From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: linmiaohe <linmiaohe@huawei.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
x86@kernel.org, pbonzini@redhat.com, rkrcmar@redhat.com,
wanpengli@tencent.com, jmattson@google.com, joro@8bytes.org,
tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
hpa@zytor.com
Subject: Re: [PATCH] KVM: apic: reuse smp_wmb() in kvm_make_request()
Date: Thu, 6 Feb 2020 07:05:23 -0800 [thread overview]
Message-ID: <20200206150523.GA13067@linux.intel.com> (raw)
In-Reply-To: <87d0asgfh5.fsf@vitty.brq.redhat.com>
On Thu, Feb 06, 2020 at 11:47:02AM +0100, Vitaly Kuznetsov wrote:
> linmiaohe <linmiaohe@huawei.com> writes:
>
> > From: Miaohe Lin <linmiaohe@huawei.com>
> >
> > There is already an smp_mb() barrier in kvm_make_request(). We reuse it
> > here.
> >
> > Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> > ---
> > arch/x86/kvm/lapic.c | 9 ++++++---
> > 1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index eafc631d305c..ea871206a370 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -1080,9 +1080,12 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
> > result = 1;
> > /* assumes that there are only KVM_APIC_INIT/SIPI */
> > apic->pending_events = (1UL << KVM_APIC_INIT);
> > - /* make sure pending_events is visible before sending
> > - * the request */
> > - smp_wmb();
> > + /*
> > + * Make sure pending_events is visible before sending
> > + * the request.
> > + * There is already an smp_wmb() in kvm_make_request(),
> > + * we reuse that barrier here.
> > + */
>
> Let me suggest an alternative wording,
>
> "kvm_make_request() provides smp_wmb() so pending_events changes are
> guaranteed to be visible"
>
> But there is nothing wrong with yours, it's just longer than it could be
> :-)
I usually lean in favor of more comments, but in thise case I'd vote to
drop the comment altogether. There are lots of places that rely on the
smp_wmb() in kvm_make_request() without a comment, e.g. the cases for
APIC_DM_STARTUP and APIC_DM_REMRD in this same switch, kvm_inject_nmi(),
etc... One might wonder what makes INIT special.
And on the flip side, APIC_DM_STARTUP is a good example of when a
smp_wmb()/smp_rmb() is needed and commented correctly (though calling out
the exactly location of the other half would be helpful).
> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>
> > kvm_make_request(KVM_REQ_EVENT, vcpu);
> > kvm_vcpu_kick(vcpu);
> > }
>
> --
> Vitaly
>
next prev parent reply other threads:[~2020-02-06 15:05 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-06 1:59 [PATCH] KVM: apic: reuse smp_wmb() in kvm_make_request() linmiaohe
2020-02-06 10:47 ` Vitaly Kuznetsov
2020-02-06 15:05 ` Sean Christopherson [this message]
2020-02-07 2:59 linmiaohe
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