibility, this must remain the default behaviour.

I think it only needs to remain the default in cases where the
alternative (bus addressing) won't work.  I think that, so far, this
is just ppc64.  But see below...

>
> My suggestion was that it might be a cleaner approach to do that by
> having the individual virtio drivers always use the dma_map_* API, and
> limiting the kludgery to a combination of virtio_pci "core" and arch
> code by selecting an appropriate set of dma_map_ops, defaulting with a
> "transparent" (or direct) one as our current default case (and thus
> overriding the iommu ones provided by the arch).

I think the cleanest way of all would be to get the bus drivers to do
the right thing so that all of the virtio code can just use the dma
api.  I don't know whether this is achievable.

>
>>   We could teach virtio_pci
>> to use physical addressing on ppc64, but that seems like a pretty
>> awful hack, and it'll start needing quirks as soon as someone tries to
>> plug a virtio-speaking PCI card into a ppc64 machine.
>
> But x86_64 is the same no ? The day it starts growing an iommu emulation
> in qemu (and I've heard it's happening) it will still want to do direct
> bypass for virtio for performance.

I don't think so.  I would argue that it's a straight-up bug for QEMU
to expose a physically-addressed virtio-pci device to the guest behind
an emulated IOMMU.  QEMU may already be doing that on ppc64, but it
isn't on x86_64 or arm (yet).

On x86_64, I'm pretty sure that QEMU can emulate an IOMMU for
everything except the virtio-pci devices.  The ACPI DMAR stuff is
quite expressive.

On ARM, I hope the QEMU will never implement a PCI IOMMU.  As far as I
could tell when I looked last week, none of the newer QEMU-emulated
ARM machines even support PCI.  Even if QEMU were to implement a PCI
IOMMU on some future ARM machine, it could continue using virtio-mmio
for virtio devices.

So ppc might actually be the only system that has or will have
physically-addressed virtio PCI devices that are behind an IOMMU.  Can
this be handled in a ppc64-specific way?  Is there any way that the
kernel can distinguish a QEMU-provided virtio PCI device from a
physical PCIe thing?  It would be kind of nice to address this without
adding complexity to the virtio spec.  Maybe virtio 1.0 devices could
be assumed to use bus addressing unless a new devicetree property says
otherwise.

--Andy

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Benjamin Herrenschmidt]

On Tue, 2014-09-02 at 16:11 -0700, Andy Lutomirski wrote:

> I don't think so.  I would argue that it's a straight-up bug for QEMU
> to expose a physically-addressed virtio-pci device to the guest behind
> an emulated IOMMU.  QEMU may already be doing that on ppc64, but it
> isn't on x86_64 or arm (yet).

Last I looked, it does on everything, it bypasses the DMA layer in qemu
which is where IOMMUs are implemented.

> On x86_64, I'm pretty sure that QEMU can emulate an IOMMU for
> everything except the virtio-pci devices.  The ACPI DMAR stuff is
> quite expressive.

Well, *except* virtio, exactly...

> On ARM, I hope the QEMU will never implement a PCI IOMMU.  As far as I
> could tell when I looked last week, none of the newer QEMU-emulated
> ARM machines even support PCI.  Even if QEMU were to implement a PCI
> IOMMU on some future ARM machine, it could continue using virtio-mmio
> for virtio devices.

Possibly...

> So ppc might actually be the only system that has or will have
> physically-addressed virtio PCI devices that are behind an IOMMU.  Can
> this be handled in a ppc64-specific way?

I wouldn't be so certain, as I said, the way virtio is implemented in
qemu bypass the DMA layer which is where IOMMUs sit. The fact that
currently x86 doesn't put an IOMMU there is not even garanteed, is it ?
What happens if you try to mix and match virtio and other emulated
devices that require the iommu on the same bus ?

If we could discriminate virtio devices to a specific host bridge and
guarantee no mix & match, we could probably add a concept of
"IOMMU-less" bus but that would require guest changes which limits the
usefulness.

>   Is there any way that the
> kernel can distinguish a QEMU-provided virtio PCI device from a
> physical PCIe thing? 

Not with existing guests which cannot be changed. Existing distros are
out with those drivers. If we add a backward compatibility mechanism,
then we could add something yes, provided we can segregate virtio onto a
dedicated host bridge (which can be a problem with the libvirt
trainwreck...)

>  It would be kind of nice to address this without
> adding complexity to the virtio spec.  Maybe virtio 1.0 devices could
> be assumed to use bus addressing unless a new devicetree property says
> otherwise.

Cheers,
Ben.

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

On Tue, Sep 2, 2014 at 4:20 PM, Benjamin Herrenschmidt <benh@au1.ibm.com> wrote:
> On Tue, 2014-09-02 at 16:11 -0700, Andy Lutomirski wrote:
>
>> I don't think so.  I would argue that it's a straight-up bug for QEMU
>> to expose a physically-addressed virtio-pci device to the guest behind
>> an emulated IOMMU.  QEMU may already be doing that on ppc64, but it
>> isn't on x86_64 or arm (yet).
>
> Last I looked, it does on everything, it bypasses the DMA layer in qemu
> which is where IOMMUs are implemented.

I believe you, but I'm not convinced that this means much from the
guest's POV, except on ppc64.

>
>> On x86_64, I'm pretty sure that QEMU can emulate an IOMMU for
>> everything except the virtio-pci devices.  The ACPI DMAR stuff is
>> quite expressive.
>
> Well, *except* virtio, exactly...

But there aren't any ACPI systems with both virtio-pci and IOMMUs,
right?  So we could say that, henceforth, ACPI systems must declare
whether virtio-pci devices live behind IOMMUs without breaking
backward compatibility.

>
>> On ARM, I hope the QEMU will never implement a PCI IOMMU.  As far as I
>> could tell when I looked last week, none of the newer QEMU-emulated
>> ARM machines even support PCI.  Even if QEMU were to implement a PCI
>> IOMMU on some future ARM machine, it could continue using virtio-mmio
>> for virtio devices.
>
> Possibly...
>
>> So ppc might actually be the only system that has or will have
>> physically-addressed virtio PCI devices that are behind an IOMMU.  Can
>> this be handled in a ppc64-specific way?
>
> I wouldn't be so certain, as I said, the way virtio is implemented in
> qemu bypass the DMA layer which is where IOMMUs sit. The fact that
> currently x86 doesn't put an IOMMU there is not even garanteed, is it ?
> What happens if you try to mix and match virtio and other emulated
> devices that require the iommu on the same bus ?

AFAIK QEMU doesn't support IOMMUs at all on x86, so current versions
of QEMU really do guarantee that virtio-pci on x86 has no IOMMU, even
if that guarantee is purely accidental.

>
> If we could discriminate virtio devices to a specific host bridge and
> guarantee no mix & match, we could probably add a concept of
> "IOMMU-less" bus but that would require guest changes which limits the
> usefulness.
>
>>   Is there any way that the
>> kernel can distinguish a QEMU-provided virtio PCI device from a
>> physical PCIe thing?
>
> Not with existing guests which cannot be changed. Existing distros are
> out with those drivers. If we add a backward compatibility mechanism,
> then we could add something yes, provided we can segregate virtio onto a
> dedicated host bridge (which can be a problem with the libvirt
> trainwreck...)

Ugh.

So here's an ugly proposal:

Step 1: Make virtio-pci use the DMA API only on x86.  This will at
least fix Xen and people experimenting with virtio hardware on x86,
and it won't break anything, since there are no emulated IOMMUs on
x86.

Step 2: Update the virtio spec.  Virtio 1.0 PCI devices should set a
new bit if they are physically addressed.  If that bit is clear, then
the device is assumed to be addressed in accordance with the
platform's standard addressing model for PCI.  Presumably this would
be something like VIRTIO_F_BUS_ADDRESSING = 33, and the spec would say
something like "Physical devices compatible with this specification
MUST offer VIRTIO_F_BUS_ADDRESSING.  Drivers MUST implement this
feature."  Alternatively, this could live in a PCI configuration
capability.

