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

[David Woodhouse]

On Wed, 2026-05-13 at 18:24 +0200, Paolo Bonzini wrote:
> 
> > See commit https://git.kernel.org/torvalds/c/49a1a2c70a7f which adds a
> > new guest-visible feature in revision 3, but allowed userspace to
> > restore the old behaviour by setting it to revision 2. All my patch
> >  above does, is make it possible to set it to revision 1 as
> > well. Because https://git.kernel.org/torvalds/c/d53c2c29ae0d previously
> > changed the behaviour and bumped the default to 2 *without* allowing
> > userspace to restore the prior behaviour, and we've been carrying a
> > *revert* of that patch.
> > 
> > Why would we *not* accept such a patch?
> 
> Agreed. Even ignoring your revert, there's no reason why any upgrade
> past 49a1a2c70a7f has to be from after d53c2c29ae0d.

So where do we go from here?

I assume you'll be taking this Documentation patch via the KVM tree?

But what about the actual fix at 
https://lore.kernel.org/all/20260511113558.3325004-2-dwmw2@infradead.org/

This is a simple and unintrusive bug fix to make KVM/arm64 follow the
"common sense" requirement that the doc patch codifies, apparently
being rejected with the rather bizarre claim that KVM has no *need* to
maintain guest-visible compatibility across host kernel changes.

So... what next? Is one of the other KVM/arm64 maintainers going to
speak up? Paolo would you consider taking the fixes through your tree
directly? 

Does Arm not actually *care* whether AArch64 is considered a stable and
mature platform for KVM hosting?

We don't have CONFIG_EXPERIMENTAL any more, do we? Or perhaps we could
mark it such. Is CONFIG_STAGING the right thing, for unstable things
which might violate the normal maturity expectations of the kernel?




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