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From: David Matlack <dmatlack@google.com>
Date: Thu, 11 Apr 2024 11:07:35 -0700
Message-ID: <CALzav=euNq2eaHYg79V=sZWytGBh-=TNoHNwRMgChy+DsCNRrw@mail.gmail.com>
Subject: Re: [PATCH v3 5/7] KVM: x86: Participate in bitmap-based PTE aging
To: James Houghton <jthoughton@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Paolo Bonzini <pbonzini@redhat.com>, 
	Yu Zhao <yuzhao@google.com>, Marc Zyngier <maz@kernel.org>, 
	Oliver Upton <oliver.upton@linux.dev>, Sean Christopherson <seanjc@google.com>, 
	Jonathan Corbet <corbet@lwn.net>, James Morse <james.morse@arm.com>, 
	Suzuki K Poulose <suzuki.poulose@arm.com>, Zenghui Yu <yuzenghui@huawei.com>, 
	Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	Dave Hansen <dave.hansen@linux.intel.com>, "H. Peter Anvin" <hpa@zytor.com>, 
	Steven Rostedt <rostedt@goodmis.org>, Masami Hiramatsu <mhiramat@kernel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, Shaoqin Huang <shahuang@redhat.com>, 
	Gavin Shan <gshan@redhat.com>, Ricardo Koller <ricarkol@google.com>, 
	Raghavendra Rao Ananta <rananta@google.com>, Ryan Roberts <ryan.roberts@arm.com>, 
	David Rientjes <rientjes@google.com>, Axel Rasmussen <axelrasmussen@google.com>, 
	linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, 
	kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-mm@kvack.org, 
	linux-trace-kernel@vger.kernel.org
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On Thu, Apr 11, 2024 at 11:00=E2=80=AFAM David Matlack <dmatlack@google.com=
> wrote:
>
> On Thu, Apr 11, 2024 at 10:28=E2=80=AFAM David Matlack <dmatlack@google.c=
om> 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 w=
as
> > > > not provided, fallback to the slowpath that takes mmu_lock, or, if =
a
> > > > bitmap was provided, inform the caller that the bitmap is unreliabl=
e.
> > > >
> > > > 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_slo=
t *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 T=
DP 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 n=
eed 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 =3D 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() wi=
ll
> > > return false immediately and indicate the bitmap is unreliable becaus=
e 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.

Nope, wrong again. Just ignore me while I try to figure out how this
actually works :)