[PATCH 2/5] vhost: involve device backends in feature negotiation

[Stefan Hajnoczi <stefanha@redhat.com>]

Many vhost devices in QEMU currently do not involve the device backend
in feature negotiation. This seems fine at first glance for device types
without their own feature bits (virtio-net has many but other device
types have none).

This overlooks the fact that QEMU's virtqueue implementation and the
device backend's implementation may support different features.  QEMU
must not report features to the guest that the the device backend
doesn't support.

For example, QEMU supports VIRTIO 1.1 packed virtqueues while many
existing vhost device backends do not. When the user sets packed=on the
device backend breaks. This should have been handled gracefully by
feature negotiation instead.

Introduce vhost_get_default_features() and update all vhost devices in
QEMU to involve the device backend in feature negotiation.

This patch fixes the following error:

  $ x86_64-softmmu/qemu-system-x86_64 \
      -drive if=virtio,file=test.img,format=raw \
      -chardev socket,path=/tmp/vhost-user-blk.sock,id=char0 \
      -device vhost-user-blk-pci,chardev=char0,packed=on \
      -object memory-backend-memfd,size=1G,share=on,id=ram0 \
      -M accel=kvm,memory-backend=ram0
  qemu-system-x86_64: Failed to set msg fds.
  qemu-system-x86_64: vhost VQ 0 ring restore failed: -1: Success (0)

The vhost-user-blk backend failed as follows:

  $ ./vhost-user-blk --socket-path=/tmp/vhost-user-blk.sock -b test2.img
  vu_panic: virtio: zero sized buffers are not allowed
  virtio-blk request missing headers

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 include/hw/virtio/vhost.h        |  1 +
 include/hw/virtio/virtio-gpu.h   |  2 ++
 include/sysemu/cryptodev-vhost.h | 11 +++++++++++
 backends/cryptodev-vhost.c       | 19 +++++++++++++++++++
 hw/display/vhost-user-gpu.c      | 17 +++++++++++++++++
 hw/display/virtio-gpu-base.c     |  2 +-
 hw/input/vhost-user-input.c      |  9 +++++++++
 hw/virtio/vhost-user-fs.c        |  5 +++--
 hw/virtio/vhost-vsock.c          |  5 +++--
 hw/virtio/vhost.c                | 22 ++++++++++++++++++++++
 hw/virtio/virtio-crypto.c        |  3 ++-
 11 files changed, 90 insertions(+), 6 deletions(-)

diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index 085450c6f8..d2e54dd4a8 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -112,6 +112,7 @@ void vhost_virtqueue_mask(struct vhost_dev *hdev, VirtIODevice *vdev, int n,
                           bool mask);
 uint64_t vhost_get_features(struct vhost_dev *hdev, const int *feature_bits,
                             uint64_t features);
+uint64_t vhost_get_default_features(struct vhost_dev *hdev, uint64_t features);
 void vhost_ack_features(struct vhost_dev *hdev, const int *feature_bits,
                         uint64_t features);
 bool vhost_has_free_slot(void);
diff --git a/include/hw/virtio/virtio-gpu.h b/include/hw/virtio/virtio-gpu.h
index 6dd57f2025..41d270d80e 100644
--- a/include/hw/virtio/virtio-gpu.h
+++ b/include/hw/virtio/virtio-gpu.h
@@ -192,6 +192,8 @@ bool virtio_gpu_base_device_realize(DeviceState *qdev,
 void virtio_gpu_base_reset(VirtIOGPUBase *g);
 void virtio_gpu_base_fill_display_info(VirtIOGPUBase *g,
                         struct virtio_gpu_resp_display_info *dpy_info);
+uint64_t virtio_gpu_base_get_features(VirtIODevice *vdev, uint64_t features,
+                                      Error **errp);
 
 /* virtio-gpu.c */
 void virtio_gpu_ctrl_response(VirtIOGPU *g,
diff --git a/include/sysemu/cryptodev-vhost.h b/include/sysemu/cryptodev-vhost.h
index f42824fbde..e629446bfb 100644
--- a/include/sysemu/cryptodev-vhost.h
+++ b/include/sysemu/cryptodev-vhost.h
@@ -122,6 +122,17 @@ int cryptodev_vhost_start(VirtIODevice *dev, int total_queues);
  */
 void cryptodev_vhost_stop(VirtIODevice *dev, int total_queues);
 
