Re: [RFC PATCH] virtio_balloon: add VIRTIO_BALLOON_F_REPORTING_PM_SAFE feature bit

[Link Lin]

On Thu, Aug 6, 2026 at 4:00 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> This makes no sense to me. So there's a bug in the guest and it crashes.
> Patch the guest.
>
> There's just no chance we'll add flags every time some guest drivers
> on some OSes have a UAF.
>
> What makes this specific bug special?
>
> Trust me when I say UAF issues e.g. around hotplug are a dime a dozen.
> Now what, let's add another one around hotplug? And so on.

Hi Michael,

Thanks for the candid feedback. I completely agree that adding feature
bits to the virtio specification to paper over guest OS bugs sets a bad
precedent, and we will drop this RFC.

To answer your question on why this felt special from our side: in public
cloud fleets with unmanaged customer images (BYOS), we cannot patch the
guest kernel. If the hypervisor turns on VIRTIO_BALLOON_F_REPORTING globally
for memory reclamation, unpatched guest kernels negotiate it blindly and
subsequently crash upon PM suspend. That makes a host-side feature rollout
act as a latent crash trigger for older guests.

We understand your point that the spec is not the place for driver bug
workarounds. We will handle this compatibility gating out-of-band in the
hypervisor / management layer instead.

Thanks for your time and review.

Sincerely,
Link