From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 1/5] KVM: x86: avoid atomic operations on APICv vmentry
Date: Fri, 14 Oct 2016 14:56:38 -0400 (EDT)
Message-ID: <770436977.3704467.1476471398385.JavaMail.zimbra@redhat.com>
References: <1476469291-5039-1-git-send-email-pbonzini@redhat.com> <1476469291-5039-2-git-send-email-pbonzini@redhat.com> <4B0C832B-BB75-40BD-85A9-9DC84DEB44E2@gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Cc: LKML <linux-kernel@vger.kernel.org>, KVM <kvm@vger.kernel.org>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Yang Zhang <yang.zhang.wz@gmail.com>,
        feng wu <feng.wu@intel.com>, mst@redhat.com
To: Nadav Amit <nadav.amit@gmail.com>
Return-path: <kvm-owner@vger.kernel.org>
Received: from mx3-phx2.redhat.com ([209.132.183.24]:58956 "EHLO
        mx3-phx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751532AbcJNS4k (ORCPT <rfc822;kvm@vger.kernel.org>);
        Fri, 14 Oct 2016 14:56:40 -0400
In-Reply-To: <4B0C832B-BB75-40BD-85A9-9DC84DEB44E2@gmail.com>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 23b99f305382..63a442aefc12 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -342,9 +342,11 @@ void __kvm_apic_update_irr(u32 *pir, void *regs)
> > 	u32 i, pir_val;
> > 
> > 	for (i = 0; i <= 7; i++) {
> > -		pir_val = xchg(&pir[i], 0);
> > -		if (pir_val)
> > +		pir_val = READ_ONCE(pir[i]);
> 
> Out of curiosity, do you really need this READ_ONCE?

The answer can only be "depends on the compiler's whims". :)
If you think of READ_ONCE as a C11 relaxed atomic load, then yes.

Paolo