Re: [PATCH] vdpa: dont check vhost_vdpa->suspended when unsupported

[Zhu, Lingshan]

On 6/7/2023 9:51 PM, Eugenio Perez Martin wrote:
> On Wed, Jun 7, 2023 at 11:09 AM Zhu Lingshan <lingshan.zhu@intel.com> wrote:
> > When read the state of a virtqueue, vhost_vdpa need
> > to check whether the device is suspended.
> >
> > This commit verifies whether VHOST_BACKEND_F_SUSPEND is
> > negotiated when checking vhost_vdpa->suspended.
> I'll add: Otherwise, qemu prints XXX error message.
>
> On second thought, not returning an error prevents the caller
> (vhost.c:vhost_virtqueue_stop) from recovering used idx from the
> guest.
>
> I'm not sure about the right fix for this, should we call
> virtio_queue_restore_last_avail_idx here? Communicate that the backend
> cannot suspend so vhost.c can print a better error message?
I agree it is better not to return an error.

Instead if neither the device does not support _F_SUSPEND nor failed to 
suspend,
I think vhost_vdpa should try to read the last_avail_idx from
the device anyway. We can print an error message here,
like: failed to suspend, the value may not reliable.

Thanks
> Thanks!
>
> > Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
> > ---
> >   hw/virtio/vhost-vdpa.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
> > index b3094e8a8b..ae176c06dd 100644
> > --- a/hw/virtio/vhost-vdpa.c
> > +++ b/hw/virtio/vhost-vdpa.c
> > @@ -1397,7 +1397,7 @@ static int vhost_vdpa_get_vring_base(struct vhost_dev 
> > *dev,
> >           return 0;
> >       }
> >
> > -    if (!v->suspended) {
> > +    if ((dev->backend_cap & BIT_ULL(VHOST_BACKEND_F_SUSPEND)) && 
> > (!v->suspended)) {
> >           /*
> >            * Cannot trust in value returned by device, let vhost recover used
> >            * idx from guest.
> > --
> > 2.39.1