Step 3: Update virtio-pci to use the DMA API for all devices on x86
and for devices that advertise bus addressing on other architectures.

I think this proposal will work, but I also think it sucks and I'd
really like to see a better counter-proposal.


--Andy

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Benjamin Herrenschmidt]

On Tue, 2014-09-02 at 16:42 -0700, Andy Lutomirski wrote:

> But there aren't any ACPI systems with both virtio-pci and IOMMUs,
> right?  So we could say that, henceforth, ACPI systems must declare
> whether virtio-pci devices live behind IOMMUs without breaking
> backward compatibility.

I don't know for sure whether that's the case and whether we can rely on
that not happening, we'll need x86 folks opinion here.

> >> On ARM, I hope the QEMU will never implement a PCI IOMMU.  As far as I
> >> could tell when I looked last week, none of the newer QEMU-emulated
> >> ARM machines even support PCI.  Even if QEMU were to implement a PCI
> >> IOMMU on some future ARM machine, it could continue using virtio-mmio
> >> for virtio devices.
> >
> > Possibly...
> >
> >> So ppc might actually be the only system that has or will have
> >> physically-addressed virtio PCI devices that are behind an IOMMU.  Can
> >> this be handled in a ppc64-specific way?
> >
> > I wouldn't be so certain, as I said, the way virtio is implemented in
> > qemu bypass the DMA layer which is where IOMMUs sit. The fact that
> > currently x86 doesn't put an IOMMU there is not even garanteed, is it ?
> > What happens if you try to mix and match virtio and other emulated
> > devices that require the iommu on the same bus ?
> 
> AFAIK QEMU doesn't support IOMMUs at all on x86, so current versions
> of QEMU really do guarantee that virtio-pci on x86 has no IOMMU, even
> if that guarantee is purely accidental.

Right.

> > If we could discriminate virtio devices to a specific host bridge and
> > guarantee no mix & match, we could probably add a concept of
> > "IOMMU-less" bus but that would require guest changes which limits the
> > usefulness.
> >
> >>   Is there any way that the
> >> kernel can distinguish a QEMU-provided virtio PCI device from a
> >> physical PCIe thing?
> >
> > Not with existing guests which cannot be changed. Existing distros are
> > out with those drivers. If we add a backward compatibility mechanism,
> > then we could add something yes, provided we can segregate virtio onto a
> > dedicated host bridge (which can be a problem with the libvirt
> > trainwreck...)
> 
> Ugh.
> 
> So here's an ugly proposal:
> 
> Step 1: Make virtio-pci use the DMA API only on x86.  This will at
> least fix Xen and people experimenting with virtio hardware on x86,
> and it won't break anything, since there are no emulated IOMMUs on
> x86.

I think we should make all virtio drivers use the DMA API and just have
different set of dma_ops. We can make a simple ifdef powerpc if needed
in virtio-pci that force the dma-ops of the device to some direct
"bypass" ops at init time.

That way no need to select whether to use the DMA API or not, just
always use it, and add a tweak to replace the DMA ops with the direct
ones on the archs/platforms that need that. That was my original
proposal and I still think it's the best approach.

> Step 2: Update the virtio spec.  Virtio 1.0 PCI devices should set a
> new bit if they are physically addressed.  If that bit is clear, then
> the device is assumed to be addressed in accordance with the
> platform's standard addressing model for PCI.  Presumably this would
> be something like VIRTIO_F_BUS_ADDRESSING = 33, and the spec would say
> something like "Physical devices compatible with this specification
> MUST offer VIRTIO_F_BUS_ADDRESSING.  Drivers MUST implement this
> feature."  Alternatively, this could live in a PCI configuration
> capability.

I'll let you sort that out with Rusty but it makes sense.

> Step 3: Update virtio-pci to use the DMA API for all devices on x86
> and for devices that advertise bus addressing on other architectures.
> 
> I think this proposal will work, but I also think it sucks and I'd
> really like to see a better counter-proposal.

As I said, make it always use the DMA API, but add a quirk to replace
the dma_ops with some NULL ops on platforms that need it.

The only issue with that is the location of the dma ops is arch
specific, so that one function will contain some ifdefs, but the rest of
the code can just use the DMA API.
 
Cheers,
Ben.

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

On Tue, Sep 2, 2014 at 5:25 PM, Benjamin Herrenschmidt <benh@au1.ibm.com> wrote:
> On Tue, 2014-09-02 at 16:42 -0700, Andy Lutomirski wrote:
>> So here's an ugly proposal:
>>
>> Step 1: Make virtio-pci use the DMA API only on x86.  This will at
>> least fix Xen and people experimenting with virtio hardware on x86,
>> and it won't break anything, since there are no emulated IOMMUs on
>> x86.
>
> I think we should make all virtio drivers use the DMA API and just have
> different set of dma_ops. We can make a simple ifdef powerpc if needed
> in virtio-pci that force the dma-ops of the device to some direct
> "bypass" ops at init time.
>
> That way no need to select whether to use the DMA API or not, just
> always use it, and add a tweak to replace the DMA ops with the direct
> ones on the archs/platforms that need that. That was my original
> proposal and I still think it's the best approach.

I agree *except* that implementing it will be a real PITA and (I
think) can't be done without changing code in arch/.  My patches plus
an ifdef powerpc will be functionally equivalent, just uglier.

>
> As I said, make it always use the DMA API, but add a quirk to replace
> the dma_ops with some NULL ops on platforms that need it.
>
> The only issue with that is the location of the dma ops is arch
> specific, so that one function will contain some ifdefs, but the rest of
> the code can just use the DMA API.

Bigger quirk: on a standard s390 virtio guest configuration,
dma_map_single etc will fail to link.  I tried this in v1 of these
patches.  So we can poke at the archdata all day, but we can't build a
kernel like that :(

So until the dma_ops pointer move into struct device and
CONFIG_HAS_DMA becomes mandatory (or mandatory enough that virtio can
depend on it), I don't think we can do it this way.

I'll send a v5 that is the same as v4 except with physical addressing
hardcoded in for powerpc.

--Andy

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Benjamin Herrenschmidt]

On Tue, 2014-09-02 at 17:32 -0700, Andy Lutomirski wrote:
> 
> I agree *except* that implementing it will be a real PITA and (I
> think) can't be done without changing code in arch/.  My patches plus
> an ifdef powerpc will be functionally equivalent, just uglier.

So for powerpc, it's a 2 liner inside virtio-pci, but yes, it might be
more of a problem for s390, I'm not too sure what they do in that area.

> Bigger quirk: on a standard s390 virtio guest configuration,
> dma_map_single etc will fail to link. 

Yuck

>  I tried this in v1 of these
> patches.  So we can poke at the archdata all day, but we can't build a
> kernel like that :(

I would like the s390 people to chime in here, it still looks like the
best way to go if they can fix things on their side :-)

> So until the dma_ops pointer move into struct device and
> CONFIG_HAS_DMA becomes mandatory (or mandatory enough that virtio can
> depend on it), I don't think we can do it this way.

I see, it's a bummer because it would be a lot cleaner.

> I'll send a v5 that is the same as v4 except with physical addressing
> hardcoded in for powerpc.

Thanks. That will do for now, but ideally we want to make it a function
of some flag from the implementation, so let's see what Rusty has to
say.

Cheers,
Ben.

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Rusty Russell]

Andy Lutomirski <luto@amacapital.net> writes:
> There really are virtio devices that are pieces of silicon and not
> figments of a hypervisor's imagination [1].

Hi Andy,

        As you're discovering, there's a reason no one has done the DMA
API before.

