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

[Yongji Xie]

On Wed, Jun 30, 2021 at 5:51 PM Stefan Hajnoczi <stefanha@xxxxxxxxxx> wrote:
>
> On Tue, Jun 29, 2021 at 10:59:51AM +0800, Yongji Xie wrote:
> > On Mon, Jun 28, 2021 at 9:02 PM Stefan Hajnoczi <stefanha@xxxxxxxxxx> wrote:
> > >
> > > On Tue, Jun 15, 2021 at 10:13:30PM +0800, Xie Yongji wrote:
> > > > +/* 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).
>

Oh, yes. I miss 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.
>
> > > > +     __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.
>

OK.

> 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?

Not sure. Maybe we might need some alignment which is less than
PAGE_SIZE sometimes.

Thanks,
Yongji