Re: [PATCH v2] virtio: refresh vring region cache after updating a virtqueue size

[Halil Pasic <pasic@linux.ibm.com>]

On Wed, 22 Mar 2023 18:24:33 +0100
Halil Pasic <pasic@linux.ibm.com> wrote:

> > > --- a/hw/s390x/virtio-ccw.c
> > > +++ b/hw/s390x/virtio-ccw.c
> > > @@ -237,6 +237,7 @@ static int virtio_ccw_set_vqs(SubchDev *sch, VqInfoBlock *info,
> > >                  return -EINVAL;
> > >              }
> > >              virtio_queue_set_num(vdev, index, num);
> > > +            virtio_init_region_cache(vdev, index);    
> > 
> > Hmm... this is not wrong, but looking at it again, I see that the guest
> > has no way to change num after our last call to
> > virtio_init_region_cache() (while setting up the queue addresses.) IOW,
> > this introduces an extra round trip that is not really needed.
> >   
> 
> I don't quite understand. AFAIU the virtio_init_region_cache() would see
> the (new) queue addresses but not the new size (num). Yes virtio-ccw
> already knows the new num but it is yet to call
> to put it into vdev->vq[n].vring.num from where
> virtio_init_region_cache() picks it up.
> 
> If we were to first virtio_queue_set_num() and only then the address
> I would understand. But with the code as is, I don't. Am I missing
> something?

Connie: have you had a chance to have yet another look at this? I
would like to understand the reason for seeing this differently.

Regards,
Halil