From 39214915f50f6ac2350355f2db63910d968fa790 Mon Sep 17 00:00:00 2001 From: Rusty Russell Date: Tue, 31 Jul 2007 19:57:47 +1000 Subject: [PATCH] KVM: kvm_vm_ioctl_get_dirty_log restore "nothing dirty" optimization 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 90cb0529dd230548a7f0d6b315997be854caea1b 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 Signed-off-by: Avi Kivity --- drivers/kvm/kvm_main.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c index bfb1b6de0584..5dee3024579d 100644 --- 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; -- 2.20.1