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

[David Hildenbrand <david@redhat.com>]

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?

What's the approach that you are thinking of?

-- 
Cheers,

David / dhildenb