KVM: x86/mmu: Revert "Revert "KVM: MMU: zap pages in batch""

Now that the fast invalidate mechanism has been reintroduced, restore
the performance tweaks for fast invalidation that existed prior to its
removal.

Paraphrashing the original changelog:

  Zap at least 10 shadow pages before releasing mmu_lock to reduce the
  overhead associated with re-acquiring the lock.

  Note: "10" is an arbitrary number, speculated to be high enough so
  that a vCPU isn't stuck zapping obsolete pages for an extended period,
  but small enough so that other vCPUs aren't starved waiting for
  mmu_lock.

This reverts commit 43d2b14b.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu.c

@@ -5671,12 +5671,12 @@ int kvm_mmu_create(struct kvm_vcpu *vcpu)
 	return ret;
 }
 
-
+#define BATCH_ZAP_PAGES	10
 static void kvm_zap_obsolete_pages(struct kvm *kvm)
 {
 	struct kvm_mmu_page *sp, *node;
 	LIST_HEAD(invalid_list);
-	int ign;
+	int nr_zapped, batch = 0;
 
 restart:
 	list_for_each_entry_safe_reverse(sp, node,
@@ -5689,28 +5689,6 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
 			break;
 
 		/*
-		 * Do not repeatedly zap a root page to avoid unnecessary
-		 * KVM_REQ_MMU_RELOAD, otherwise we may not be able to
-		 * progress:
-		 *    vcpu 0                        vcpu 1
-		 *                         call vcpu_enter_guest():
-		 *                            1): handle KVM_REQ_MMU_RELOAD
-		 *                                and require mmu-lock to
-		 *                                load mmu
-		 * repeat:
-		 *    1): zap root page and
-		 *        send KVM_REQ_MMU_RELOAD
-		 *
-		 *    2): if (cond_resched_lock(mmu-lock))
-		 *
-		 *                            2): hold mmu-lock and load mmu
-		 *
-		 *                            3): see KVM_REQ_MMU_RELOAD bit
-		 *                                on vcpu->requests is set
-		 *                                then return 1 to call
-		 *                                vcpu_enter_guest() again.
-		 *            goto repeat;
-		 *
 		 * Since we are reversely walking the list and the invalid
 		 * list will be moved to the head, skip the invalid page
 		 * can help us to avoid the infinity list walking.
@@ -5718,15 +5696,20 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
 		if (sp->role.invalid)
 			continue;
 
-		if (need_resched() || spin_needbreak(&kvm->mmu_lock)) {
+		if (batch >= BATCH_ZAP_PAGES &&
+		    (need_resched() || spin_needbreak(&kvm->mmu_lock))) {
+			batch = 0;
 			kvm_mmu_commit_zap_page(kvm, &invalid_list);
 			cond_resched_lock(&kvm->mmu_lock);
 			goto restart;
 		}
 
-		if (__kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list, &ign))
+		if (__kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list,
+					       &nr_zapped)) {
+			batch += nr_zapped;
 			goto restart;
 		}
+	}
 
 	kvm_mmu_commit_zap_page(kvm, &invalid_list);
 }