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

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

On Fri, Mar 20, 2015 at 08:39:24AM +0100, Cornelia Huck wrote:
> On Wed, 18 Mar 2015 14:08:56 +0100
> "Michael S. Tsirkin" <mst@redhat.com> 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 <mst@redhat.com>
> > > Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
> > > CC: Christian Borntraeger <borntraeger@de.ibm.com>
> > > Cc: Richard Henderson <rth@twiddle.net>
> > > Cc: Alexander Graf <agraf@suse.de>
> > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > 
> > 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.

But VIRTIO_PCI_QUEUE_MAX only makes sense as a VQ number.
Why does it make sense as a vector number?
Jason's patches actually change VIRTIO_PCI_QUEUE_MAX
so we need to figure our what to do for this code.


> > 
> > These changes need some testing though.
> 
> My identical patch seemed to work for me.