So the problem is that ppc64's IOMMU is a platform thing, not a bus
thing.  They really do carve out an exception for virtio devices,
because performance (LOTS of performance).  It remains to be seen if
other platforms have the same performance issues, but in absence of
other evidence, the answer is yes.

It's a hack.  But having specific virtual-only devices are an even
bigger hack.

Physical virtio devices have been talked about, but don't actually exist
in Real Life.  And someone a virtio PCI card is going to have serious
performance issues: mainly because they'll want the rings in the card's
MMIO region, not allocated by the driver.  Being broken on PPC is really
the least of their problems.

So, what do we do?  It'd be nice if Linux virtio Just Worked under Xen,
though Xen's IOMMU is outside the virtio spec.  Since virtio_pci can be
a module, obvious hacks like having xen_arch_setup initialize a dma_ops pointer
exposed by virtio_pci.c is out.

I think the best approach is to have a new feature bit (25 is free),
VIRTIO_F_USE_BUS_MAPPING which indicates that a device really wants to
use the mapping for the bus it is on.  A real device would set this,
or it won't work behind an IOMMU.  A Xen device would also set this.

Thoughts?
Rusty.

PS.  I cc'd OASIS virtio-dev: it's subscriber only for IP reasons (to
     subscribe you have to promise we can use your suggestion in the
     standard).  Feel free to remove in any replies, but it's part of
     the world we live in...

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Paolo Bonzini]

Il 03/09/2014 01:20, Benjamin Herrenschmidt ha scritto:
> I wouldn't be so certain, as I said, the way virtio is implemented in
> qemu bypass the DMA layer which is where IOMMUs sit. The fact that
> currently x86 doesn't put an IOMMU there is not even garanteed, is it ?
> What happens if you try to mix and match virtio and other emulated
> devices that require the iommu on the same bus ?

As far as QEMU is concerned, it's trivial to add a property like
"direct-ram-access" that selects whether to bypass the IOMMU or not.
And it would have zero performance cost if direct RAM access is enabled,
compared to the current code.

If possible, I would quirk it in the PPC code.

Paolo

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Paolo Bonzini]

Il 03/09/2014 02:25, Benjamin Herrenschmidt ha scritto:
> > But there aren't any ACPI systems with both virtio-pci and IOMMUs,
> > right?  So we could say that, henceforth, ACPI systems must declare
> > whether virtio-pci devices live behind IOMMUs without breaking
> > backward compatibility.
> 
> I don't know for sure whether that's the case and whether we can rely on
> that not happening, we'll need x86 folks opinion here.

IOMMU support for x86 is going to go in this week.

However, it is and likely will remain niche enough that I don't really
care about performance loss from IOMMU support.  If you enable it, you
want it.

So from the QEMU point of view we can simply add the direct-ram-access
property, and have the pseries machine turn it on by default (while
other machines can leave it off by default---they have no IOMMU and thus
no performance cost).

Paolo

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

On Sep 2, 2014 11:53 PM, "Rusty Russell" <rusty@rustcorp.com.au> wrote:
>
> Andy Lutomirski <luto@amacapital.net> writes:
> > There really are virtio devices that are pieces of silicon and not
> > figments of a hypervisor's imagination [1].
>
> Hi Andy,
>
>         As you're discovering, there's a reason no one has done the DMA
> API before.
>
> So the problem is that ppc64's IOMMU is a platform thing, not a bus
> thing.  They really do carve out an exception for virtio devices,
> because performance (LOTS of performance).  It remains to be seen if
> other platforms have the same performance issues, but in absence of
> other evidence, the answer is yes.
>
> It's a hack.  But having specific virtual-only devices are an even
> bigger hack.
>
> Physical virtio devices have been talked about, but don't actually exist
> in Real Life.  And someone a virtio PCI card is going to have serious
> performance issues: mainly because they'll want the rings in the card's
> MMIO region, not allocated by the driver.  Being broken on PPC is really
> the least of their problems.
>
> So, what do we do?  It'd be nice if Linux virtio Just Worked under Xen,
> though Xen's IOMMU is outside the virtio spec.  Since virtio_pci can be
> a module, obvious hacks like having xen_arch_setup initialize a dma_ops pointer
> exposed by virtio_pci.c is out.

Xen does expose dma_ops.  The trick is knowing when to use it.

>
> I think the best approach is to have a new feature bit (25 is free),
> VIRTIO_F_USE_BUS_MAPPING which indicates that a device really wants to
> use the mapping for the bus it is on.  A real device would set this,
> or it won't work behind an IOMMU.  A Xen device would also set this.

The devices I care about aren't actually Xen devices.  They're devices
supplied by QEMU/KVM, booting a Xen hypervisor, which in turn passes
the virtio device (along with every other PCI device) through to dom0.
So this is exactly the same virtio device that regular x86 KVM guests
would see.  The reason that current code fails is that Xen guest
physical addresses aren't the same as the addresses seen by the outer
hypervisor.

These devices don't know that physical addresses != bus addresses, so
they can't advertise that fact.

If we ever end up with a virtio_pci device with physical addressing,
behind an IOMMU (but ignoring it), on Xen, we'll have a problem, since
neither "physical" addressing nor dma ops will work.

That being said, there are also proposals for virtio devices supplied
by Xen dom0 to domU, and these will presumably work the same way,
except that the device implementation will know that it's on Xen.

Grr.  This is mostly a result of the fact that virtio_pci devices
aren't really PCI devices.  I still think that virtio_pci shouldn't
have to worry about this; ideally this would all be handled higher up
in the device hierarchy.  x86 already gets this right.


Are there any hypervisors except PPC that use virtio_pci, have IOMMUs
on the pci slot that virtio_pci lives in, and that use physical
addressing?  If not, I think that just quirking PPC will work (at
least until someone wants IOMMU support in virtio_pci on PPC, in which
case doing something using devicetree seems like a reasonable
solution).

--Andy

>
> Thoughts?
> Rusty.
>
> PS.  I cc'd OASIS virtio-dev: it's subscriber only for IP reasons (to
>      subscribe you have to promise we can use your suggestion in the
>      standard).  Feel free to remove in any replies, but it's part of
>      the world we live in...

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

On Wed, Sep 3, 2014 at 12:47 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> Il 03/09/2014 02:25, Benjamin Herrenschmidt ha scritto:
>> > But there aren't any ACPI systems with both virtio-pci and IOMMUs,
>> > right?  So we could say that, henceforth, ACPI systems must declare
>> > whether virtio-pci devices live behind IOMMUs without breaking
>> > backward compatibility.
>>
>> I don't know for sure whether that's the case and whether we can rely on
>> that not happening, we'll need x86 folks opinion here.
>
> IOMMU support for x86 is going to go in this week.
>

Can you try to make sure that qemu-system-x86_64 -device iommu -device
virtio-balloon-pci (or whatever the syntax is) doesn't put the
virtio-pci device behind the IOMMU?  Because, if it does, then the
kernel will have to support that, and it'll be messy.

--Andy

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Paolo Bonzini]

Il 03/09/2014 09:52, Andy Lutomirski ha scritto:
>> > IOMMU support for x86 is going to go in this week.
>> >
> Can you try to make sure that qemu-system-x86_64 -device iommu -device
> virtio-balloon-pci (or whatever the syntax is) doesn't put the
> virtio-pci device behind the IOMMU?  Because, if it does, then the
> kernel will have to support that, and it'll be messy.

Right now it will not put the device behind the IOMMU, but I'm fairly
sure that the DMAR will show the device as being behind the IOMMU.

We have time till QEMU 2.2 is out to make it use the IOMMU for
virtio-pci devices on x86.  I'm not worried about that.

The virtio-pci devices do set the "bus master" bit in the command
register, right?  I think they do, because otherwise MSIs will not be
received by the guest.

