Re: [Qemu-devel] [RFC 13/19] fuzz: add ctrl vq support to virtio-net in libqos

[Oleinik, Alexander]

On Thu, 2019-07-25 at 12:25 -0400, John Snow wrote:
> 
> On 7/24/19 11:23 PM, Oleinik, Alexander wrote:
> > Signed-off-by: Alexander Oleinik <address@hidden>
> 
> Is there some explanation for why the below patch does what the
> subject
> line claims for the uninitiated?
When multiqueue mode (VIRTIO_NET_F_MQ) is disabled, virtio-net sets up
three queues. 0:receiveq, 1:transmitq and 2:controlq. 
> I don't know why increasing the number of queues from 2 to 3 here is
> correct in the general case, OR why it would "add ctrl vq support".
> (Or what it has to do with fuzzing, in general.)

Prior to the change, accessing the ctrl vq through QOS, would trigger a
segfault, since only two queues were allocated to QVirtioDevice*
interface->queues.

Also, when VIRTIO_NET_F_MQ is enabled, the number of queues is 2*N + 1,
so I think in that case n->n_queues is also short by one in the code
below.

> [Only responding because this landed in tests/libqos, which I do try
> to
> keep an eye on, but this patch is opaque to me. --js]
> 
> > ---
> >  tests/libqos/virtio-net.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/tests/libqos/virtio-net.c b/tests/libqos/virtio-net.c
> > index 66405b646e..247a0a17a8 100644
> > --- a/tests/libqos/virtio-net.c
> > +++ b/tests/libqos/virtio-net.c
> > @@ -51,7 +51,7 @@ static void virtio_net_setup(QVirtioNet
> > *interface)
> >      if (features & (1u << VIRTIO_NET_F_MQ)) {
> >          interface->n_queues = qvirtio_config_readw(vdev, 8) * 2;
> >      } else {
> > -        interface->n_queues = 2;
> > +        interface->n_queues = 3;
> >      }
> >  
> >      interface->queues = g_new(QVirtQueue *, interface->n_queues);
> >