Re: [PATCH v3 2/2] vhost: Fix last vq queue index of devices with no cvq

[Jason Wang]

On Wed, Nov 3, 2021 at 6:01 PM Eugenio Pérez <eperezma@redhat.com> wrote:
>
> The -1 assumes that last index counts all vhost device models as having
> two queues, but they count only the ones that models the data queues.
>
> Because of that, the right change in last_index is to actually add the
> cvq, not to remove the missing one.
>
> This is not a problem to vhost-net, but it is to vhost-vdpa, which
> device model trust to reach the last index to finish starting the
> device.
>
> Tested with vp_vdpa with host's vhost=on and vhost=off, with ctrl_vq=on
> and ctrl_vq=off.
>
> Fixes: 049eb15b5fc9 ("vhost: record the last virtqueue index for the virtio 
> device")
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>  hw/net/vhost_net.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> index 081946dc93..fe2b8a2b83 100644
> --- a/hw/net/vhost_net.c
> +++ b/hw/net/vhost_net.c
> @@ -329,8 +329,8 @@ int vhost_net_start(VirtIODevice *dev, NetClientState 
> *ncs,
>      int r, e, i, last_index = data_queue_pairs * 2;
>      NetClientState *peer;
>
> -    if (!cvq) {
> -        last_index -= 1;
> +    if (cvq) {
> +        last_index += 1;

Patch looks ok.

Not a native speaker but I guess "last" should be inside the closed
interval. If this is true, I still prefer to change the check in
vhost_vdpa_dev_start().

Thanks

>      }
>
>      if (!k->set_guest_notifiers) {
> --
> 2.27.0
>