[RFC PATCH 4/9] vfio-pci: register default dynamic-trap-bar-info region

[Yan Zhao <yan.y.zhao@intel.com>]

Dynamic trap bar info region is a channel for QEMU and vendor driver to
communicate dynamic trap info. It is of type
VFIO_REGION_TYPE_DYNAMIC_TRAP_BAR_INFO and subtype
VFIO_REGION_SUBTYPE_DYNAMIC_TRAP_BAR_INFO.

This region has two fields: dt_fd and trap.
When QEMU detects a device regions of this type, it will create an
eventfd and write its eventfd id to dt_fd field.
When vendor drivre signals this eventfd, QEMU reads trap field of this
info region.
- If trap is true, QEMU would search the device's PCI BAR
regions and disable all the sparse mmaped subregions (if the sparse
mmaped subregion is disablable).
- If trap is false, QEMU would re-enable those subregions.

A typical usage is
1. vendor driver first cuts its bar 0 into several sections, all in a
sparse mmap array. So initally, all its bar 0 are passthroughed.
2. vendor driver specifys part of bar 0 sections to be disablable.
3. on migration starts, vendor driver signals dt_fd and set trap to true
to notify QEMU disabling the bar 0 sections of disablable flags on.
4. QEMU disables those bar 0 section and hence let vendor driver be able
to trap access of bar 0 registers and make dirty page tracking possible.
5. on migration failure, vendor driver signals dt_fd to QEMU again.
QEMU reads trap field of this info region which is false and QEMU
re-passthrough the whole bar 0 region.

Vendor driver specifies whether it supports dynamic-trap-bar-info region
through cap VFIO_PCI_DEVICE_CAP_DYNAMIC_TRAP_BAR in
vfio_pci_mediate_ops->open().

If vfio-pci detects this cap, it will create a default
dynamic_trap_bar_info region on behalf of vendor driver with region len=0
and region->ops=null.
Vvendor driver should override this region's len, flags, rw, mmap in its
vfio_pci_mediate_ops.

Cc: Kevin Tian <kevin.tian@intel.com>

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
---
 drivers/vfio/pci/vfio_pci.c | 16 ++++++++++++++++
 include/linux/vfio.h        |  3 ++-
 include/uapi/linux/vfio.h   | 11 +++++++++++
 3 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index 059660328be2..62b811ca43e4 100644
--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c
@@ -127,6 +127,19 @@ void init_migration_region(struct vfio_pci_device *vdev)
 		NULL);
 }
 
+/**
+ * register a region to hold info for dynamically trap bar regions
+ */
+void init_dynamic_trap_bar_info_region(struct vfio_pci_device *vdev)
+{
+	vfio_pci_register_dev_region(vdev,
+		VFIO_REGION_TYPE_DYNAMIC_TRAP_BAR_INFO,
+		VFIO_REGION_SUBTYPE_DYNAMIC_TRAP_BAR_INFO,
+		NULL, 0,
+		VFIO_REGION_INFO_FLAG_READ | VFIO_REGION_INFO_FLAG_WRITE,
+		NULL);
+}
+
 static void vfio_pci_probe_mmaps(struct vfio_pci_device *vdev)
 {
 	struct resource *res;
@@ -538,6 +551,9 @@ static int vfio_pci_open(void *device_data)
 				if (caps & VFIO_PCI_DEVICE_CAP_MIGRATION)
 					init_migration_region(vdev);
 
+				if (caps & VFIO_PCI_DEVICE_CAP_DYNAMIC_TRAP_BAR)
+					init_dynamic_trap_bar_info_region(vdev);
+
 				pr_info("vfio pci found mediate_ops %s, caps=%llx, handle=%x for %x:%x\n",
 						vdev->mediate_ops->name, caps,
 						handle, vdev->pdev->vendor,
diff --git a/include/linux/vfio.h b/include/linux/vfio.h
index cddea8e9dcb2..cf8ecf687bee 100644
--- a/include/linux/vfio.h
+++ b/include/linux/vfio.h
@@ -197,7 +197,8 @@ extern void vfio_virqfd_disable(struct virqfd **pvirqfd);
 
 struct vfio_pci_mediate_ops {
 	char	*name;
-#define VFIO_PCI_DEVICE_CAP_MIGRATION (0x01)
+#define VFIO_PCI_DEVICE_CAP_MIGRATION		(0x01)
+#define VFIO_PCI_DEVICE_CAP_DYNAMIC_TRAP_BAR	(0x02)
 	int	(*open)(struct pci_dev *pdev, u64 *caps, u32 *handle);
 	void	(*release)(int handle);
 	void	(*get_region_info)(int handle,
diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
index caf8845a67a6..74a2d0b57741 100644
--- a/include/uapi/linux/vfio.h
+++ b/include/uapi/linux/vfio.h
@@ -258,6 +258,9 @@ struct vfio_region_info {
 struct vfio_region_sparse_mmap_area {
 	__u64	offset;	/* Offset of mmap'able area within region */
 	__u64	size;	/* Size of mmap'able area */
+	__u32	disablable;	/* whether this mmap'able are able to
+				 *  be dynamically disabled
+				 */
 };
 
 struct vfio_region_info_cap_sparse_mmap {
@@ -454,6 +457,14 @@ struct vfio_device_migration_info {
 #define VFIO_DEVICE_DIRTY_PFNS_ALL	(~0ULL)
 } __attribute__((packed));
 
+/* Region type and sub-type to hold info to dynamically trap bars */
+#define VFIO_REGION_TYPE_DYNAMIC_TRAP_BAR_INFO		(4)
+#define VFIO_REGION_SUBTYPE_DYNAMIC_TRAP_BAR_INFO	(1)
+
+struct vfio_device_dt_bar_info_region {
+	__u32 dt_fd; /* fd of eventfd to notify qemu trap/untrap bars*/
+	__u32 trap;   /* trap/untrap bar regions */
+};
 
 /* sub-types for VFIO_REGION_TYPE_PCI_* */
 
-- 
2.17.1