[2/2] KVM: Stop using extra buffer for copying dirty_bitmap to user-space

Commit Message

Takuya Yoshikawa Aug. 30, 2013, 3:52 a.m. UTC

Now that mmu_lock is held only inside kvm_mmu_write_protect_pt_masked(),
we can do __put_user() for copying each 64/32 dirty bits to user-space.

This eliminates the need to copy the whole bitmap to an extra buffer and
the resulting code is much more cache friendly than before.

Signed-off-by: Takuya Yoshikawa <[e-mail]>
Cc: Xiao Guangrong <[e-mail]>
---
 arch/x86/kvm/x86.c  |   18 ++++++++----------
 virt/kvm/kvm_main.c |    6 +-----
 2 files changed, 9 insertions(+), 15 deletions(-)

Patch

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1d1f6df..79e8ad0 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3522,7 +3522,7 @@  int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log)
 	struct kvm_memory_slot *memslot;
 	unsigned long n, i;
 	unsigned long *dirty_bitmap;
-	unsigned long *dirty_bitmap_buffer;
+	unsigned long __user *p_user;
 	bool is_dirty = false;
 
 	mutex_lock(&kvm->slots_lock);
@@ -3539,11 +3539,12 @@  int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log)
 		goto out;
 
 	n = kvm_dirty_bitmap_bytes(memslot);
+	r = -EFAULT;
+	if (clear_user(log->dirty_bitmap, n))
+		goto out;
 
-	dirty_bitmap_buffer = dirty_bitmap + n / sizeof(long);
-	memset(dirty_bitmap_buffer, 0, n);
-
-	for (i = 0; i < n / sizeof(long); i++) {
+	p_user = (unsigned long __user *)log->dirty_bitmap;
+	for (i = 0; i < n / sizeof(long); i++, p_user++) {
 		unsigned long mask;
 		gfn_t offset;
 
@@ -3553,7 +3554,8 @@  int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log)
 		is_dirty = true;
 
 		mask = xchg(&dirty_bitmap[i], 0);
-		dirty_bitmap_buffer[i] = mask;
+		if (__put_user(mask, p_user))
+			goto out;
 
 		offset = i * BITS_PER_LONG;
 		kvm_mmu_write_protect_pt_masked(kvm, memslot, offset, mask);
@@ -3561,10 +3563,6 @@  int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log)
 	if (is_dirty)
 		kvm_flush_remote_tlbs(kvm);
 
-	r = -EFAULT;
-	if (copy_to_user(log->dirty_bitmap, dirty_bitmap_buffer, n))
-		goto out;
-
 	r = 0;
 out:
 	mutex_unlock(&kvm->slots_lock);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index bf040c4..c919f58 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -626,14 +626,10 @@  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.
- * See x86's kvm_vm_ioctl_get_dirty_log() why this is needed.
- */
 static int kvm_create_dirty_bitmap(struct kvm_memory_slot *memslot)
 {
 #ifndef CONFIG_S390
-	unsigned long dirty_bytes = 2 * kvm_dirty_bitmap_bytes(memslot);
+	unsigned long dirty_bytes = kvm_dirty_bitmap_bytes(memslot);
 
 	memslot->dirty_bitmap = kvm_kvzalloc(dirty_bytes);
 	if (!memslot->dirty_bitmap)