Paolo

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Benjamin Herrenschmidt]

On Wed, 2014-09-03 at 09:47 +0200, Paolo Bonzini wrote:
> 
> IOMMU support for x86 is going to go in this week.

But won't that break virtio on x86 ? Or will virtio continue bypassing
it ? IE, the guest side virtio doesn't expect an IOMMU and doesn't call
the dma mappings ops.

> However, it is and likely will remain niche enough that I don't really
> care about performance loss from IOMMU support.  If you enable it, you
> want it.
> 
> So from the QEMU point of view we can simply add the direct-ram-access
> property, and have the pseries machine turn it on by default (while
> other machines can leave it off by default---they have no IOMMU and
> thus no performance cost).

Well, it's only for virtio and should be on by default on x86 as well if
an iommu is installed no ?

Cheers,
Ben.

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Paolo Bonzini]

Il 03/09/2014 10:05, Benjamin Herrenschmidt ha scritto:
> On Wed, 2014-09-03 at 09:47 +0200, Paolo Bonzini wrote:
>>
>> IOMMU support for x86 is going to go in this week.
> 
> But won't that break virtio on x86 ? Or will virtio continue bypassing
> it ? IE, the guest side virtio doesn't expect an IOMMU and doesn't call
> the dma mappings ops.
> 
>> However, it is and likely will remain niche enough that I don't really
>> care about performance loss from IOMMU support.  If you enable it, you
>> want it.
>>
>> So from the QEMU point of view we can simply add the direct-ram-access
>> property, and have the pseries machine turn it on by default (while
>> other machines can leave it off by default---they have no IOMMU and
>> thus no performance cost).
> 
> Well, it's only for virtio and should be on by default on x86 as well if
> an iommu is installed no ?

Yes, only for virtio---but for x86 I think it should be off by default,
even if that means virtio+IOMMU requires a new kernel.

Paolo

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Michael S. Tsirkin]

On Wed, Sep 03, 2014 at 04:12:01PM +0930, Rusty Russell wrote:
> Andy Lutomirski <luto@amacapital.net> writes:
> > There really are virtio devices that are pieces of silicon and not
> > figments of a hypervisor's imagination [1].
> 
> Hi Andy,
> 
>         As you're discovering, there's a reason no one has done the DMA
> API before.
> 
> So the problem is that ppc64's IOMMU is a platform thing, not a bus
> thing.  They really do carve out an exception for virtio devices,
> because performance (LOTS of performance).  It remains to be seen if
> other platforms have the same performance issues, but in absence of
> other evidence, the answer is yes.
> 
> It's a hack.  But having specific virtual-only devices are an even
> bigger hack.
> 
> Physical virtio devices have been talked about, but don't actually exist
> in Real Life.  And someone a virtio PCI card is going to have serious
> performance issues: mainly because they'll want the rings in the card's
> MMIO region, not allocated by the driver.

Why? What's wrong with rings in memory?

>  Being broken on PPC is really
> the least of their problems.
> 
> So, what do we do?  It'd be nice if Linux virtio Just Worked under Xen,
> though Xen's IOMMU is outside the virtio spec.  Since virtio_pci can be
> a module, obvious hacks like having xen_arch_setup initialize a dma_ops pointer
> exposed by virtio_pci.c is out.

Well virtio could probe for xen, it's not a lot of code.

> I think the best approach is to have a new feature bit (25 is free),
> VIRTIO_F_USE_BUS_MAPPING which indicates that a device really wants to
> use the mapping for the bus it is on.  A real device would set this,
> or it won't work behind an IOMMU.  A Xen device would also set this.
> 
> Thoughts?
> Rusty.

OK and it should then be active even if guest does not ack
the feature (so in fact, it would have to be a mandatory feature).
That can work, but I still find this a bit inelegant: this is
a property of the platform, not of the device.


> PS.  I cc'd OASIS virtio-dev: it's subscriber only for IP reasons (to
>      subscribe you have to promise we can use your suggestion in the
>      standard).  Feel free to remove in any replies, but it's part of
>      the world we live in...

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

On Sep 3, 2014 5:11 AM, "Paolo Bonzini" <pbonzini@redhat.com> wrote:
>
> Il 03/09/2014 10:05, Benjamin Herrenschmidt ha scritto:
> > On Wed, 2014-09-03 at 09:47 +0200, Paolo Bonzini wrote:
> >>
> >> IOMMU support for x86 is going to go in this week.
> >
> > But won't that break virtio on x86 ? Or will virtio continue bypassing
> > it ? IE, the guest side virtio doesn't expect an IOMMU and doesn't call
> > the dma mappings ops.
> >
> >> However, it is and likely will remain niche enough that I don't really
> >> care about performance loss from IOMMU support.  If you enable it, you
> >> want it.
> >>
> >> So from the QEMU point of view we can simply add the direct-ram-access
> >> property, and have the pseries machine turn it on by default (while
> >> other machines can leave it off by default---they have no IOMMU and
> >> thus no performance cost).
> >
> > Well, it's only for virtio and should be on by default on x86 as well if
> > an iommu is installed no ?
>
> Yes, only for virtio---but for x86 I think it should be off by default,
> even if that means virtio+IOMMU requires a new kernel.

Just to clarify: is "it" the direct-ram-access property?  If so, I
think I might agree.

Alternatively, could QEMU easily teach the IOMMU code to generate the
ACPI tables such that virtio-pci devices aren't advertised as living
behind the IOMMU?  This would work both with and without my patches.
On the other hand, maybe this gets complicated when hotplug is
involved.

--Andy

>
> Paolo

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Paolo Bonzini]

Il 03/09/2014 17:07, Andy Lutomirski ha scritto:
>> > Yes, only for virtio---but for x86 I think it should be off by default,
>> > even if that means virtio+IOMMU requires a new kernel.
> Just to clarify: is "it" the direct-ram-access property?  If so, I
> think I might agree.

Yes.

> Alternatively, could QEMU easily teach the IOMMU code to generate the
> ACPI tables such that virtio-pci devices aren't advertised as living
> behind the IOMMU?  This would work both with and without my patches.
> On the other hand, maybe this gets complicated when hotplug is
> involved.

That could be possible.  For hot-plug, you can simply forbid hotplugging
with direct-ram-access=on.  If you want hotplug, then we should add the
direct-ram-access property to PCI bridges too (with the same limitation
on hotplug).  All devices under such a bridge would be outside the
IOMMU, including virtio devices with direct-ram-access=off.

Paolo

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Michael S. Tsirkin]

On Wed, Sep 03, 2014 at 08:07:15AM -0700, Andy Lutomirski wrote:
> On Sep 3, 2014 5:11 AM, "Paolo Bonzini" <pbonzini@redhat.com> wrote:
> >
> > Il 03/09/2014 10:05, Benjamin Herrenschmidt ha scritto:
> > > On Wed, 2014-09-03 at 09:47 +0200, Paolo Bonzini wrote:
> > >>
> > >> IOMMU support for x86 is going to go in this week.
> > >
> > > But won't that break virtio on x86 ? Or will virtio continue bypassing
> > > it ? IE, the guest side virtio doesn't expect an IOMMU and doesn't call
> > > the dma mappings ops.
> > >
> > >> However, it is and likely will remain niche enough that I don't really
> > >> care about performance loss from IOMMU support.  If you enable it, you
> > >> want it.
> > >>
> > >> So from the QEMU point of view we can simply add the direct-ram-access
> > >> property, and have the pseries machine turn it on by default (while
> > >> other machines can leave it off by default---they have no IOMMU and
> > >> thus no performance cost).
> > >
> > > Well, it's only for virtio and should be on by default on x86 as well if
> > > an iommu is installed no ?
> >
> > Yes, only for virtio---but for x86 I think it should be off by default,
> > even if that means virtio+IOMMU requires a new kernel.
> 
> Just to clarify: is "it" the direct-ram-access property?  If so, I
> think I might agree.
> 
> Alternatively, could QEMU easily teach the IOMMU code to generate the
> ACPI tables such that virtio-pci devices aren't advertised as living
> behind the IOMMU?  This would work both with and without my patches.

