mmu-notifier-add-clear_young-callback-fix

Cc: Andres Lagar-Cavilla <andreslc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index ff4173ce6924..e69a5cb99571 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -397,6 +397,19 @@ static int kvm_mmu_notifier_clear_young(struct mmu_notifier *mn,
 
 	idx = srcu_read_lock(&kvm->srcu);
 	spin_lock(&kvm->mmu_lock);
+	/*
+	 * Even though we do not flush TLB, this will still adversely
+	 * affect performance on pre-Haswell Intel EPT, where there is
+	 * no EPT Access Bit to clear so that we have to tear down EPT
+	 * tables instead. If we find this unacceptable, we can always
+	 * add a parameter to kvm_age_hva so that it effectively doesn't
+	 * do anything on clear_young.
+	 *
+	 * Also note that currently we never issue secondary TLB flushes
+	 * from clear_young, leaving this job up to the regular system
+	 * cadence. If we find this inaccurate, we might come up with a
+	 * more sophisticated heuristic later.
+	 */
 	young = kvm_age_hva(kvm, start, end);
 	spin_unlock(&kvm->mmu_lock);
 	srcu_read_unlock(&kvm->srcu, idx);