Re: [PATCH RFC] virtio-balloon: make it spec compliant

[Michael S. Tsirkin]

On Wed, Jul 10, 2024 at 05:15:18AM +0200, David Hildenbrand wrote:
> On 04.07.24 23:30, Michael S. Tsirkin wrote:
> > Currently, if VIRTIO_BALLOON_F_FREE_PAGE_HINT is off but
> > VIRTIO_BALLOON_F_REPORTING is on, then the reporting vq
> > gets number 3 while spec says it's number 4.
> > It happens to work because the linux virtio pci driver
> > is *also* out of spec.
> > 
> > To fix:
> > 1. add vq4 as per spec
> > 2. to help out the buggy Linux driver, in the above configuration,
> >     also create vq3, and handle it exactly as we do vq4.
> > 
> > I think that some clever hack is doable to address the issue
> > for existing machine types (which would get it in user's hands
> > sooner), but I'm not 100% sure what, exactly.
> > 
> > This is a simpler, straight-forward approach.
> > 
> > Reported-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> > 
> > I don't think I'll stop here, I want to fix exiting machine types,
> > but sending this here for comparison.
> > I'll send a Linux patch later.
> 
> The downside is that new machine types will stop working with mainline Linux
> / major distros in that feature combination, right?

The should keep working, that's why I put in part 2. above.

> What's the approach that you are thinking of?
> 
> -- 
> Cheers,
> 
> David / dhildenb