[PATCH] kvm_vm_ioctl_get_dirty_log restore "nothing dirty" optimization

[Rusty Russell <rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>]

kvm_vm_ioctl_get_dirty_log scans bitmap to see it it's all zero, but
doesn't use that information.

Avi says:
	Looks like it was used to guard	kvm_mmu_slot_remove_write_access();
	optimizing the case where the guest just leaves the screen alone (which
	it usually does, especially in benchmarks).

	I'd rather reinstate that optimization.  See
	66d8a4e4d4bd470216028daabb9d887b73259c96 where the damage was done.

It's pretty simple: if the bitmap is all zero, we don't need to do anything to
clean it.

Signed-off-by: Rusty Russell <rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>

diff -r 37f0dd1eaa0d drivers/kvm/kvm_main.c
--- a/drivers/kvm/kvm_main.c	Tue Jul 31 19:30:22 2007 +1000
+++ b/drivers/kvm/kvm_main.c	Tue Jul 31 19:34:26 2007 +1000
@@ -803,11 +803,14 @@ static int kvm_vm_ioctl_get_dirty_log(st
 	if (copy_to_user(log->dirty_bitmap, memslot->dirty_bitmap, n))
 		goto out;
 
-	mutex_lock(&kvm->lock);
-	kvm_mmu_slot_remove_write_access(kvm, log->slot);
-	kvm_flush_remote_tlbs(kvm);
-	memset(memslot->dirty_bitmap, 0, n);
-	mutex_unlock(&kvm->lock);
+	/* If nothing is dirty, don't bother messing with page tables. */
+	if (any) {
+		mutex_lock(&kvm->lock);
+		kvm_mmu_slot_remove_write_access(kvm, log->slot);
+		kvm_flush_remote_tlbs(kvm);
+		memset(memslot->dirty_bitmap, 0, n);
+		mutex_unlock(&kvm->lock);
+	}
 
 	r = 0;
 



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