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From: linmiaohe <linmiaohe@huawei.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>,
	"rkrcmar@redhat.com" <rkrcmar@redhat.com>,
	"wanpengli@tencent.com" <wanpengli@tencent.com>,
	"jmattson@google.com" <jmattson@google.com>,
	"joro@8bytes.org" <joro@8bytes.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"bp@alien8.de" <bp@alien8.de>, "hpa@zytor.com" <hpa@zytor.com>
Subject: Re: [PATCH] KVM: apic: reuse smp_wmb() in kvm_make_request()
Date: Fri, 7 Feb 2020 02:59:14 +0000	[thread overview]
Message-ID: <0f8ae71f59cd4b80ac93b9e1aa3b2428@huawei.com> (raw)

Sean Christopherson <sean.j.christopherson@intel.com> wrote:
> 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.
>> > +			/*
>> > +			 * 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
>> :-)

Thanks for your alternative wording. It looks much better.

>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).

Yeh, I think the comment should be dropped too. :)

Thanks to both for review! I would send v2.

             reply	other threads:[~2020-02-07  2:59 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-07  2:59 linmiaohe [this message]
  -- strict thread matches above, loose matches on Subject: below --
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

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