Subject: Re: [PATCH 7/11] KVM: Remove redundant check in the kvm_get_dirty_log_protect()

From: Sean Christopherson <[email-masked]>
Date: Fri, 4 Jan 2019 13:27:02 -0800

On Fri, Jan 04, 2019 at 07:50:36AM -0800, Sean Christopherson wrote:
> On Fri, Jan 04, 2019 at 04:54:01PM +0800, [email-masked] wrote:
> > From: Lan Tianyu <[email-masked]>
> > 
> > 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 <[email-masked]>
> > ---
> >  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.

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