Re: [PATCH] vhost: Fix last queue index of devices with no cvq

[Eugenio Perez Martin]

On Sun, Oct 31, 2021 at 10:47 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Fri, Oct 29, 2021 at 04:16:08PM +0200, Eugenio Pérez wrote:
> > The -1 assumes that all devices with no cvq have an spare vq allocated
> > for them, but with no offer of VIRTIO_NET_F_CTRL_VQ. This may not be the
> > case, and the device may have a pair number of queues.
> >
> > To fix this, just resort to the lower even number of queues.
> >
> > 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 | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> > index 0d888f29a6..edf56a597f 100644
> > --- a/hw/net/vhost_net.c
> > +++ b/hw/net/vhost_net.c
> > @@ -330,7 +330,7 @@ int vhost_net_start(VirtIODevice *dev, NetClientState 
> > *ncs,
> >      NetClientState *peer;
> >
> >      if (!cvq) {
> > -        last_index -= 1;
> > +        last_index &= ~1ULL;
> >      }
> >
> >      if (!k->set_guest_notifiers) {
>
> could this code be made clearer?
>

I can expand both for sure, but do you mean clearer because the
operation is obscure (to remove the last bit to make last_index even)
or because the need of that operation is not clear enough?

Thanks!

> > --
> > 2.27.0
>