KVM: x86/mmu: Don't rebuild page when the page is synced and no tlb flushing is required

Before Commit c3e5e415 ("KVM: X86: Change kvm_sync_page()
to return true when remote flush is needed"), the return value
of kvm_sync_page() indicates whether the page is synced, and
kvm_mmu_get_page() would rebuild page when the sync fails.
But now, kvm_sync_page() returns false when the page is
synced and no tlb flushing is required, which leads to
rebuild page in kvm_mmu_get_page(). So return the return
value of mmu->sync_page() directly and check it in
kvm_mmu_get_page(). If the sync fails, the page will be
zapped and the invalid_list is not empty, so set flush as
true is accepted in mmu_sync_children().

Cc: stable@vger.kernel.org
Fixes: c3e5e415 ("KVM: X86: Change kvm_sync_page() to return true when remote flush is needed")
Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
Acked-by: Lai Jiangshan <jiangshanlai@gmail.com>
Message-Id: <0dabeeb789f57b0d793f85d073893063e692032d.1647336064.git.houwenlong.hwl@antgroup.com>
[mmu_sync_children should not flush if the page is zapped. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -1866,17 +1866,14 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
 	  &(_kvm)->arch.mmu_page_hash[kvm_page_table_hashfn(_gfn)])	\
 		if ((_sp)->gfn != (_gfn) || (_sp)->role.direct) {} else
 
-static bool kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
+static int kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 			 struct list_head *invalid_list)
 {
 	int ret = vcpu->arch.mmu->sync_page(vcpu, sp);
 
-	if (ret < 0) {
+	if (ret < 0)
 		kvm_mmu_prepare_zap_page(vcpu->kvm, sp, invalid_list);
-		return false;
-	}
-
-	return !!ret;
+	return ret;
 }
 
 static bool kvm_mmu_remote_flush_or_zap(struct kvm *kvm,
@@ -1998,7 +1995,7 @@ static int mmu_sync_children(struct kvm_vcpu *vcpu,
 
 		for_each_sp(pages, sp, parents, i) {
 			kvm_unlink_unsync_page(vcpu->kvm, sp);
-			flush |= kvm_sync_page(vcpu, sp, &invalid_list);
+			flush |= kvm_sync_page(vcpu, sp, &invalid_list) > 0;
 			mmu_pages_clear_parents(&parents);
 		}
 		if (need_resched() || rwlock_needbreak(&vcpu->kvm->mmu_lock)) {
@@ -2039,6 +2036,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
 	struct hlist_head *sp_list;
 	unsigned quadrant;
 	struct kvm_mmu_page *sp;
+	int ret;
 	int collisions = 0;
 	LIST_HEAD(invalid_list);
 
@@ -2091,11 +2089,13 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
 			 * If the sync fails, the page is zapped.  If so, break
 			 * in order to rebuild it.
 			 */
-			if (!kvm_sync_page(vcpu, sp, &invalid_list))
+			ret = kvm_sync_page(vcpu, sp, &invalid_list);
+			if (ret < 0)
 				break;
 
 			WARN_ON(!list_empty(&invalid_list));
-			kvm_flush_remote_tlbs(vcpu->kvm);
+			if (ret > 0)
+				kvm_flush_remote_tlbs(vcpu->kvm);
 		}
 
 		__clear_sp_write_flooding_count(sp);