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Message-ID: <20240807064131.rf7f2ve324hg434r@yy-desk-7060>
Date: Wed, 7 Aug 2024 14:41:31 +0800
From: Yuan Yao <yuan.yao@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 9/9] KVM: x86/mmu: Track SPTE accessed info across
mmu_notifier PROT changes
On Mon, Aug 05, 2024 at 03:59:11PM +0800, Yuan Yao wrote:
> On Thu, Aug 01, 2024 at 11:34:53AM -0700, Sean Christopherson wrote:
> > Preserve Accessed information when zapping SPTEs in response to an
> > mmu_notifier protection change, e.g. if KVM is zapping SPTEs because
> > NUMA balancing kicked in. KVM is not required to fully unmap the SPTE,
> > and the core VMA information isn't changing, i.e. the information is
> > still fresh and useful.
> >
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> > arch/x86/kvm/mmu/tdp_mmu.c | 31 +++++++++++++++++++++++++------
> > 1 file changed, 25 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index ac3200ce00f9..780f35a22c05 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -838,7 +838,8 @@ bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
> > * operation can cause a soft lockup.
> > */
> > static bool tdp_mmu_zap_leafs(struct kvm *kvm, struct kvm_mmu_page *root,
> > - gfn_t start, gfn_t end, bool can_yield, bool flush)
> > + gfn_t start, gfn_t end, bool can_yield,
> > + bool keep_accessed_bit, bool flush)
> > {
> > struct tdp_iter iter;
> >
> > @@ -849,17 +850,29 @@ static bool tdp_mmu_zap_leafs(struct kvm *kvm, struct kvm_mmu_page *root,
> > rcu_read_lock();
> >
> > for_each_tdp_pte_min_level(iter, root, PG_LEVEL_4K, start, end) {
> > + u64 new_spte = SHADOW_NONPRESENT_VALUE;
> > +
> > if (can_yield &&
> > tdp_mmu_iter_cond_resched(kvm, &iter, flush, false)) {
> > flush = false;
> > continue;
> > }
> >
> > + /*
> > + * Note, this will fail to clear non-present, accessed SPTEs,
> > + * but that isn't a functional problem, it can only result in
> > + * a _potential_ false positive in the unlikely scenario that
> > + * the primary MMU zaps an hva, reinstalls a new hva, and ages
> > + * the new hva, all before KVM accesses the hva.
> > + */
> > if (!is_shadow_present_pte(iter.old_spte) ||
> > !is_last_spte(iter.old_spte, iter.level))
> > continue;
> >
> > - tdp_mmu_iter_set_spte(kvm, &iter, SHADOW_NONPRESENT_VALUE);
> > + if (keep_accessed_bit)
> > + new_spte |= iter.old_spte & shadow_accessed_mask;
> > +
> > + tdp_mmu_iter_set_spte(kvm, &iter, new_spte);
> >
> > /*
> > * Zappings SPTEs in invalid roots doesn't require a TLB flush,
> > @@ -889,7 +902,7 @@ bool kvm_tdp_mmu_zap_leafs(struct kvm *kvm, gfn_t start, gfn_t end, bool flush)
> >
> > lockdep_assert_held_write(&kvm->mmu_lock);
> > for_each_valid_tdp_mmu_root_yield_safe(kvm, root, -1)
> > - flush = tdp_mmu_zap_leafs(kvm, root, start, end, true, flush);
> > + flush = tdp_mmu_zap_leafs(kvm, root, start, end, true, false, flush);
> >
> > return flush;
> > }
> > @@ -1180,11 +1193,13 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> > bool kvm_tdp_mmu_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range,
> > bool flush)
> > {
> > + bool keep_a_bit = range->arg.event == MMU_NOTIFY_PROTECTION_VMA ||
> > + range->arg.event == MMU_NOTIFY_PROTECTION_PAGE;
> > struct kvm_mmu_page *root;
> >
> > __for_each_tdp_mmu_root_yield_safe(kvm, root, range->slot->as_id, false)
> > flush = tdp_mmu_zap_leafs(kvm, root, range->start, range->end,
> > - range->may_block, flush);
> > + range->may_block, keep_a_bit, flush);
> >
> > return flush;
> > }
> > @@ -1201,7 +1216,11 @@ static void kvm_tdp_mmu_age_spte(struct tdp_iter *iter)
> > {
> > u64 new_spte;
> >
> > - if (spte_ad_enabled(iter->old_spte)) {
> > + if (spte_ad_enabled(iter->old_spte) ||
> > + !is_shadow_present_pte(iter->old_spte)) {
> > + KVM_MMU_WARN_ON(!is_shadow_present_pte(iter->old_spte) &&
> > + iter->old_spte != (SHADOW_NONPRESENT_VALUE | shadow_accessed_mask));
>
> Is that possible some sptes are zapped by
> kvm_tdp_mmu_zap_leafs(keep_accessed_bit = false) i.e. from kvm_post_set_cr0(),
> then handled by __kvm_tdp_mmu_age_gfn_range() for aging before
> accessed by guest again ?
> In this scenario the spte is non-present w/o A bit set.
>
> > +
> > iter->old_spte = tdp_mmu_clear_spte_bits(iter->sptep,
> > iter->old_spte,
> > shadow_accessed_mask,
> > @@ -1235,7 +1254,7 @@ static bool __kvm_tdp_mmu_age_gfn_range(struct kvm *kvm,
> > for_each_valid_tdp_mmu_root(kvm, root, range->slot->as_id) {
> > rcu_read_lock();
> >
> > - tdp_root_for_each_leaf_pte(iter, root, range->start, range->end) {
> > + tdp_root_for_each_pte(iter, root, range->start, range->end) {
>
> This also clears the A bit of non-leaf entries for aging, I remember
> KVM doesn't care them before, could you please explain the reason of
> this ?
The new __kvm_tdp_mmu_age_gfn_range() covers aging and testing, so here
it allows testing on non-present sptes, becasue A bit is preserved there.
I worried before that the access state is updated by
handle_changed_spte() in case of zapping, preserve A bit
gives the inaccurate access state to in future .test_young()
if no one access the zapped guest again. But this should be
addressed by patch 8 and 81 in the 'massive "follow pfn"
rework' patch set mentioned in the cover letter.
>
> > if (!is_accessed_spte(iter.old_spte))
> > continue;
> >
> > --
> > 2.46.0.rc1.232.g9752f9e123-goog
> >
> >
>
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