On Fri, Jan 04, 2019 at 04:54:01PM +0800, lantianyu1986@xxxxxxxxx wrote:
> From: Lan Tianyu <Tianyu.Lan@xxxxxxxxxxxxx>
>
> 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 <Tianyu.Lan@xxxxxxxxxxxxx>
> ---
> 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.
Tangentially related, does mmu_lock actually need to be held while we
walk dirty_bitmap in kvm_{clear,get}_dirty_log_protect()? The bitmap
itself is protected by slots_lock (a lockdep assertion would be nice
too), e.g. can we grab the lock iff dirty_bitmap[i] != 0?
> }
> spin_unlock(&kvm->mmu_lock);
> }
> --
> 2.14.4
>
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