Re: [PATCH v3 5/7] KVM: x86: Participate in bitmap-based PTE aging

[David Matlack <dmatlack@google.com>]

On Thu, Apr 11, 2024 at 10:28 AM David Matlack <dmatlack@google.com> wrote:
>
> On 2024-04-11 10:08 AM, David Matlack wrote:
> > On 2024-04-01 11:29 PM, James Houghton wrote:
> > > Only handle the TDP MMU case for now. In other cases, if a bitmap was
> > > not provided, fallback to the slowpath that takes mmu_lock, or, if a
> > > bitmap was provided, inform the caller that the bitmap is unreliable.
> > >
> > > Suggested-by: Yu Zhao <yuzhao@google.com>
> > > Signed-off-by: James Houghton <jthoughton@google.com>
> > > ---
> > >  arch/x86/include/asm/kvm_host.h | 14 ++++++++++++++
> > >  arch/x86/kvm/mmu/mmu.c          | 16 ++++++++++++++--
> > >  arch/x86/kvm/mmu/tdp_mmu.c      | 10 +++++++++-
> > >  3 files changed, 37 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > > index 3b58e2306621..c30918d0887e 100644
> > > --- a/arch/x86/include/asm/kvm_host.h
> > > +++ b/arch/x86/include/asm/kvm_host.h
> > > @@ -2324,4 +2324,18 @@ int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages);
> > >   */
> > >  #define KVM_EXIT_HYPERCALL_MBZ             GENMASK_ULL(31, 1)
> > >
> > > +#define kvm_arch_prepare_bitmap_age kvm_arch_prepare_bitmap_age
> > > +static inline bool kvm_arch_prepare_bitmap_age(struct mmu_notifier *mn)
> > > +{
> > > +   /*
> > > +    * Indicate that we support bitmap-based aging when using the TDP MMU
> > > +    * and the accessed bit is available in the TDP page tables.
> > > +    *
> > > +    * We have no other preparatory work to do here, so we do not need to
> > > +    * redefine kvm_arch_finish_bitmap_age().
> > > +    */
> > > +   return IS_ENABLED(CONFIG_X86_64) && tdp_mmu_enabled
> > > +                                    && shadow_accessed_mask;
> > > +}
> > > +
> > >  #endif /* _ASM_X86_KVM_HOST_H */
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index 992e651540e8..fae1a75750bb 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -1674,8 +1674,14 @@ bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
> > >  {
> > >     bool young = false;
> > >
> > > -   if (kvm_memslots_have_rmaps(kvm))
> > > +   if (kvm_memslots_have_rmaps(kvm)) {
> > > +           if (range->lockless) {
> > > +                   kvm_age_set_unreliable(range);
> > > +                   return false;
> > > +           }
> >
> > If a VM has TDP MMU enabled, supports A/D bits, and is using nested
> > virtualization, MGLRU will effectively be blind to all accesses made by
> > the VM.
> >
> > kvm_arch_prepare_bitmap_age() will return true indicating that the
> > bitmap is supported. But then kvm_age_gfn() and kvm_test_age_gfn() will
> > return false immediately and indicate the bitmap is unreliable because a
> > shadow root is allocate. The notfier will then return
> > MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE.

Ah no, I'm wrong here. Setting args.unreliable causes the notifier to
return 0 instead of MMU_NOTIFIER_YOUNG_FAST.
MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE is used for something else.

The control flow of all this and naming of functions and macros is
overall confusing. args.unreliable and
MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE for one. Also I now realize
kvm_arch_prepare/finish_bitmap_age() are used even when the bitmap is
_not_ provided, so those names are also misleading.