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

[Jason Wang]

On Mon, Nov 1, 2021 at 4:59 PM Eugenio Perez Martin <eperezma@redhat.com> wrote:
>
> On Mon, Nov 1, 2021 at 4:34 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> > On Fri, Oct 29, 2021 at 10:16 PM 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.
> > >
> > > 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;
> > >      }
> >
> > The math here looks correct but we need to fix vhost_vdpa_dev_start() 
> > instead?
> >
> > if (dev->vq_index + dev->nvqs - 1 != dev->last_index) {
> > ...
> > }
> >
>
> If we just do that, devices that offer an odd number of queues but do
> not offer ctrl vq would never enable the last vq pair, isn't it?

For vq pair, you assume that it's a networking device, so the device
you described here violates the spec.

>
> Also, I would say that the right place for the solution of this
> problem should not be virtio/vhost-vdpa: This is highly dependent on
> having cvq, and this implies a knowledge about the use of each
> virtqueue. Another kind of device could have an odd number of
> virtqueues naturally, and that (-1) would not work for them, isn't it?

It actually depends on how multiqueue is modeled for each specific
type of device. They need to initialize the vq_index and nvqs
correctly:

E.g if we had a device with 3 queues, we could model it with the following:

vhost_dev 1, vq_index = 0, nvqs = 2
vhost_dev 2, vq_index = 2, nvqs = 1

In this case the last_index should be initialized to 2, then we know
all the vhost_dev is initialized and we can start the hardware.

Thanks

>
> Thanks!
>
> > Thanks
> >
> > >
> > >      if (!k->set_guest_notifiers) {
> > > --
> > > 2.27.0
> > >
> >
>