KVM: x86: avoid unnecessary bitmap allocation when memslot is clean

Although we always allocate a new dirty bitmap in x86's get_dirty_log(),
it is only used as a zero-source of copy_to_user() and freed right after
that when memslot is clean. This patch uses clear_user() instead of doing
this unnecessary zero-source allocation.

Performance improvement: as we can expect easily, the time needed to
allocate a bitmap is completely reduced. In my test, the improved ioctl
was about 4 to 10 times faster than the original one for clean slots.
Furthermore, reducing memory allocations and copies will produce good
effects to caches too.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/x86.c

@@ -2797,7 +2797,6 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
 	struct kvm_memory_slot *memslot;
 	unsigned long n;
 	unsigned long is_dirty = 0;
-	unsigned long *dirty_bitmap = NULL;
 
 	mutex_lock(&kvm->slots_lock);
 
@@ -2812,27 +2811,30 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
 
 	n = kvm_dirty_bitmap_bytes(memslot);
 
-	r = -ENOMEM;
-	dirty_bitmap = vmalloc(n);
-	if (!dirty_bitmap)
-		goto out;
-	memset(dirty_bitmap, 0, n);
-
 	for (i = 0; !is_dirty && i < n/sizeof(long); i++)
 		is_dirty = memslot->dirty_bitmap[i];
 
 	/* If nothing is dirty, don't bother messing with page tables. */
 	if (is_dirty) {
 		struct kvm_memslots *slots, *old_slots;
+		unsigned long *dirty_bitmap;
 
 		spin_lock(&kvm->mmu_lock);
 		kvm_mmu_slot_remove_write_access(kvm, log->slot);
 		spin_unlock(&kvm->mmu_lock);
 
-		slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
-		if (!slots)
-			goto out_free;
+		r = -ENOMEM;
+		dirty_bitmap = vmalloc(n);
+		if (!dirty_bitmap)
+			goto out;
+		memset(dirty_bitmap, 0, n);
 
+		r = -ENOMEM;
+		slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
+		if (!slots) {
+			vfree(dirty_bitmap);
+			goto out;
+		}
 		memcpy(slots, kvm->memslots, sizeof(struct kvm_memslots));
 		slots->memslots[log->slot].dirty_bitmap = dirty_bitmap;
 
@@ -2841,13 +2843,20 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
 		synchronize_srcu_expedited(&kvm->srcu);
 		dirty_bitmap = old_slots->memslots[log->slot].dirty_bitmap;
 		kfree(old_slots);
+
+		r = -EFAULT;
+		if (copy_to_user(log->dirty_bitmap, dirty_bitmap, n)) {
+			vfree(dirty_bitmap);
+			goto out;
+		}
+		vfree(dirty_bitmap);
+	} else {
+		r = -EFAULT;
+		if (clear_user(log->dirty_bitmap, n))
+			goto out;
 	}
 
 	r = 0;
-	if (copy_to_user(log->dirty_bitmap, dirty_bitmap, n))
-		r = -EFAULT;
-out_free:
-	vfree(dirty_bitmap);
 out:
 	mutex_unlock(&kvm->slots_lock);
 	return r;