KVM: x86: enable dirty log gradually in small chunks

It could take kvm->mmu_lock for an extended period of time when
enabling dirty log for the first time. The main cost is to clear
all the D-bits of last level SPTEs. This situation can benefit from
manual dirty log protect as well, which can reduce the mmu_lock
time taken. The sequence is like this:

1. Initialize all the bits of the dirty bitmap to 1 when enabling
   dirty log for the first time
2. Only write protect the huge pages
3. KVM_GET_DIRTY_LOG returns the dirty bitmap info
4. KVM_CLEAR_DIRTY_LOG will clear D-bit for each of the leaf level
   SPTEs gradually in small chunks

Under the Intel(R) Xeon(R) Gold 6152 CPU @ 2.10GHz environment,
I did some tests with a 128G windows VM and counted the time taken
of memory_global_dirty_log_start, here is the numbers:

VM Size        Before    After optimization
128G           460ms     10ms

Signed-off-by: Jay Zhou <jianjay.zhou@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 17 insertions, 4 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ddd1d296bd20..864d0aded0b8 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9916,7 +9916,7 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
 {
 	/* Still write protect RO slot */
 	if (new->flags & KVM_MEM_READONLY) {
-		kvm_mmu_slot_remove_write_access(kvm, new);
+		kvm_mmu_slot_remove_write_access(kvm, new, PT_PAGE_TABLE_LEVEL);
 		return;
 	}
 
@@ -9951,10 +9951,23 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
 	 * See the comments in fast_page_fault().
 	 */
 	if (new->flags & KVM_MEM_LOG_DIRTY_PAGES) {
-		if (kvm_x86_ops->slot_enable_log_dirty)
+		if (kvm_x86_ops->slot_enable_log_dirty) {
 			kvm_x86_ops->slot_enable_log_dirty(kvm, new);
-		else
-			kvm_mmu_slot_remove_write_access(kvm, new);
+		} else {
+			int level =
+				kvm_dirty_log_manual_protect_and_init_set(kvm) ?
+				PT_DIRECTORY_LEVEL : PT_PAGE_TABLE_LEVEL;
+
+			/*
+			 * If we're with initial-all-set, we don't need
+			 * to write protect any small page because
+			 * they're reported as dirty already.  However
+			 * we still need to write-protect huge pages
+			 * so that the page split can happen lazily on
+			 * the first write to the huge page.
+			 */
+			kvm_mmu_slot_remove_write_access(kvm, new, level);
+		}
 	} else {
 		if (kvm_x86_ops->slot_disable_log_dirty)
 			kvm_x86_ops->slot_disable_log_dirty(kvm, new);