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

[Will Deacon <will@kernel.org>]

On Tue, May 19, 2026 at 11:41:26AM +0100, David Woodhouse wrote:
> 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?

Hey, come on. Marc cares more about this stuff than anybody else on the
planet. He's been single-handedly maintaining the tree for the past
couple of releases while Oliver was out and he's on the end of a _lot_
of patches. I'm only cc'd on a fraction of the KVM/arm64 changes and
it's bedlam.

Will