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

[Yu Zhao <yuzhao@google.com>]

On Thu, Feb 23, 2023 at 11:24 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Thu, Feb 23, 2023, Yu Zhao wrote:
> > On Thu, Feb 23, 2023 at 10:09 AM Sean Christopherson <seanjc@google.com> 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.

Sorry, let me rephrase:
1. Clearing the A-bit in a non-leaf entry is technically wrong because
we didn't make sure there is the A-bit there --  the bit we are
clearing can be something else. (Yes, we know it's not, but we didn't
define this behavior, e.g., a macro to designate that bit for non-leaf
entries. Also I didn't check the spec -- does EPT actually support the
A-bit in non-leaf entries? My guess is that NPT does.)