On Fri, Jan 04, 2019 at 07:50:36AM -0800, Sean Christopherson wrote:

On Fri, Jan 04, 2019 at 04:54:01PM +0800, lantianyu1986@gmail.com wrote:

quoted

From: Lan Tianyu <redacted>

The dirty bits have already been checked in the previous check of
"dirty_bitmap" and mask must be non-zero value at this point.

Signed-off-by: Lan Tianyu <redacted>
---
 virt/kvm/kvm_main.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index cf7cc0554094..e75dbb15fd09 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c

@@ -1206,11 +1206,9 @@ int kvm_get_dirty_log_protect(struct kvm *kvm,
 			mask = xchg(&dirty_bitmap[i], 0);
 			dirty_bitmap_buffer[i] = mask;
 
-			if (mask) {
-				offset = i * BITS_PER_LONG;
-				kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot,
-									offset, mask);
-			}
+			offset = i * BITS_PER_LONG;
+			kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot,
+								offset, mask);

Hmm, the check against mask was explicitly added by commit 58d2930f4ee3
("KVM: Eliminate extra function calls in kvm_get_dirty_log_protect()").
AFAIK KVM only *sets* bits in dirty_bitmap without holding slots_lock
and/or mmu_lock, so I agree that checking mask is redundant, but it'd be
nice to elaborate a bit more in the changelog.

At the very least this needs a Fixes tag for the aforementioned commit.

Actually, this can be a straight revert of 58d2930f4ee3.