Re: [PATCH RFC 2/3] vhost-vdpa: introduce descriptor group backend feature

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On 8/9/2023 8:49 PM, Jason Wang wrote:
On Wed, Aug 9, 2023 at 8:56 PM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> wrote:
Userspace knows if the device has dedicated descriptor group or not
by checking this feature bit.

It's only exposed if the vdpa driver backend implements the
.get_vq_desc_group() operation callback. Userspace trying to negotiate
this feature when it or the dependent _F_IOTLB_ASID feature hasn't
been exposed will result in an error.

Signed-off-by: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>
  drivers/vhost/vdpa.c             | 17 +++++++++++++++++
  include/uapi/linux/vhost_types.h |  5 +++++
  2 files changed, 22 insertions(+)

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index b43e868..f2e5dce 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -389,6 +389,14 @@ static bool vhost_vdpa_can_resume(const struct vhost_vdpa *v)
         return ops->resume;

+static bool vhost_vdpa_has_desc_group(const struct vhost_vdpa *v)
+       struct vdpa_device *vdpa = v->vdpa;
+       const struct vdpa_config_ops *ops = vdpa->config;
+       return ops->get_vq_desc_group;
  static long vhost_vdpa_get_features(struct vhost_vdpa *v, u64 __user *featurep)
         struct vdpa_device *vdpa = v->vdpa;
@@ -679,6 +687,7 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
                 if (copy_from_user(&features, featurep, sizeof(features)))
                         return -EFAULT;
                 if (features & ~(VHOST_VDPA_BACKEND_FEATURES |
+                                BIT_ULL(VHOST_BACKEND_F_DESC_ASID) |
                                  BIT_ULL(VHOST_BACKEND_F_SUSPEND) |
                         return -EOPNOTSUPP;
@@ -688,6 +697,12 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
                 if ((features & BIT_ULL(VHOST_BACKEND_F_RESUME)) &&
                         return -EOPNOTSUPP;
+               if ((features & BIT_ULL(VHOST_BACKEND_F_DESC_ASID)) &&
+                   !(features & BIT_ULL(VHOST_BACKEND_F_IOTLB_ASID)))
+                       return -EINVAL;
+               if ((features & BIT_ULL(VHOST_BACKEND_F_DESC_ASID)) &&
+                    !vhost_vdpa_has_desc_group(v))
+                       return -EOPNOTSUPP;
                 vhost_set_backend_features(&v->vdev, features);
                 return 0;
@@ -741,6 +756,8 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
                         features |= BIT_ULL(VHOST_BACKEND_F_SUSPEND);
                 if (vhost_vdpa_can_resume(v))
                         features |= BIT_ULL(VHOST_BACKEND_F_RESUME);
+               if (vhost_vdpa_has_desc_group(v))
+                       features |= BIT_ULL(VHOST_BACKEND_F_DESC_ASID);
                 if (copy_to_user(featurep, &features, sizeof(features)))
                         r = -EFAULT;
diff --git a/include/uapi/linux/vhost_types.h b/include/uapi/linux/vhost_types.h
index d3aad12a..0856f84 100644
--- a/include/uapi/linux/vhost_types.h
+++ b/include/uapi/linux/vhost_types.h
@@ -181,5 +181,10 @@ struct vhost_vdpa_iova_range {
  /* Device can be resumed */
+/* Device may expose the descriptor table, avail and used ring in a
+ * different group for ASID binding than the buffers it contains.

s/a different group/different groups/?
Yep, I will try to rephrase. Would below work?

"Device may expose virtqueue's descriptor table, avail and used ring in a
different group for ASID binding than where buffers it contains reside."

Btw, not a native speaker but I think "descriptor" might be confusing
since as you explained above, it contains more than just a descriptor
Yep. I chose "descriptor" because packed virtqueue doesn't have "physical" avail and used ring other than descriptor table, but I think I am open to a better name. I once thought of "descriptor ring" but that might be too specific to packed virtqueue. Any suggestion?



+ */


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