2017-04-07 12:55+0200, Christian Borntraeger:
> On 04/06/2017 10:20 PM, Radim Krčmář wrote:
>> static inline bool kvm_check_request(int req, struct kvm_vcpu *vcpu)
>> {
>> - if (test_bit(req, &vcpu->requests)) {
>> - clear_bit(req, &vcpu->requests);
>> + if (kvm_test_request(req, vcpu)) {
>> + kvm_clear_request(req, vcpu);
>
> This looks fine. I am just asking myself why we do not use
> test_and_clear_bit? Do we expect gcc to merge all test bits as
> a fast path? This does not seem to work as far as I can tell and
> almost everybody does a fast path like in
test_and_clear_bit() is a slower operation even if the test is false (at
least on x86), because it needs to be fully atomic.
> arch/s390/kvm/kvm-s390.c:
> if (!vcpu->requests)
> return 0;
>
> arch/x86/kvm/x86.c:
> if (vcpu->requests) {
We'll mostly have only one request set, so splitting the test_and_clear
improves the performance of many subsequent tests_and_clear()s even if
the compiler doesn't optimize.
GCC couldn't even optimize if we used test_and_clear_bit(), because that
instruction adds barriers, but the forward check for vcpu->requests is
there because we do not trust the optimizer to do it for us and it would
make a big difference.
