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

[David Matlack <dmatlack@google.com>]

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.
> 
> Looking at the callers, MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE is never
> consumed or used. So I think MGLRU will assume all memory is
> unaccessed?
> 
> One way to improve the situation would be to re-order the TDP MMU
> function first and return young instead of false, so that way MGLRU at
> least has visibility into accesses made by L1 (and L2 if EPT is disable
> in L2). But that still means MGLRU is blind to accesses made by L2.
> 
> What about grabbing the mmu_lock if there's a shadow root allocated and
> get rid of MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE altogether?
> 
> 	if (kvm_memslots_have_rmaps(kvm)) {
> 		write_lock(&kvm->mmu_lock);
> 		young |= kvm_handle_gfn_range(kvm, range, kvm_age_rmap);
> 		write_unlock(&kvm->mmu_lock);
> 	}
> 
> The TDP MMU walk would still be lockless. KVM only has to take the
> mmu_lock to collect accesses made by L2.
> 
> kvm_age_rmap() and kvm_test_age_rmap() will need to become bitmap-aware
> as well, but that seems relatively simple with the helper functions.

Wait, even simpler, just check kvm_memslots_have_rmaps() in
kvm_arch_prepare_bitmap_age() and skip the shadow MMU when processing a
bitmap request.

i.e.

static inline bool kvm_arch_prepare_bitmap_age(struct kvm *kvm, 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
		&& !kvm_memslots_have_rmaps(kvm);
}

bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
{
        bool young = false;

        if (!range->arg.metadata->bitmap && kvm_memslots_have_rmaps(kvm))
                young = kvm_handle_gfn_range(kvm, range, kvm_age_rmap);

        if (tdp_mmu_enabled)
                young |= kvm_tdp_mmu_age_gfn_range(kvm, range);

        return young;
}

bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
{
        bool young = false;

        if (!range->arg.metadata->bitmap && kvm_memslots_have_rmaps(kvm))
                young = kvm_handle_gfn_range(kvm, range, kvm_test_age_rmap);

        if (tdp_mmu_enabled)
                young |= kvm_tdp_mmu_test_age_gfn(kvm, range);

        return young;
}

Sure this could race with the creation of a shadow root but so can the
non-bitmap code.