How exactly does this look in ACPI?

> On the other hand, maybe this gets complicated when hotplug is
> involved.
> 
> --Andy
> 
> >
> > Paolo

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

On Wed, Sep 3, 2014 at 9:39 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Wed, Sep 03, 2014 at 08:07:15AM -0700, Andy Lutomirski wrote:
>> On Sep 3, 2014 5:11 AM, "Paolo Bonzini" <pbonzini@redhat.com> wrote:
>> >
>> > Il 03/09/2014 10:05, Benjamin Herrenschmidt ha scritto:
>> > > On Wed, 2014-09-03 at 09:47 +0200, Paolo Bonzini wrote:
>> > >>
>> > >> IOMMU support for x86 is going to go in this week.
>> > >
>> > > But won't that break virtio on x86 ? Or will virtio continue bypassing
>> > > it ? IE, the guest side virtio doesn't expect an IOMMU and doesn't call
>> > > the dma mappings ops.
>> > >
>> > >> However, it is and likely will remain niche enough that I don't really
>> > >> care about performance loss from IOMMU support.  If you enable it, you
>> > >> want it.
>> > >>
>> > >> So from the QEMU point of view we can simply add the direct-ram-access
>> > >> property, and have the pseries machine turn it on by default (while
>> > >> other machines can leave it off by default---they have no IOMMU and
>> > >> thus no performance cost).
>> > >
>> > > Well, it's only for virtio and should be on by default on x86 as well if
>> > > an iommu is installed no ?
>> >
>> > Yes, only for virtio---but for x86 I think it should be off by default,
>> > even if that means virtio+IOMMU requires a new kernel.
>>
>> Just to clarify: is "it" the direct-ram-access property?  If so, I
>> think I might agree.
>>
>> Alternatively, could QEMU easily teach the IOMMU code to generate the
>> ACPI tables such that virtio-pci devices aren't advertised as living
>> behind the IOMMU?  This would work both with and without my patches.
>
> How exactly does this look in ACPI?

I think that all you need is a PCI device or segment that isn't
included in the scope of any DRHD.  I could be wrong.

--Andy

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

On Tue, Sep 2, 2014 at 5:43 PM, Benjamin Herrenschmidt <benh@au1.ibm.com> wrote:
> On Tue, 2014-09-02 at 17:32 -0700, Andy Lutomirski wrote:
>>
>> I agree *except* that implementing it will be a real PITA and (I
>> think) can't be done without changing code in arch/.  My patches plus
>> an ifdef powerpc will be functionally equivalent, just uglier.
>
> So for powerpc, it's a 2 liner inside virtio-pci, but yes, it might be
> more of a problem for s390, I'm not too sure what they do in that area.
>
>> Bigger quirk: on a standard s390 virtio guest configuration,
>> dma_map_single etc will fail to link.
>
> Yuck
>
>>  I tried this in v1 of these
>> patches.  So we can poke at the archdata all day, but we can't build a
>> kernel like that :(
>
> I would like the s390 people to chime in here, it still looks like the
> best way to go if they can fix things on their side :-)
>
>> So until the dma_ops pointer move into struct device and
>> CONFIG_HAS_DMA becomes mandatory (or mandatory enough that virtio can
>> depend on it), I don't think we can do it this way.
>
> I see, it's a bummer because it would be a lot cleaner.
>
>> I'll send a v5 that is the same as v4 except with physical addressing
>> hardcoded in for powerpc.
>
> Thanks. That will do for now, but ideally we want to make it a function
> of some flag from the implementation, so let's see what Rusty has to
> say.

I've confirmed that ppc64 (on QEMU) breaks without the ppc special
case and that ppc64 keeps working with the special case.  Once Rusty's
patches settle down, I'll rebase onto them and send v5.

--Andy

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Rusty Russell]

Andy Lutomirski <luto@amacapital.net> writes:
> On Sep 2, 2014 11:53 PM, "Rusty Russell" <rusty@rustcorp.com.au> wrote:
>>
>> Andy Lutomirski <luto@amacapital.net> writes:
>> > There really are virtio devices that are pieces of silicon and not
>> > figments of a hypervisor's imagination [1].
>>
>> Hi Andy,
>>
>>         As you're discovering, there's a reason no one has done the DMA
>> API before.
>>
>> So the problem is that ppc64's IOMMU is a platform thing, not a bus
>> thing.  They really do carve out an exception for virtio devices,
>> because performance (LOTS of performance).  It remains to be seen if
>> other platforms have the same performance issues, but in absence of
>> other evidence, the answer is yes.
>>
>> It's a hack.  But having specific virtual-only devices are an even
>> bigger hack.
>>
>> Physical virtio devices have been talked about, but don't actually exist
>> in Real Life.  And someone a virtio PCI card is going to have serious
>> performance issues: mainly because they'll want the rings in the card's
>> MMIO region, not allocated by the driver.  Being broken on PPC is really
>> the least of their problems.
>>
>> So, what do we do?  It'd be nice if Linux virtio Just Worked under Xen,
>> though Xen's IOMMU is outside the virtio spec.  Since virtio_pci can be
>> a module, obvious hacks like having xen_arch_setup initialize a dma_ops pointer
>> exposed by virtio_pci.c is out.
>
> Xen does expose dma_ops.  The trick is knowing when to use it.
>
>>
>> I think the best approach is to have a new feature bit (25 is free),
>> VIRTIO_F_USE_BUS_MAPPING which indicates that a device really wants to
>> use the mapping for the bus it is on.  A real device would set this,
>> or it won't work behind an IOMMU.  A Xen device would also set this.
>
> The devices I care about aren't actually Xen devices.  They're devices
> supplied by QEMU/KVM, booting a Xen hypervisor, which in turn passes
> the virtio device (along with every other PCI device) through to dom0.
> So this is exactly the same virtio device that regular x86 KVM guests
> would see.  The reason that current code fails is that Xen guest
> physical addresses aren't the same as the addresses seen by the outer
> hypervisor.
>
> These devices don't know that physical addresses != bus addresses, so
> they can't advertise that fact.

Ah, I see.  Then we will need a Xen-specific hack.

> Grr.  This is mostly a result of the fact that virtio_pci devices
> aren't really PCI devices.  I still think that virtio_pci shouldn't
> have to worry about this; ideally this would all be handled higher up
> in the device hierarchy.  x86 already gets this right.

Yes.  Adding a feature to say "I am a real PCI device" is possible, but
has other issues (particularly as Michael Tsirkin pointed out, what do
you do if the driver doesn't understand the feature).

> Are there any hypervisors except PPC that use virtio_pci, have IOMMUs
> on the pci slot that virtio_pci lives in, and that use physical
> addressing?  If not, I think that just quirking PPC will work (at
> least until someone wants IOMMU support in virtio_pci on PPC, in which
> case doing something using devicetree seems like a reasonable
> solution).

We can either patch to make PPC weird or make Xen weird.  I'm on the
fence.

Two questions for Paulo:
1) When QEMU support IOMMU on x86, will the virtio devices behind it
   respect the IOMMU (do they use the right memory access primitives?).

2) Are we really going to be able to exclude virtio devices from using
   the x86 IOMMU in a portable way which will always work?  If it's
   per-bus granularity, will qemu really put them on their own PCI bus
   and get this right?  Or will it sometimes get it wrong and users will
   end up using virtio devices via IOMMU by accident?

If the answers are both "yes", then x86 is going to be able to use
virtio+IOMMU, so PPC looks like the odd one out.  Otherwise it looks
like we're really going to want to stick with the "ignore IOMMU" rule
until (handwave future), and we make an exception for Xen.

