KVM: Eliminate extra function calls in kvm_get_dirty_log_protect()

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>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

virt/kvm/kvm_main.c

@@ -1053,9 +1053,11 @@ 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);