[PATCH] Documentation: KVM: Document guest-visible compatibility expectations

[Paolo Bonzini]

On 5/11/26 18:38, David Woodhouse wrote:
> Not *everything* is in CPUID; one recent exception that comes to mind
> is the SUPPRESS_EOI_BROADCAST quirk. But on x86 we preserve the
> existing behaviour of older kernels — even when that behaviour doesn't
> make much sense, as with SUPPRESS_EOI_BROADCAST where older KVM would
> *advertise* the feature, but not actually *implement* it. Nevertheless,
> that remains the default behaviour of future kernels unless userspace
> explicitly opts in to fully enable (or disable) the feature.
> 
> But this documentation update isn't even asking for that compatible-by-
> default behaviour, even though that is the right thing to do. It's only
> asking that it be *possible* to reinstate the old behaviour, for
> userspace that *knows* about the change and explicitly wants to go back
> to the old way to remain compatible.

Yep, these are the "quirks"---if it's too early for Arm to commit to 
that, I guess it's fine.

However, independent of this patch which I (obviously) believe is a good 
idea, I'd like to understand how far it is, assuming 1) no quirks 2) 
same CPU host.

By the way, you didn't Cc Marc...

Paolo