Subject: Re: [PATCH mm-unstable v1 2/5] kvm/x86: add kvm_arch_test_clear_young()

From: Sean Christopherson <[email-masked]>
Date: Thu, 23 Feb 2023 10:23:56 -0800

On Thu, Feb 23, 2023, Yu Zhao wrote:
> On Thu, Feb 23, 2023 at 10:09 AM Sean Christopherson <[email-masked]> wrote:
> > > I'll take a look at that series. clear_bit() probably won't cause any
> > > practical damage but is technically wrong because, for example, it can
> > > end up clearing the A-bit in a non-leaf PMD. (cmpxchg will just fail
> > > in this case, obviously.)
> >
> > Eh, not really.  By that argument, clearing an A-bit in a huge PTE is also technically
> > wrong because the target gfn may or may not have been accessed.
> 
> Sorry, I don't understand. You mean clear_bit() on a huge PTE is
> technically wrong? Yes, that's what I mean. (cmpxchg() on a huge PTE
> is not.)
> 
> > The only way for
> > KVM to clear a A-bit in a non-leaf entry is if the entry _was_ a huge PTE, but was
> > replaced between the "is leaf" and the clear_bit().
> 
> I think there is a misunderstanding here. Let me be more specific:
> 1. Clearing the A-bit in a non-leaf entry is technically wrong because
> that's not our intention.
> 2. When we try to clear_bit() on a leaf PMD, it can at the same time
> become a non-leaf PMD, which causes 1) above, and therefore is
> technically wrong.
> 3. I don't think 2) could do any real harm, so no practically no problem.
> 4. cmpxchg() can avoid 2).
> 
> Does this make sense?

I understand what you're saying, but clearing an A-bit on a non-leaf PMD that
_just_ got converted from a leaf PMD is "wrong" if and only if the intented
behavior is nonsensical.

Without an explicit granluarity from the caller, the intent is to either (a) reap
A-bit on leaf PTEs, or (b) reap A-bit at the highest possible granularity.  (a) is
nonsensical because because it provides zero guarantees to the caller as to the
granularity of the information.  Leaf vs. non-leaf matters for the life cycle of
page tables and guest accesses, e.g. KVM needs to zap _only_ leaf SPTEs when
handling an mmu_notifier invalidation, but when it comes to the granularity of the
A-bit, leaf vs. non-leaf has no meaning.  On KVM x86, a PMD covers 2MiB of GPAs
regardless of whether it's a leaf or non-leaf PMD.

If the intent is (b), then clearing the A-bit on a PMD a few cycles after the PMD
was converted from leaf to non-leaf is a pointless distinction, because it yields
the same end result as clearing the A-bit just a few cycles earlier, when the PMD
was a leaf.

Actually, if I'm reading patch 5 correctly, this is all much ado about nothing,
because the MGLRU code only kicks in only for non-huge PTEs, and KVM cannot have
larger mappings than the primary MMU, i.e. should not encounter huge PTEs.

On that topic, if the assumption is that the bitmap is used only for non-huge PTEs,
then x86's kvm_arch_test_clear_young() needs to be hardened to process only 4KiB
PTEs, and probably to WARN if a huge PTE is encountered.  That assumption should
also be documented.

If that assumption is incorrect, then kvm_arch_test_clear_young() is broken and/or
the expected behavior of the bitmap isn't fully defined.