Cheers,
Rusty.

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Rusty Russell]

"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Wed, Sep 03, 2014 at 04:12:01PM +0930, Rusty Russell wrote:
>> Andy Lutomirski <luto@amacapital.net> writes:
>> > There really are virtio devices that are pieces of silicon and not
>> > figments of a hypervisor's imagination [1].
>> 
>> Hi Andy,
>> 
>>         As you're discovering, there's a reason no one has done the DMA
>> API before.
>> 
>> So the problem is that ppc64's IOMMU is a platform thing, not a bus
>> thing.  They really do carve out an exception for virtio devices,
>> because performance (LOTS of performance).  It remains to be seen if
>> other platforms have the same performance issues, but in absence of
>> other evidence, the answer is yes.
>> 
>> It's a hack.  But having specific virtual-only devices are an even
>> bigger hack.
>> 
>> Physical virtio devices have been talked about, but don't actually exist
>> in Real Life.  And someone a virtio PCI card is going to have serious
>> performance issues: mainly because they'll want the rings in the card's
>> MMIO region, not allocated by the driver.
>
> Why? What's wrong with rings in memory?

AFAICT, the card would have to access guest memory to read it, using
multiple DMA cycles.  That's going to be slow.

>>  Being broken on PPC is really
>> the least of their problems.
>> 
>> So, what do we do?  It'd be nice if Linux virtio Just Worked under Xen,
>> though Xen's IOMMU is outside the virtio spec.  Since virtio_pci can be
>> a module, obvious hacks like having xen_arch_setup initialize a dma_ops pointer
>> exposed by virtio_pci.c is out.
>
> Well virtio could probe for xen, it's not a lot of code.

We could, but I think this is going to be a more general problem in
future.  x86 is heading down the IOMMU path, and they're likely to
suffer similarly.

>> I think the best approach is to have a new feature bit (25 is free),
>> VIRTIO_F_USE_BUS_MAPPING which indicates that a device really wants to
>> use the mapping for the bus it is on.  A real device would set this,
>> or it won't work behind an IOMMU.  A Xen device would also set this.
>> 
>> Thoughts?
>> Rusty.
>
> OK and it should then be active even if guest does not ack
> the feature (so in fact, it would have to be a mandatory feature).
> That can work, but I still find this a bit inelegant: this is
> a property of the platform, not of the device.

True.  If a device needs it though, we're no worse of having a device
which doesn't work if the driver understand the feature than we were
before.

Cheers,
Rusty.

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

On Thu, Sep 4, 2014 at 7:31 PM, Rusty Russell <rusty@rustcorp.com.au> wrote:
> Andy Lutomirski <luto@amacapital.net> writes:
>> On Sep 2, 2014 11:53 PM, "Rusty Russell" <rusty@rustcorp.com.au> wrote:
>>>
>>> Andy Lutomirski <luto@amacapital.net> writes:
>>> > There really are virtio devices that are pieces of silicon and not
>>> > figments of a hypervisor's imagination [1].
>>>
>>> Hi Andy,
>>>
>>>         As you're discovering, there's a reason no one has done the DMA
>>> API before.
>>>
>>> So the problem is that ppc64's IOMMU is a platform thing, not a bus
>>> thing.  They really do carve out an exception for virtio devices,
>>> because performance (LOTS of performance).  It remains to be seen if
>>> other platforms have the same performance issues, but in absence of
>>> other evidence, the answer is yes.
>>>
>>> It's a hack.  But having specific virtual-only devices are an even
>>> bigger hack.
>>>
>>> Physical virtio devices have been talked about, but don't actually exist
>>> in Real Life.  And someone a virtio PCI card is going to have serious
>>> performance issues: mainly because they'll want the rings in the card's
>>> MMIO region, not allocated by the driver.  Being broken on PPC is really
>>> the least of their problems.
>>>
>>> So, what do we do?  It'd be nice if Linux virtio Just Worked under Xen,
>>> though Xen's IOMMU is outside the virtio spec.  Since virtio_pci can be
>>> a module, obvious hacks like having xen_arch_setup initialize a dma_ops pointer
>>> exposed by virtio_pci.c is out.
>>
>> Xen does expose dma_ops.  The trick is knowing when to use it.
>>
>>>
>>> I think the best approach is to have a new feature bit (25 is free),
>>> VIRTIO_F_USE_BUS_MAPPING which indicates that a device really wants to
>>> use the mapping for the bus it is on.  A real device would set this,
>>> or it won't work behind an IOMMU.  A Xen device would also set this.
>>
>> The devices I care about aren't actually Xen devices.  They're devices
>> supplied by QEMU/KVM, booting a Xen hypervisor, which in turn passes
>> the virtio device (along with every other PCI device) through to dom0.
>> So this is exactly the same virtio device that regular x86 KVM guests
>> would see.  The reason that current code fails is that Xen guest
>> physical addresses aren't the same as the addresses seen by the outer
>> hypervisor.
>>
>> These devices don't know that physical addresses != bus addresses, so
>> they can't advertise that fact.
>
> Ah, I see.  Then we will need a Xen-specific hack.
>
>> Grr.  This is mostly a result of the fact that virtio_pci devices
>> aren't really PCI devices.  I still think that virtio_pci shouldn't
>> have to worry about this; ideally this would all be handled higher up
>> in the device hierarchy.  x86 already gets this right.
>
> Yes.  Adding a feature to say "I am a real PCI device" is possible, but
> has other issues (particularly as Michael Tsirkin pointed out, what do
> you do if the driver doesn't understand the feature).
>
>> Are there any hypervisors except PPC that use virtio_pci, have IOMMUs
>> on the pci slot that virtio_pci lives in, and that use physical
>> addressing?  If not, I think that just quirking PPC will work (at
>> least until someone wants IOMMU support in virtio_pci on PPC, in which
>> case doing something using devicetree seems like a reasonable
>> solution).
>
> We can either patch to make PPC weird or make Xen weird.  I'm on the
> fence.
>
> Two questions for Paulo:
> 1) When QEMU support IOMMU on x86, will the virtio devices behind it
>    respect the IOMMU (do they use the right memory access primitives?).
>
> 2) Are we really going to be able to exclude virtio devices from using
>    the x86 IOMMU in a portable way which will always work?  If it's
>    per-bus granularity, will qemu really put them on their own PCI bus
>    and get this right?  Or will it sometimes get it wrong and users will
>    end up using virtio devices via IOMMU by accident?
>
> If the answers are both "yes", then x86 is going to be able to use
> virtio+IOMMU, so PPC looks like the odd one out.  Otherwise it looks
> like we're really going to want to stick with the "ignore IOMMU" rule
> until (handwave future), and we make an exception for Xen.

There's a third option: try to make virtio-mmio work everywhere
(except s390), at least in the long run.  This other benefits: it
makes minimal hypervisors simpler, I think it'll get rid of the limits
on the number of virtio devices in a system.  ARM is already going
this direction, and I imagine that PPC support would be
straightforward (it's already using devicetree).

Does virtio-mmio have any reasonable way of doing hotplug?  It could
also eventually make sense to have a standard for virtio on virtio.

