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, 5 insertions, 0 deletions

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index e6e66c5e56f2..6f0f389f5f9c 100644
--- a/include/linux/kvm_host.h
+++ b/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;