Re: [RFC PATCH 06/17] KVM: arm64: Implement break-before-make sequence for parallel walks

[Sean Christopherson <seanjc@google.com>]

On Mon, Apr 25, 2022, Oliver Upton wrote:
> On Mon, Apr 25, 2022 at 8:13 AM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Fri, Apr 15, 2022, Oliver Upton wrote:
> > > The ARM architecture requires that software use the 'break-before-make'
> > > sequence whenever memory is being remapped.
> >
> > What does "remapped" mean here?  Changing the pfn?  Promoting/demoting to/from a
> > huge page?
> 
> Both, but in the case of this series it is mostly concerned with
> promotion/demotion. I'll make this language a bit more precise next
> time around.

Please be very precise :-)  It matters because it should be impossible for KVM to
actually change a PFN in a valid PTE.  Callers of mmu_notifier_change_pte() are
required to bookend it with mmu_notifier_invalidate_range_start/end(), i.e. KVM
should have zapped all PTEs and should not establish new PTEs.  I'd actually like
to drop mmu_notifier_change_pte() altogether, because for all intents and purposes,
it's dead code.  But convincing "everyone" that dropping it instead of trying to
salvage it for KSM is too much work :-)