On Wed, Jul 29, 2026 at 12:25:30PM +0300, Michael Tokarev wrote:
> On 7/29/26 08:12, Michael Tokarev wrote:
> > On 7/29/26 02:22, Michael S. Tsirkin wrote:
> > > On Tue, Jul 28, 2026 at 11:17:34PM +0300, Michael Tokarev wrote:
> > ..
> > > > So this whole change, for older versions, becomes just:
> > > >
> > > > --- a/hw/virtio/virtio.c
> > > > +++ b/hw/virtio/virtio.c
> > > > @@ -2572,6 +2572,8 @@ VirtQueue *virtio_add_queue(VirtIODevice *vdev,
> > > > int queue_size,
> > > > if (i == VIRTIO_QUEUE_MAX || queue_size > VIRTQUEUE_MAX_SIZE)
> > > > abort();
> > > >
> > > > + queue_size = VIRTQUEUE_MAX_SIZE; /* fixed properly in 11.1. */
> > > > +
> > >
> > > Or you can stick to upstream and add the compat entry for 11.0.
> >
> > Compat entry for 11.0 which will never be used?
> > With a non-obvious logic which boils down to this single line?
> >
> > Or what do you mean?
> >
> > BTW, can't this logic be implemented without a compat property in
> > the first place? This x-override-queue-size seems like a hack.
> > or a.. twisted way to compare a machine version.
> >
> > Can't this very place become
> >
> > if (machine_version < 11.1) {
> > queue_size = VIRTQUEUE_MAX_SIZE;
> > }
> >
> > instead of using this indirect property?
>
> Michael, I still don't understand what we're doing here and - now -
> why do we need to distinguish between old and new machine types at all.
>
> Behavior before the fixes:
>
> qemu always reported queue size to be the maximum allowed one (1024),
> even if actual queue size requested and allocated was smaller
> qemu accepted requests to elements past the allocated size, which
> allowed to read/write past the allocated buffer - which can't be
> considered part of ABI, obviously :)
>
> Behavior after the fixes, for new machine types:
>
> qemu reports actually requested and allocated queue size
> qemu properly disallows access past the allocated queue size
>
> What would break if we switch to the new behavior unconditionally,
> regardless of the machine types?
It's guest visible. You will get two reads from same register
suddenly returning different values.
> I don't understand why do you suggest to implement the same logic for
> older/stable qemu versions if it will always evaluate to allocating
> the max size for the queue regardless of the requested size.
>
> Thanks,
>
> /mjt
My preference, normally, is to just stick to upstream as much as
possibly. I just do not want slightly different code bases when we can
trivially have one. If nothing else, less of a chance a follow up patch
will cause conflicts, and then it snowballs from there.
--
MST
