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

[Marc Zyngier]

On Tue, 19 May 2026 13:38:57 +0100,
Marc Zyngier <maz at kernel.org> wrote:
> 
> As I said before, I'd be OK with something that would restore IIDR to
> REV1. But not something that actively breaks the GIC emulation by
> reintroducing a bug. That's, by construction, dead code that will only
> bitrot, because there is no SW that can make use of this nonsense.

I will also add that if we make it a policy to preserve buggy
behaviours that the guest cannot be relying on, then I question
whether we should be fixing anything at all.

For example, 6.19 fixed a totally buggy behaviour where a guest
couldn't not have more than (on most HW) 4 interrupts in flight at any
given time. This was obviously totally bogus, and this was fixed
unconditionally, as legitimate guests could experience gold-platted
lock-ups.

Should we revert to the previous behaviour? In the affirmative, I will
simply stop fixing things, and someone else can have fun retrofitting
buggy crap.

	M.

-- 
Without deviation from the norm, progress is not possible.