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

[Cornelia Huck]

On Sat, 21 Mar 2015 19:27:49 +0100
"Michael S. Tsirkin" <address@hidden> wrote:

> 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" <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.
> 
> 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.

We don't have "vectors" for virtio-ccw. It's just a concept mandated by
common code, we just do an identity mapping. Using this value just
pushes the notifier bits for config changes to behind the highest
possible virtqueue.