KVM: x86: Optimize mmio spte zapping when creating/moving memslot

When we create or move a memory slot, we need to zap mmio sptes.
Currently, zap_all() is used for this and this is causing two problems:
 - extra page faults after zapping mmu pages
 - long mmu_lock hold time during zapping mmu pages

For the latter, Marcelo reported a disastrous mmu_lock hold time during
hot-plug, which made the guest unresponsive for a long time.

This patch takes a simple way to fix these problems: do not zap mmu
pages unless they are marked mmio cached.  On our test box, this took
only 50us for the 4GB guest and we did not see ms of mmu_lock hold time
any more.

Note that we still need to do zap_all() for other cases.  So another
work is also needed: Xiao's work may be the one.

Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 61a5bb60af86..d3c478742e2c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6991,7 +6991,7 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
 	 * mmio sptes.
 	 */
 	if ((change == KVM_MR_CREATE) || (change == KVM_MR_MOVE)) {
-		kvm_mmu_zap_all(kvm);
+		kvm_mmu_zap_mmio_sptes(kvm);
 		kvm_reload_remote_mmus(kvm);
 	}
 }