Re: Re: [PATCH v8 09/10] vduse: Introduce VDUSE - vDPA Device in Userspace

On Tue, Jun 29, 2021 at 10:59:51AM +0800, Yongji Xie wrote:

On Mon, Jun 28, 2021 at 9:02 PM Stefan Hajnoczi [off-list ref] wrote:

quoted

On Tue, Jun 15, 2021 at 10:13:30PM +0800, Xie Yongji wrote:

quoted

+/* ioctls */
+
+struct vduse_dev_config {
+     char name[VDUSE_NAME_MAX]; /* vduse device name */
+     __u32 vendor_id; /* virtio vendor id */
+     __u32 device_id; /* virtio device id */
+     __u64 features; /* device features */
+     __u64 bounce_size; /* bounce buffer size for iommu */
+     __u16 vq_size_max; /* the max size of virtqueue */

The VIRTIO specification allows per-virtqueue sizes. A device can have
two virtqueues, where the first one allows up to 1024 descriptors and
the second one allows only 128 descriptors, for example.

Good point! But it looks like virtio-vdpa/virtio-pci doesn't support
that now. All virtqueues have the same maximum size.

I see struct vpda_config_ops only supports a per-device max vq size:
u16 (*get_vq_num_max)(struct vdpa_device *vdev);

virtio-pci supports per-virtqueue sizes because the struct
virtio_pci_common_cfg->queue_size register is per-queue (controlled by
queue_select).

I guess this is a question for Jason: will vdpa will keep this limitation?
If yes, then VDUSE can stick to it too without running into problems in
the future.

quoted

quoted

+     __u16 padding; /* padding */
+     __u32 vq_num; /* the number of virtqueues */
+     __u32 vq_align; /* the allocation alignment of virtqueue's metadata */

I'm not sure what this is?

 This will be used by vring_create_virtqueue() too.

If there is no official definition for the meaning of this value then
"/* same as vring_create_virtqueue()'s vring_align parameter */" would
be clearer. That way the reader knows what to research in order to
understand how this field works.

I don't remember but maybe it was used to support vrings when the
host/guest have non-4KB page sizes. I wonder if anyone has an official
definition for this value?