Re: [PATCH v2 4/6] KVM: x86/mmu: Track the number of TDP MMU pages, but not the actual pages

[Sean Christopherson <seanjc@google.com>]

On Wed, Jul 27, 2022, Yan Zhao wrote:
> On Sat, Jul 23, 2022 at 01:23:23AM +0000, Sean Christopherson wrote:
> 
> <snip>
> 
> > @@ -386,16 +385,18 @@ static void handle_changed_spte_dirty_log(struct kvm *kvm, int as_id, gfn_t gfn,
> >  static void tdp_mmu_unlink_sp(struct kvm *kvm, struct kvm_mmu_page *sp,
> >  			      bool shared)
> >  {
> > +	atomic64_dec(&kvm->arch.tdp_mmu_pages);
> > +
> > +	if (!sp->nx_huge_page_disallowed)
> > +		return;
> > +
> Does this read of sp->nx_huge_page_disallowed also need to be protected by
> tdp_mmu_pages_lock in shared path?


No, because only one CPU can call tdp_mmu_unlink_sp() for a shadow page.  E.g. in
a shared walk, the SPTE is zapped atomically and only the CPU that "wins" gets to
unlink the s[.  The extra lock is needed to prevent list corruption, but the
sp itself is thread safe.

FWIW, even if that guarantee didn't hold, checking the flag outside of tdp_mmu_pages_lock
is safe because false positives are ok.  untrack_possible_nx_huge_page() checks that
the shadow page is actually on the list, i.e. it's a nop if a different task unlinks
the page first.

False negatives need to be avoided, but nx_huge_page_disallowed is cleared only
when untrack_possible_nx_huge_page() is guaranteed to be called, i.e. true false
negatives can't occur.

Hmm, but I think there's a missing smp_rmb(), which is needed to ensure
nx_huge_page_disallowed is read after observing the shadow-present SPTE (that's
being unlinked).  I'll add that in the next version.