Re: [PATCH] vdpa: Support setting vring_base for packed svq

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Nov 04, 2024 at 09:51:24PM +0530, Sahil Siddiq wrote:
> Linux commit v5.14-rc1~30^2~8 enabled the vp_vdpa driver to set the
> vq state to the device's initial state. This works differently for
> split and packed vqs.
> 
> With shadow virtqueues enabled, vhost-vdpa sets the vring base using
> the VHOST_SET_VRING_BASE ioctl. The payload (vhost_vring_state)
> differs for split and packed vqs. The implementation in QEMU currently
> uses the payload required for split vqs (i.e., the num field of
> vhost_vring_state is set to 0). The kernel throws EOPNOTSUPP when this
> payload is used with packed vqs.
> 
> This patch sets the num field in the payload appropriately so vhost-vdpa
> (with the vp_vdpa driver) can use packed svqs.
> 
> Link: https://lists.nongnu.org/archive/html/qemu-devel/2024-10/msg05106.html
> Link: https://lore.kernel.org/r/20210602021536.39525-4-jasowang@redhat.com
> Signed-off-by: Sahil Siddiq <sahilcdq@proton.me>

Looks like a feature, not a bugfix to me, so I guess - next release?

> ---
> QEMU currently does not support packed vhost shadow virtqueues. I am
> working on adding support for packed svqs [1]. The test environment
> that I am using [2] requires vhost-vdpa to use the relevant payload
> when setting vring base.
> 
> [1] https://wiki.qemu.org/Internships/ProjectIdeas/PackedShadowVirtqueue
> [2] https://www.redhat.com/en/blog/hands-vdpa-what-do-you-do-when-you-aint-got-hardware-part-2
> 
>  hw/virtio/vhost-vdpa.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
> index 3cdaa12ed5..5f81945109 100644
> --- a/hw/virtio/vhost-vdpa.c
> +++ b/hw/virtio/vhost-vdpa.c
> @@ -1230,6 +1230,10 @@ static bool vhost_vdpa_svq_setup(struct vhost_dev *dev,
>      };
>      int r;
>  
> +    if (virtio_vdev_has_feature(dev->vdev, VIRTIO_F_RING_PACKED)) {
> +        s.num = 0x80008000;
> +    }
> +
>      r = vhost_vdpa_set_dev_vring_base(dev, &s);
>      if (unlikely(r)) {
>          error_setg_errno(errp, -r, "Cannot set vring base");
> -- 
> 2.47.0