--Andy

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

On Thu, Sep 4, 2014 at 7:32 PM, Rusty Russell <rusty@rustcorp.com.au> wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
>> On Wed, Sep 03, 2014 at 04:12:01PM +0930, Rusty Russell wrote:
>>> Andy Lutomirski <luto@amacapital.net> writes:
>>> > There really are virtio devices that are pieces of silicon and not
>>> > figments of a hypervisor's imagination [1].
>>>
>>> Hi Andy,
>>>
>>>         As you're discovering, there's a reason no one has done the DMA
>>> API before.
>>>
>>> So the problem is that ppc64's IOMMU is a platform thing, not a bus
>>> thing.  They really do carve out an exception for virtio devices,
>>> because performance (LOTS of performance).  It remains to be seen if
>>> other platforms have the same performance issues, but in absence of
>>> other evidence, the answer is yes.
>>>
>>> It's a hack.  But having specific virtual-only devices are an even
>>> bigger hack.
>>>
>>> Physical virtio devices have been talked about, but don't actually exist
>>> in Real Life.  And someone a virtio PCI card is going to have serious
>>> performance issues: mainly because they'll want the rings in the card's
>>> MMIO region, not allocated by the driver.
>>
>> Why? What's wrong with rings in memory?
>
> AFAICT, the card would have to access guest memory to read it, using
> multiple DMA cycles.  That's going to be slow.

I don't personally know all the considerations, but AFAICT NVMe puts
its rings in memory, and NMVe is very much focused on performance.

There might be an argument for trying to avoid using indirect rings on
real hardware to reduce the number of DMA round-trips needed for a
comment.

--Andy

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Benjamin Herrenschmidt]

On Fri, 2014-09-05 at 12:01 +0930, Rusty Russell wrote:
> If the answers are both "yes", then x86 is going to be able to use
> virtio+IOMMU, so PPC looks like the odd one out. 

Well, yes and no ... ppc will be able to do that too, it's just
pointless and will suck performances.

Additionally, it will be incompatible with existing guests since
today, the guest assumes physical (doesn't use the dma mapping
routines), so even if x86 grows the ability to have virtio behind an
iommu in qemu, that will break existing guests.

>  Otherwise it looks
> like we're really going to want to stick with the "ignore IOMMU" rule
> until (handwave future), and we make an exception for Xen.

Either that or we have a capability that can be negociated.

There are other reasons for wanting to allow the use of the DMA ops,
such as people using virtio as a transport between two physically
connected machines (such as a CPU running a PCIe endpoint to a CPU
running a PCIe host, or two hosts connected to a non-transparent switch,
essentially using PCIe as a fast network fabric).

Cheers,
Ben.

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Benjamin Herrenschmidt]

On Thu, 2014-09-04 at 19:57 -0700, Andy Lutomirski wrote:

> There's a third option: try to make virtio-mmio work everywhere
> (except s390), at least in the long run.  This other benefits: it
> makes minimal hypervisors simpler, I think it'll get rid of the limits
> on the number of virtio devices in a system.  ARM is already going
> this direction, and I imagine that PPC support would be
> straightforward (it's already using devicetree).

PCI has advantages though. Management stacks know about PCI and nothing
else really. We already have all the infra to do hotplug with PCI,
etc...

> Does virtio-mmio have any reasonable way of doing hotplug?  It could
> also eventually make sense to have a standard for virtio on virtio.

That would be very platform specific.

Cheers,
Ben.

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Christian Borntraeger]

On 05/09/14 04:57, Andy Lutomirski wrote:
> There's a third option: try to make virtio-mmio work everywhere
> (except s390), at least in the long run.  This other benefits: it
> makes minimal hypervisors simpler, I think it'll get rid of the limits
> on the number of virtio devices in a system.  ARM is already going
> this direction, and I imagine that PPC support would be
> straightforward (it's already using devicetree).

Well this chance is gone.
When virtio was first introduced we though about abstraction (mmio,hypercalls, pci ops depending on the platform as part of the transport. There was even a virtio over serial line as potential implementation), but we had to do a fully PCI variant to please windows guests IIRC.

Christian

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Christopher Covington]

On 09/04/2014 10:57 PM, Andy Lutomirski wrote:
> On Thu, Sep 4, 2014 at 7:31 PM, Rusty Russell <rusty@rustcorp.com.au> wrote:
>> Andy Lutomirski <luto@amacapital.net> writes:
>>> On Sep 2, 2014 11:53 PM, "Rusty Russell" <rusty@rustcorp.com.au> wrote:
>>>>
>>>> Andy Lutomirski <luto@amacapital.net> writes:
>>>>> There really are virtio devices that are pieces of silicon and not
>>>>> figments of a hypervisor's imagination [1].
>>>>
>>>> Hi Andy,
>>>>
>>>>         As you're discovering, there's a reason no one has done the DMA
>>>> API before.
>>>>
>>>> So the problem is that ppc64's IOMMU is a platform thing, not a bus
>>>> thing.  They really do carve out an exception for virtio devices,
>>>> because performance (LOTS of performance).  It remains to be seen if
>>>> other platforms have the same performance issues, but in absence of
>>>> other evidence, the answer is yes.
>>>>
>>>> It's a hack.  But having specific virtual-only devices are an even
>>>> bigger hack.
>>>>
>>>> Physical virtio devices have been talked about, but don't actually exist
>>>> in Real Life.  And someone a virtio PCI card is going to have serious
>>>> performance issues: mainly because they'll want the rings in the card's
>>>> MMIO region, not allocated by the driver.  Being broken on PPC is really
>>>> the least of their problems.
>>>>
>>>> So, what do we do?  It'd be nice if Linux virtio Just Worked under Xen,
>>>> though Xen's IOMMU is outside the virtio spec.  Since virtio_pci can be
>>>> a module, obvious hacks like having xen_arch_setup initialize a dma_ops pointer
>>>> exposed by virtio_pci.c is out.
>>>
>>> Xen does expose dma_ops.  The trick is knowing when to use it.
>>>
>>>>
>>>> I think the best approach is to have a new feature bit (25 is free),
>>>> VIRTIO_F_USE_BUS_MAPPING which indicates that a device really wants to
>>>> use the mapping for the bus it is on.  A real device would set this,
>>>> or it won't work behind an IOMMU.  A Xen device would also set this.
>>>
>>> The devices I care about aren't actually Xen devices.  They're devices
>>> supplied by QEMU/KVM, booting a Xen hypervisor, which in turn passes
>>> the virtio device (along with every other PCI device) through to dom0.
>>> So this is exactly the same virtio device that regular x86 KVM guests
>>> would see.  The reason that current code fails is that Xen guest
>>> physical addresses aren't the same as the addresses seen by the outer
>>> hypervisor.
>>>
>>> These devices don't know that physical addresses != bus addresses, so
>>> they can't advertise that fact.
>>
>> Ah, I see.  Then we will need a Xen-specific hack.
>>
>>> Grr.  This is mostly a result of the fact that virtio_pci devices
>>> aren't really PCI devices.  I still think that virtio_pci shouldn't
>>> have to worry about this; ideally this would all be handled higher up
>>> in the device hierarchy.  x86 already gets this right.
>>
>> Yes.  Adding a feature to say "I am a real PCI device" is possible, but
>> has other issues (particularly as Michael Tsirkin pointed out, what do
>> you do if the driver doesn't understand the feature).
>>
>>> Are there any hypervisors except PPC that use virtio_pci, have IOMMUs
>>> on the pci slot that virtio_pci lives in, and that use physical
>>> addressing?  If not, I think that just quirking PPC will work (at
>>> least until someone wants IOMMU support in virtio_pci on PPC, in which
>>> case doing something using devicetree seems like a reasonable
>>> solution).
>>
>> We can either patch to make PPC weird or make Xen weird.  I'm on the
>> fence.
>>
>> Two questions for Paulo:
>> 1) When QEMU support IOMMU on x86, will the virtio devices behind it
>>    respect the IOMMU (do they use the right memory access primitives?).
>>
>> 2) Are we really going to be able to exclude virtio devices from using
>>    the x86 IOMMU in a portable way which will always work?  If it's
>>    per-bus granularity, will qemu really put them on their own PCI bus
>>    and get this right?  Or will it sometimes get it wrong and users will
>>    end up using virtio devices via IOMMU by accident?
>>
>> If the answers are both "yes", then x86 is going to be able to use
>> virtio+IOMMU, so PPC looks like the odd one out.  Otherwise it looks
>> like we're really going to want to stick with the "ignore IOMMU" rule
>> until (handwave future), and we make an exception for Xen.
> 
> There's a third option: try to make virtio-mmio work everywhere
> (except s390), at least in the long run.  This other benefits: it
> makes minimal hypervisors simpler, I think it'll get rid of the limits
> on the number of virtio devices in a system.  ARM is already going
> this direction, and I imagine that PPC support would be
> straightforward (it's already using devicetree).

