Re: [Qemu-devel] [PATCH V4 06/19] virtio-ccw: using VIRTIO_NO_VECTOR instead of 0 for invalid virtqueue

[Cornelia Huck]

On Wed, 18 Mar 2015 14:08:56 +0100
"Michael S. Tsirkin" <address@hidden> wrote:

> On Wed, Mar 18, 2015 at 05:34:56PM +0800, Jason Wang wrote:
> > There's no need to use vector 0 for invalid virtqueue. So this patch
> > changes to use VIRTIO_NO_VECTOR instead.
> > 
> > Cc: Michael S. Tsirkin <address@hidden>
> > Cc: Cornelia Huck <address@hidden>
> > CC: Christian Borntraeger <address@hidden>
> > Cc: Richard Henderson <address@hidden>
> > Cc: Alexander Graf <address@hidden>
> > Signed-off-by: Jason Wang <address@hidden>
> 
> I don't know what does this actually do.
> Cornelia?

I actually have the same patch somewhere in my queue. The point here is
that 0 is plain wrong (it's a valid queue), while VIRTIO_NO_VECTOR is
most certainly no valid queue.

> 
> > ---
> >  hw/s390x/virtio-ccw.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
> > index 130535c..c8b87aa 100644
> > --- a/hw/s390x/virtio-ccw.c
> > +++ b/hw/s390x/virtio-ccw.c
> > @@ -281,7 +281,7 @@ static int virtio_ccw_set_vqs(SubchDev *sch, uint64_t 
> > addr, uint32_t align,
> >  
> >      virtio_queue_set_addr(vdev, index, addr);
> >      if (!addr) {
> > -        virtio_queue_set_vector(vdev, index, 0);
> > +        virtio_queue_set_vector(vdev, index, VIRTIO_NO_VECTOR);
> >      } else {
> >          /* Fail if we don't have a big enough queue. */
> >          /* TODO: Add interface to handle vring.num changing */
> 
> Right below this, we have
>     /* tell notify handler in case of config change */
>     vdev->config_vector = VIRTIO_PCI_QUEUE_MAX;
> 
> which also does not seem to make sense.

Basically we have:

- at most 64 virtqueues with their own indicators (always 64 indicator
bits when using classic I/O interrupts, up to 64 indicator bits when
using adapter interrupts)
- another indicator bit for configuration changes (bit 0 of the
secondary indicator bits)

That way, the configuration change indicator is always one bit behind
the last possible queue indicator.

> 
> These changes need some testing though.

My identical patch seemed to work for me.