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: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NTakuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: NGleb Natapov <gleb@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -767,6 +767,7 @@ void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
 				     struct kvm_memory_slot *slot,
 				     gfn_t gfn_offset, unsigned long mask);
 void kvm_mmu_zap_all(struct kvm *kvm);
+void kvm_mmu_zap_mmio_sptes(struct kvm *kvm);
 unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm);
 void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned int kvm_nr_mmu_pages);
 

arch/x86/kvm/mmu.c

@@ -4189,6 +4189,24 @@ void kvm_mmu_zap_all(struct kvm *kvm)
 	spin_unlock(&kvm->mmu_lock);
 }
 
+void kvm_mmu_zap_mmio_sptes(struct kvm *kvm)
+{
+	struct kvm_mmu_page *sp, *node;
+	LIST_HEAD(invalid_list);
+
+	spin_lock(&kvm->mmu_lock);
+restart:
+	list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link) {
+		if (!sp->mmio_cached)
+			continue;
+		if (kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list))
+			goto restart;
+	}
+
+	kvm_mmu_commit_zap_page(kvm, &invalid_list);
+	spin_unlock(&kvm->mmu_lock);
+}
+
 static int mmu_shrink(struct shrinker *shrink, struct shrink_control *sc)
 {
 	struct kvm *kvm;

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);
 	}
 }