linux-kernel - Re: [PATCH v2] KVM: x86: add paging gcc optimization

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Date:	Thu, 08 Mar 2012 16:47:27 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Davidlohr Bueso <dave@....org>
CC:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] KVM: x86: add paging gcc optimization

On 08/03/12 12:45, Davidlohr Bueso wrote:
> From: Davidlohr Bueso <dave@....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

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