linux-kernel - Re: [PATCH 2/6] KVM: use kvm_{test,clear}_request instead of {test,clear}

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Message-ID: <20170407122413.GC23559@potion>
Date:   Fri, 7 Apr 2017 14:24:14 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Christoffer Dall <cdall@...aro.org>,
        Andrew Jones <drjones@...hat.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Cornelia Huck <cornelia.huck@...ibm.com>,
        James Hogan <james.hogan@...tec.com>,
        Paul Mackerras <paulus@...abs.org>
Subject: Re: [PATCH 2/6] KVM: use kvm_{test,clear}_request instead of
 {test,clear}_bit

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.