+/**
+ * cryptodev_vhost_get_features:
+ * @dev: the virtio crypto object
+ * @requested_features: the features being offered
+ *
+ * Returns: the requested features bits that are supported by the vhost device,
+ * or the original request feature bits if vhost is disabled
+ *
+ */
+uint64_t cryptodev_vhost_get_features(VirtIODevice *dev, uint64_t features);
+
 /**
  * cryptodev_vhost_virtqueue_mask:
  * @dev: the virtio crypto object
diff --git a/backends/cryptodev-vhost.c b/backends/cryptodev-vhost.c
index 8337c9a495..5f5a4fda7b 100644
--- a/backends/cryptodev-vhost.c
+++ b/backends/cryptodev-vhost.c
@@ -266,6 +266,20 @@ void cryptodev_vhost_stop(VirtIODevice *dev, int total_queues)
     assert(r >= 0);
 }
 
+uint64_t cryptodev_vhost_get_features(VirtIODevice *dev, uint64_t features)
+{
+    VirtIOCrypto *vcrypto = VIRTIO_CRYPTO(dev);
+    CryptoDevBackend *b = vcrypto->cryptodev;
+    CryptoDevBackendClient *cc = b->conf.peers.ccs[0];
+    CryptoDevBackendVhost *vhost_crypto = cryptodev_get_vhost(cc, b, 0);
+
+    if (!vhost_crypto) {
+        return features; /* vhost disabled */
+    }
+
+    return vhost_get_default_features(&vhost_crypto->dev, features);
+}
+
 void cryptodev_vhost_virtqueue_mask(VirtIODevice *dev,
                                            int queue,
                                            int idx, bool mask)
@@ -333,6 +347,11 @@ void cryptodev_vhost_stop(VirtIODevice *dev, int total_queues)
 {
 }
 
+uint64_t cryptodev_vhost_get_features(VirtIODevice *dev, uint64_t features)
+{
+    return features;
+}
+
 void cryptodev_vhost_virtqueue_mask(VirtIODevice *dev,
                                     int queue,
                                     int idx, bool mask)
diff --git a/hw/display/vhost-user-gpu.c b/hw/display/vhost-user-gpu.c
index 4cdaee1bde..e483df2a9e 100644
--- a/hw/display/vhost-user-gpu.c
+++ b/hw/display/vhost-user-gpu.c
@@ -466,6 +466,22 @@ vhost_user_gpu_set_config(VirtIODevice *vdev,
     }
 }
 
+static uint64_t
+vhost_user_gpu_get_features(VirtIODevice *vdev, uint64_t features,
+                            Error **errp)
+{
+    VhostUserGPU *g = VHOST_USER_GPU(vdev);
+    Error *local_err = NULL;
+
+    features = virtio_gpu_base_get_features(vdev, features, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        return 0;
+    }
+
+    return vhost_get_default_features(&g->vhost->dev, features);
+}
+
 static void
 vhost_user_gpu_set_status(VirtIODevice *vdev, uint8_t val)
 {
@@ -582,6 +598,7 @@ vhost_user_gpu_class_init(ObjectClass *klass, void *data)
 
     vdc->realize = vhost_user_gpu_device_realize;
     vdc->reset = vhost_user_gpu_reset;
+    vdc->get_features = vhost_user_gpu_get_features;
     vdc->set_status   = vhost_user_gpu_set_status;
     vdc->guest_notifier_mask = vhost_user_gpu_guest_notifier_mask;
     vdc->guest_notifier_pending = vhost_user_gpu_guest_notifier_pending;
diff --git a/hw/display/virtio-gpu-base.c b/hw/display/virtio-gpu-base.c
index c159351be3..05d1ff2db2 100644
--- a/hw/display/virtio-gpu-base.c
+++ b/hw/display/virtio-gpu-base.c
@@ -176,7 +176,7 @@ virtio_gpu_base_device_realize(DeviceState *qdev,
     return true;
 }
 
-static uint64_t
+uint64_t
 virtio_gpu_base_get_features(VirtIODevice *vdev, uint64_t features,
                              Error **errp)
 {
diff --git a/hw/input/vhost-user-input.c b/hw/input/vhost-user-input.c
index 63984a8ba7..1371fb32cc 100644
--- a/hw/input/vhost-user-input.c
+++ b/hw/input/vhost-user-input.c
@@ -45,6 +45,14 @@ static void vhost_input_change_active(VirtIOInput *vinput)
     }
 }
 
