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

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Date:   Fri, 7 Apr 2017 12:55:15 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     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

On 04/06/2017 10:20 PM, Radim Krčmář wrote:
> Users were expected to use kvm_check_request() for testing and clearing,
> but request have expanded their use since then and some users want to
> only test or do a faster clear.
> 
> Make sure that requests are not directly accessed with bit operations, because
> we'll be clearing them later.
> 
> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>

Patch itself looks sane
Reviewed-by: Christian Borntraeger <borntraeger@...ibm.com>


one question:
>  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



arch/s390/kvm/kvm-s390.c:
        if (!vcpu->requests)
                return 0;

arch/x86/kvm/x86.c:
    if (vcpu->requests) {

> 
>  		/*
>  		 * Ensure the rest of the request is visible to kvm_check_request's
>