KVM: kvm_vm_ioctl_get_dirty_log restore "nothing dirty" optimization

[~andy/linux] / drivers / kvm / kvm_main.c

diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
index bfb1b6de05843beb95a80d31f0547b4d5bde89f1..5dee3024579dc828b30a129f9647fe5913aa36c8 100644 (file)
--- a/drivers/kvm/kvm_main.c
+++ b/drivers/kvm/kvm_main.c
@@ -803,11 +803,14 @@ static int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
        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;