Re: [PATCH] KVM: Eliminate extra function calls in kvm_get_dirty_log_protect()

Paolo Bonzini
2015-03-17T08:58:21Z

On 17/03/2015 08:19, Takuya Yoshikawa wrote:
> When all bits in mask are not set,
> kvm_arch_mmu_enable_log_dirty_pt_masked() has nothing to do. But since
> it needs to be called from the generic code, it cannot be inlined, and
> a few function calls, two when PML is enabled, are wasted.
>
> Since it is common to see many pages remain clean, e.g. framebuffers can
> stay calm for a long time, it is worth eliminating this overhead.
>
> Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
> ---
> virt/kvm/kvm_main.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index a109370..420d8cf 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1061,9 +1061,11 @@ int kvm_get_dirty_log_protect(struct kvm *kvm,
> mask = xchg(&dirty_bitmap[i], 0);
> dirty_bitmap_buffer[i] = mask;
>
> - offset = i * BITS_PER_LONG;
> - kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot, offset,
> - mask);
> + if (mask) {
> + offset = i * BITS_PER_LONG;
> + kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot,
> + offset, mask);
> + }
> }
>
> spin_unlock(&kvm->mmu_lock);
>

Good catch!

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>