Re: [PATCH v5 8/9] mm: multi-gen LRU: Have secondary MMUs participate in aging

[Oliver Upton <oliver.upton@linux.dev>]

On Wed, Jun 12, 2024 at 10:23:38AM -0700, Sean Christopherson wrote:
> On Wed, Jun 12, 2024, Yu Zhao wrote:
> > I do think there can be false negatives but we have not been able to
> > measure their practical impacts since we disabled the flush on some
> > host MMUs long ago (NOT by MGLRU), e.g., on x86 and ppc,
> > ptep_clear_flush_young() is just ptep_test_andclear_young().
> 
> Aha!  That's what I was missing, I somehow didn't see x86's ptep_clear_flush_young().

Heh, well the helper name isn't exactly giving any hints...

> That begs the question, why does KVM flush TLBs on architectures that don't need
> to?  And since kvm_mmu_notifier_clear_young() explicitly doesn't flush, are there
> even any KVM-supported architectures for which the flush is mandatory?
> 
> Skipping the flush on KVM x86 seems like a complete no-brainer.
> 
> Will, Marc and/or Oliver, what are arm64's requirements in this area?  E.g. I see
> that arm64's version of __ptep_clear_flush_young() does TLBI but not DSB.  Should
> KVM be doing something similar?  Can KVM safely skip even the TBLI?

Short answer, yes, KVM can elide TLBIs when clearing AF.

Long answer: Software needs to be extremely careful to ensure that TLBI
elision doesn't lead to a failure to uphold break-before-make requirements,
if we're only concerned with architecture-specific requirements. IOW, the AF
cannot be used as a hint for the presence of TLB entries for a given PTE.

There's the obvious failure of skipping TLBIs for old pages when
unmapping, but that isn't an architecture-specific issue.

So, since KVM/arm64 doesn't play any games with the AF at stage-2, leaving
out a TLBI when aging ought to be fine.

-- 
Thanks,
Oliver