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>

1 files changed, 19 insertions, 3 deletions

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index cfff74685a25..4736d7849c60 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/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))