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KVM: x86: avoid memslot check in NX hugepage recovery if it cannot succeed
Since gfn_to_memslot() is relatively expensive, it helps to skip it if it the memslot cannot possibly have dirty logging enabled. In order to do this, add to struct kvm a counter of the number of log-page memslots. While the correct value can only be read with slots_lock taken, the NX recovery thread is content with using an approximate value. Therefore, the counter is an atomic_t. Based on https://lore.kernel.org/kvm/20221027200316.2221027-2-dmatlack@google.com/ by David Matlack. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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@ -6878,16 +6878,32 @@ static void kvm_recover_nx_huge_pages(struct kvm *kvm)
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WARN_ON_ONCE(!sp->nx_huge_page_disallowed);
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WARN_ON_ONCE(!sp->role.direct);
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slot = gfn_to_memslot(kvm, sp->gfn);
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WARN_ON_ONCE(!slot);
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/*
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* Unaccount and do not attempt to recover any NX Huge Pages
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* that are being dirty tracked, as they would just be faulted
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* back in as 4KiB pages. The NX Huge Pages in this slot will be
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* recovered, along with all the other huge pages in the slot,
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* when dirty logging is disabled.
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*
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* Since gfn_to_memslot() is relatively expensive, it helps to
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* skip it if it the test cannot possibly return true. On the
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* other hand, if any memslot has logging enabled, chances are
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* good that all of them do, in which case unaccount_nx_huge_page()
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* is much cheaper than zapping the page.
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*
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* If a memslot update is in progress, reading an incorrect value
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* of kvm->nr_memslots_dirty_logging is not a problem: if it is
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* becoming zero, gfn_to_memslot() will be done unnecessarily; if
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* it is becoming nonzero, the page will be zapped unnecessarily.
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* Either way, this only affects efficiency in racy situations,
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* and not correctness.
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*/
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slot = NULL;
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if (atomic_read(&kvm->nr_memslots_dirty_logging)) {
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slot = gfn_to_memslot(kvm, sp->gfn);
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WARN_ON_ONCE(!slot);
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}
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if (slot && kvm_slot_dirty_track_enabled(slot))
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unaccount_nx_huge_page(kvm, sp);
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else if (is_tdp_mmu_page(sp))
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@ -722,6 +722,11 @@ struct kvm {
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/* The current active memslot set for each address space */
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struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM];
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struct xarray vcpu_array;
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/*
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* Protected by slots_lock, but can be read outside if an
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* incorrect answer is acceptable.
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*/
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atomic_t nr_memslots_dirty_logging;
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/* Used to wait for completion of MMU notifiers. */
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spinlock_t mn_invalidate_lock;
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@ -1641,6 +1641,8 @@ static void kvm_commit_memory_region(struct kvm *kvm,
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const struct kvm_memory_slot *new,
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enum kvm_mr_change change)
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{
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int old_flags = old ? old->flags : 0;
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int new_flags = new ? new->flags : 0;
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/*
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* Update the total number of memslot pages before calling the arch
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* hook so that architectures can consume the result directly.
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@ -1650,6 +1652,12 @@ static void kvm_commit_memory_region(struct kvm *kvm,
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else if (change == KVM_MR_CREATE)
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kvm->nr_memslot_pages += new->npages;
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if ((old_flags ^ new_flags) & KVM_MEM_LOG_DIRTY_PAGES) {
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int change = (new_flags & KVM_MEM_LOG_DIRTY_PAGES) ? 1 : -1;
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atomic_set(&kvm->nr_memslots_dirty_logging,
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atomic_read(&kvm->nr_memslots_dirty_logging) + change);
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}
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kvm_arch_commit_memory_region(kvm, old, new, change);
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switch (change) {
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