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>

arch/x86/kvm/mmu/mmu.c

@@ -6878,16 +6878,32 @@ static void kvm_recover_nx_huge_pages(struct kvm *kvm)
 		WARN_ON_ONCE(!sp->nx_huge_page_disallowed);
 		WARN_ON_ONCE(!sp->role.direct);
 
-		slot = gfn_to_memslot(kvm, sp->gfn);
-		WARN_ON_ONCE(!slot);
-
 		/*
 		 * Unaccount and do not attempt to recover any NX Huge Pages
 		 * that are being dirty tracked, as they would just be faulted
 		 * back in as 4KiB pages. The NX Huge Pages in this slot will be
 		 * recovered, along with all the other huge pages in the slot,
 		 * when dirty logging is disabled.
+		 *
+		 * Since gfn_to_memslot() is relatively expensive, it helps to
+		 * skip it if it the test cannot possibly return true.  On the
+		 * other hand, if any memslot has logging enabled, chances are
+		 * good that all of them do, in which case unaccount_nx_huge_page()
+		 * is much cheaper than zapping the page.
+		 *
+		 * If a memslot update is in progress, reading an incorrect value
+		 * of kvm->nr_memslots_dirty_logging is not a problem: if it is
+		 * becoming zero, gfn_to_memslot() will be done unnecessarily; if
+		 * it is becoming nonzero, the page will be zapped unnecessarily.
+		 * Either way, this only affects efficiency in racy situations,
+		 * and not correctness.
 		 */
+		slot = NULL;
+		if (atomic_read(&kvm->nr_memslots_dirty_logging)) {
+			slot = gfn_to_memslot(kvm, sp->gfn);
+			WARN_ON_ONCE(!slot);
+		}
+
 		if (slot && kvm_slot_dirty_track_enabled(slot))
 			unaccount_nx_huge_page(kvm, sp);
 		else if (is_tdp_mmu_page(sp))

include/linux/kvm_host.h

@@ -722,6 +722,11 @@ struct kvm {
 	/* The current active memslot set for each address space */
 	struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM];
 	struct xarray vcpu_array;
+	/*
+	 * Protected by slots_lock, but can be read outside if an
+	 * incorrect answer is acceptable.
+	 */
+	atomic_t nr_memslots_dirty_logging;
 
 	/* Used to wait for completion of MMU notifiers.  */
 	spinlock_t mn_invalidate_lock;

virt/kvm/kvm_main.c

@@ -1641,6 +1641,8 @@ static void kvm_commit_memory_region(struct kvm *kvm,
 				     const struct kvm_memory_slot *new,
 				     enum kvm_mr_change change)
 {
+	int old_flags = old ? old->flags : 0;
+	int new_flags = new ? new->flags : 0;
 	/*
 	 * Update the total number of memslot pages before calling the arch
 	 * hook so that architectures can consume the result directly.
@@ -1650,6 +1652,12 @@ static void kvm_commit_memory_region(struct kvm *kvm,
 	else if (change == KVM_MR_CREATE)
 		kvm->nr_memslot_pages += new->npages;
 
+	if ((old_flags ^ new_flags) & KVM_MEM_LOG_DIRTY_PAGES) {
+		int change = (new_flags & KVM_MEM_LOG_DIRTY_PAGES) ? 1 : -1;
+		atomic_set(&kvm->nr_memslots_dirty_logging,
+			   atomic_read(&kvm->nr_memslots_dirty_logging) + change);
+	}
+
 	kvm_arch_commit_memory_region(kvm, old, new, change);
 
 	switch (change) {