In my opinion, a uniform "virt" machine for every instruction set would be
very beneficial. I would guess that MMIO is more universally available than
PCI, and as you point out, simpler to implement.

> Does virtio-mmio have any reasonable way of doing hotplug?  It could
> also eventually make sense to have a standard for virtio on virtio.

I don't think so, but it seems possible. My bystander understanding is that
QEMU allocates some fixed number of VirtIO-MMIO devices, maybe a dozen, in the
device tree. The ones that don't actually get hooked up to something real like
a block device or network interface are populated with a dummy device. One
naive approach might be to allow the dummy devices to tell the kernel that
they are now changing to a real device.

Also, higher level hotplug for at least SCSI sounds possible.

https://bugzilla.redhat.com/show_bug.cgi?id=1123390

Christopher

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by the Linux Foundation.

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski]

On Wed, Sep 10, 2014 at 8:36 AM, Christopher Covington
<cov@codeaurora.org> wrote:
> On 09/04/2014 10:57 PM, Andy Lutomirski wrote:
>> There's a third option: try to make virtio-mmio work everywhere
>> (except s390), at least in the long run.  This other benefits: it
>> makes minimal hypervisors simpler, I think it'll get rid of the limits
>> on the number of virtio devices in a system.  ARM is already going
>> this direction, and I imagine that PPC support would be
>> straightforward (it's already using devicetree).
>
> In my opinion, a uniform "virt" machine for every instruction set would be
> very beneficial. I would guess that MMIO is more universally available than
> PCI, and as you point out, simpler to implement.

Except for x86 :(  That's presumably fixable, though.

>
>> Does virtio-mmio have any reasonable way of doing hotplug?  It could
>> also eventually make sense to have a standard for virtio on virtio.
>
> I don't think so, but it seems possible. My bystander understanding is that
> QEMU allocates some fixed number of VirtIO-MMIO devices, maybe a dozen, in the
> device tree. The ones that don't actually get hooked up to something real like
> a block device or network interface are populated with a dummy device. One
> naive approach might be to allow the dummy devices to tell the kernel that
> they are now changing to a real device.

My thought (which I completely failed to articulate) was to have a
spec for a virtio device that exposes a complete virtio bus along with
hotplug and per-cpu interrupts (a la MSI-X).  This might be a bit
complicated, but it would work everywhere without any firmware or
platform issues.

--Andy

Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Michael S. Tsirkin]

On Fri, Sep 05, 2014 at 12:01:33PM +0930, Rusty Russell wrote:
> Andy Lutomirski <luto@amacapital.net> writes:
> > On Sep 2, 2014 11:53 PM, "Rusty Russell" <rusty@rustcorp.com.au> wrote:
> >>
> >> Andy Lutomirski <luto@amacapital.net> writes:
> >> > There really are virtio devices that are pieces of silicon and not
> >> > figments of a hypervisor's imagination [1].
> >>
> >> Hi Andy,
> >>
> >>         As you're discovering, there's a reason no one has done the DMA
> >> API before.
> >>
> >> So the problem is that ppc64's IOMMU is a platform thing, not a bus
> >> thing.  They really do carve out an exception for virtio devices,
> >> because performance (LOTS of performance).  It remains to be seen if
> >> other platforms have the same performance issues, but in absence of
> >> other evidence, the answer is yes.
> >>
> >> It's a hack.  But having specific virtual-only devices are an even
> >> bigger hack.
> >>
> >> Physical virtio devices have been talked about, but don't actually exist
> >> in Real Life.  And someone a virtio PCI card is going to have serious
> >> performance issues: mainly because they'll want the rings in the card's
> >> MMIO region, not allocated by the driver.  Being broken on PPC is really
> >> the least of their problems.
> >>
> >> So, what do we do?  It'd be nice if Linux virtio Just Worked under Xen,
> >> though Xen's IOMMU is outside the virtio spec.  Since virtio_pci can be
> >> a module, obvious hacks like having xen_arch_setup initialize a dma_ops pointer
> >> exposed by virtio_pci.c is out.
> >
> > Xen does expose dma_ops.  The trick is knowing when to use it.
> >
> >>
> >> I think the best approach is to have a new feature bit (25 is free),
> >> VIRTIO_F_USE_BUS_MAPPING which indicates that a device really wants to
> >> use the mapping for the bus it is on.  A real device would set this,
> >> or it won't work behind an IOMMU.  A Xen device would also set this.
> >
> > The devices I care about aren't actually Xen devices.  They're devices
> > supplied by QEMU/KVM, booting a Xen hypervisor, which in turn passes
> > the virtio device (along with every other PCI device) through to dom0.
> > So this is exactly the same virtio device that regular x86 KVM guests
> > would see.  The reason that current code fails is that Xen guest
> > physical addresses aren't the same as the addresses seen by the outer
> > hypervisor.
> >
> > These devices don't know that physical addresses != bus addresses, so
> > they can't advertise that fact.
> 
> Ah, I see.  Then we will need a Xen-specific hack.
> 
> > Grr.  This is mostly a result of the fact that virtio_pci devices
> > aren't really PCI devices.  I still think that virtio_pci shouldn't
> > have to worry about this; ideally this would all be handled higher up
> > in the device hierarchy.  x86 already gets this right.
> 
> Yes.  Adding a feature to say "I am a real PCI device" is possible, but
> has other issues (particularly as Michael Tsirkin pointed out, what do
> you do if the driver doesn't understand the feature).
> 
> > Are there any hypervisors except PPC that use virtio_pci, have IOMMUs
> > on the pci slot that virtio_pci lives in, and that use physical
> > addressing?  If not, I think that just quirking PPC will work (at
> > least until someone wants IOMMU support in virtio_pci on PPC, in which
> > case doing something using devicetree seems like a reasonable
> > solution).
> 
> We can either patch to make PPC weird or make Xen weird.  I'm on the
> fence.
> 
> Two questions for Paulo:
> 1) When QEMU support IOMMU on x86, will the virtio devices behind it
>    respect the IOMMU (do they use the right memory access primitives?).
> 
> 2) Are we really going to be able to exclude virtio devices from using
>    the x86 IOMMU in a portable way which will always work?  If it's
>    per-bus granularity, will qemu really put them on their own PCI bus
>    and get this right?  Or will it sometimes get it wrong and users will
>    end up using virtio devices via IOMMU by accident?
> 
> If the answers are both "yes", then x86 is going to be able to use
> virtio+IOMMU, so PPC looks like the odd one out.  Otherwise it looks
> like we're really going to want to stick with the "ignore IOMMU" rule
> until (handwave future), and we make an exception for Xen.
> 
> Cheers,
> Rusty.

In theory, it's yes to both questions.
In practice, with patches merged recently it's no to both questions :).

It's a work in progress, but some extra effort to support miltiple
PCI roots will be needed on the QEMU side.
What problems will surface when we try to do multiple roots?
Only time will tell.

If it's felt that it's much cleaner to make PPC the odd one out, we can
defer enabling iommu in qemu on x86 until ways to bypass it are
implemented.

But I would be inclined, for pre-1.0 drivers, to make Xen weird.
For 1.0 drivers, we have a bit of time to consider this,
and maybe PPC guys can come up with some way (can be PV)
to tell guest "these devices bypass the IOMMU".

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