3 files changed, 15 insertions, 13 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a2a7854..35f82f2 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3208,18 +3208,15 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
 		struct kvm_memslots *slots, *old_slots;
 		unsigned long *dirty_bitmap;
 
-		r = -ENOMEM;
-		dirty_bitmap = vmalloc(n);
-		if (!dirty_bitmap)
-			goto out;
+		dirty_bitmap = memslot->dirty_bitmap_head;
+		if (memslot->dirty_bitmap == dirty_bitmap)
+			dirty_bitmap += n / sizeof(long);
 		memset(dirty_bitmap, 0, n);
 
 		r = -ENOMEM;
 		slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
-		if (!slots) {
-			vfree(dirty_bitmap);
+		if (!slots)
 			goto out;
-		}
 		memcpy(slots, kvm->memslots, sizeof(struct kvm_memslots));
 		slots->memslots[log->slot].dirty_bitmap = dirty_bitmap;
 		slots->generation++;
@@ -3235,11 +3232,8 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
 		spin_unlock(&kvm->mmu_lock);
 
 		r = -EFAULT;
-		if (copy_to_user(log->dirty_bitmap, dirty_bitmap, n)) {
-			vfree(dirty_bitmap);
+		if (copy_to_user(log->dirty_bitmap, dirty_bitmap, n))
 			goto out;
-		}
-		vfree(dirty_bitmap);
 	} else {
 		r = -EFAULT;
 		if (clear_user(log->dirty_bitmap, n))
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 462b982..bcf71c7 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -150,6 +150,7 @@ struct kvm_memory_slot {
 	unsigned long flags;
 	unsigned long *rmap;
 	unsigned long *dirty_bitmap;
+	unsigned long *dirty_bitmap_head;
 	struct {
 		unsigned long rmap_pde;
 		int write_count;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 0021c28..27649fd 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -449,8 +449,9 @@ static void kvm_destroy_dirty_bitmap(struct kvm_memory_slot *memslot)
 	if (!memslot->dirty_bitmap)
 		return;
 
-	vfree(memslot->dirty_bitmap);
+	vfree(memslot->dirty_bitmap_head);
 	memslot->dirty_bitmap = NULL;
+	memslot->dirty_bitmap_head = NULL;
 }
 
 /*
@@ -537,15 +538,21 @@ static int kvm_vm_release(struct inode *inode, struct file *filp)
 	return 0;
 }
 
+/*
+ * Allocation size is twice as large as the actual dirty bitmap size.
+ * This makes it possible to do double buffering: see x86's
+ * kvm_vm_ioctl_get_dirty_log().
+ */
 static int kvm_create_dirty_bitmap(struct kvm_memory_slot *memslot)
 {
-	unsigned long dirty_bytes = kvm_dirty_bitmap_bytes(memslot);
+	unsigned long dirty_bytes = 2 * kvm_dirty_bitmap_bytes(memslot);
 
 	memslot->dirty_bitmap = vmalloc(dirty_bytes);
 	if (!memslot->dirty_bitmap)
 		return -ENOMEM;
 
 	memset(memslot->dirty_bitmap, 0, dirty_bytes);
+	memslot->dirty_bitmap_head = memslot->dirty_bitmap;
 	return 0;
 }