Re: [RFC PATCH v5 02/26] vhost: Fix last queue index of devices with no cvq

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

On Tue, Nov 02, 2021 at 08:25:27AM +0100, Juan Quintela wrote:
> Eugenio Pérez <eperezma@redhat.com> 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.
>                                   ^^^^
> even
> 
> I know, I know, I am Spanish myself O:-)

Nobody expects the Spanish ;)

> > To fix this, just resort to the lower even number of queues.
> 
> I don't understand what you try to achieve here.
> 
> > 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;
> >      }
> 
> As far as I can see, that is a nop. last_index is defined as an int.
> 
> $ cat kk.c
> #include <stdio.h>
> 
> int main(void)
> {
> 	int i = 7;
> 	i &= -1ULL;

Stefano's patch has ~1ULL , not -1ULL here.

> 	printf("%d\n", i);
> 	i = 8;
> 	i &= -1ULL;
> 	printf("%d\n", i);
> 	i = 0;
> 	i &= -1ULL;
> 	printf("%d\n", i);
> 	i = -2;
> 	i &= -1ULL;
> 	printf("%d\n", i);
> 	return 0;
> }
> $ ./kk
> 7
> 8
> 0
> -2