+static uint64_t vhost_input_get_features(VirtIODevice *vdev, uint64_t features,
+                                         Error **errp)
+{
+    VHostUserInput *vhi = VHOST_USER_INPUT(vdev);
+
+    return vhost_get_default_features(&vhi->vhost->dev, features);
+}
+
 static void vhost_input_get_config(VirtIODevice *vdev, uint8_t *config_data)
 {
     VirtIOInput *vinput = VIRTIO_INPUT(vdev);
@@ -89,6 +97,7 @@ static void vhost_input_class_init(ObjectClass *klass, void *data)
     DeviceClass *dc = DEVICE_CLASS(klass);
 
     dc->vmsd = &vmstate_vhost_input;
+    vdc->get_features = vhost_input_get_features;
     vdc->get_config = vhost_input_get_config;
     vdc->set_config = vhost_input_set_config;
     vic->realize = vhost_input_realize;
diff --git a/hw/virtio/vhost-user-fs.c b/hw/virtio/vhost-user-fs.c
index 1bc5d03a00..56015ca3d4 100644
--- a/hw/virtio/vhost-user-fs.c
+++ b/hw/virtio/vhost-user-fs.c
@@ -130,8 +130,9 @@ static uint64_t vuf_get_features(VirtIODevice *vdev,
                                       uint64_t requested_features,
                                       Error **errp)
 {
-    /* No feature bits used yet */
-    return requested_features;
+    VHostUserFS *fs = VHOST_USER_FS(vdev);
+
+    return vhost_get_default_features(&fs->vhost_dev, requested_features);
 }
 
 static void vuf_handle_output(VirtIODevice *vdev, VirtQueue *vq)
diff --git a/hw/virtio/vhost-vsock.c b/hw/virtio/vhost-vsock.c
index 4a228f5168..7276587be6 100644
--- a/hw/virtio/vhost-vsock.c
+++ b/hw/virtio/vhost-vsock.c
@@ -180,8 +180,9 @@ static uint64_t vhost_vsock_get_features(VirtIODevice *vdev,
                                          uint64_t requested_features,
                                          Error **errp)
 {
-    /* No feature bits used yet */
-    return requested_features;
+    VHostVSock *vsock = VHOST_VSOCK(vdev);
+
+    return vhost_get_default_features(&vsock->vhost_dev, requested_features);
 }
 
 static void vhost_vsock_handle_output(VirtIODevice *vdev, VirtQueue *vq)
diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index aff98a0ede..f8a144dcd0 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -48,6 +48,23 @@ static unsigned int used_memslots;
 static QLIST_HEAD(, vhost_dev) vhost_devices =
     QLIST_HEAD_INITIALIZER(vhost_devices);
 
+/*
+ * Feature bits that device backends must explicitly report. Feature bits not
+ * listed here maybe set by QEMU without checking with the device backend.
+ * Ideally all feature bits would be listed here but existing vhost device
+ * implementations do not explicitly report bits like VIRTIO_F_VERSION_1, so we
+ * can only assume they are supported.
+ *
+ * New feature bits added to the VIRTIO spec should usually be included here
+ * so that existing vhost device backends that do not support them yet continue
+ * to work.
+ */
+static const int vhost_default_feature_bits[] = {
+    VIRTIO_F_IOMMU_PLATFORM,
+    VIRTIO_F_RING_PACKED,
+    VHOST_INVALID_FEATURE_BIT
+};
+
 bool vhost_has_free_slot(void)
 {
     unsigned int slots_limit = ~0U;
@@ -1468,6 +1485,11 @@ uint64_t vhost_get_features(struct vhost_dev *hdev, const int *feature_bits,
     return features;
 }
 
+uint64_t vhost_get_default_features(struct vhost_dev *hdev, uint64_t features)
+{
+    return vhost_get_features(hdev, vhost_default_feature_bits, features);
+}
+
 void vhost_ack_features(struct vhost_dev *hdev, const int *feature_bits,
                         uint64_t features)
 {
diff --git a/hw/virtio/virtio-crypto.c b/hw/virtio/virtio-crypto.c
index bd9165c565..ef711b56f4 100644
--- a/hw/virtio/virtio-crypto.c
+++ b/hw/virtio/virtio-crypto.c
@@ -739,7 +739,8 @@ static uint64_t virtio_crypto_get_features(VirtIODevice *vdev,
                                            uint64_t features,
                                            Error **errp)
 {
-    return features;
+    /* Just returns features when vhost is disabled */
+    return cryptodev_vhost_get_features(vdev, features);
 }
 
 static void virtio_crypto_reset(VirtIODevice *vdev)
-- 
2.25.3