KVM: Remove test-before-set optimization for dirty bits

As Avi pointed out, testing bit part in mark_page_dirty() was important
in the days of shadow paging, but currently EPT and NPT has already become
common and the chance of faulting a page more that once per iteration is
small. So let's remove the test bit to avoid extra access.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 6dc9404..9ab1a77 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1192,9 +1192,7 @@ void mark_page_dirty(struct kvm *kvm, gfn_t gfn)
        if (memslot && memslot->dirty_bitmap) {
                unsigned long rel_gfn = gfn - memslot->base_gfn;
 
-               /* avoid RMW */
-               if (!generic_test_le_bit(rel_gfn, memslot->dirty_bitmap))
-                       generic___set_le_bit(rel_gfn, memslot->dirty_bitmap);
+               generic___set_le_bit(rel_gfn, memslot->dirty_bitmap);
        }
 }