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

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Date:	Mon, 12 Mar 2012 14:12:59 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
CC:	Davidlohr Bueso <dave@....org>,
	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 03/08/2012 05:47 PM, Christian Borntraeger wrote:
> 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.

Agree.

> 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".

In fact this is a classic example.  Almost no guests use real mode (the
last guests to use real mode extensively was DOS; I think Win9x switches
to real mode pretty often).  As it's a user-controlled setting, we're
penalizing users who do things differently.

However the majority if is_paging() == true guests is so huge, and since
non-paging guests don't really expect 2012 performance levels anyway
(being so old) that I think in practice this is a good optimization here.

-- 
error compiling committee.c: too many arguments to function

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