Revert "KVM: MMU: zap pages in batch"

Unwinding optimizations related to obsolete pages is a step towards
removing x86 KVM's fast invalidate mechanism, i.e. this is one part of
a revert all patches from the series that introduced the mechanism[1].

This reverts commit e7d11c7a.

[1] https://lkml.kernel.org/r/1369960590-14138-1-git-send-email-xiaoguangrong@linux.vnet.ibm.com

Cc: Xiao Guangrong <guangrong.xiao@gmail.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu.c

@@ -5846,18 +5846,14 @@ void kvm_mmu_slot_set_dirty(struct kvm *kvm,
 }
 EXPORT_SYMBOL_GPL(kvm_mmu_slot_set_dirty);
 
-#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 batch = 0;
 
 restart:
 	list_for_each_entry_safe_reverse(sp, node,
 	      &kvm->arch.active_mmu_pages, link) {
-		int ret;
-
 		/*
 		 * No obsolete page exists before new created page since
 		 * active_mmu_pages is the FIFO list.
@@ -5866,6 +5862,28 @@ 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.
@@ -5873,18 +5891,13 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
 		if (sp->role.invalid)
 			continue;
 
-		if (batch >= BATCH_ZAP_PAGES &&
-		      (need_resched() || spin_needbreak(&kvm->mmu_lock))) {
-			batch = 0;
+		if (need_resched() || spin_needbreak(&kvm->mmu_lock)) {
 			kvm_mmu_commit_zap_page(kvm, &invalid_list);
 			cond_resched_lock(&kvm->mmu_lock);
 			goto restart;
 		}
 
-		ret = kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list);
-		batch += ret;
-
-		if (ret)
+		if (kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list))
 			goto restart;
 	}