From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: [PATCH v2] KVM: x86: add paging gcc optimization
Date: Thu, 08 Mar 2012 16:47:27 +0100
Message-ID: <4F58D48F.8050807@de.ibm.com>
References: <1331207154.28711.2.camel@offworld>
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Cc: Avi Kivity <avi@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>,
	KVM <kvm@vger.kernel.org>, lkml <linux-kernel@vger.kernel.org>
To: Davidlohr Bueso <dave@gnu.org>
Return-path: <kvm-owner@vger.kernel.org>
Received: from e06smtp16.uk.ibm.com ([195.75.94.112]:57061 "EHLO
	e06smtp16.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1757400Ab2CHPrf (ORCPT <rfc822;kvm@vger.kernel.org>);
	Thu, 8 Mar 2012 10:47:35 -0500
Received: from /spool/local
	by e06smtp16.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted
	for <kvm@vger.kernel.org> from <borntraeger@de.ibm.com>;
	Thu, 8 Mar 2012 15:47:33 -0000
In-Reply-To: <1331207154.28711.2.camel@offworld>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

On 08/03/12 12:45, Davidlohr Bueso wrote:
> From: Davidlohr Bueso <dave@gnu.org>
> 
> Since most guests will have paging enabled for memory management, add likely() optimization
> around CR0.PG checks.

>  {
> -	return kvm_read_cr0_bits(vcpu, X86_CR0_PG);
> +	return likely(kvm_read_cr0_bits(vcpu, X86_CR0_PG));


IMHO likely/unlikely should be considered more as fast-path/slow-path and not as often/less often.
Is that the case here? This patch  might cause a mis-prediction for non-paging guests all 
the time. 

Non-paging might be really irrelevant, so I am just making a point, since
likely/unlikely is mis-used too often especially for "most users do it that way".


Christian