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

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

On Mon, Nov 01, 2021 at 10:03:19AM +0100, Eugenio Perez Martin wrote:
> 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!

I'm not sure ... both?

> > > --
> > > 2.27.0
> >