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

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Date:	Fri, 23 Mar 2012 22:51:53 +0100
From:	Davidlohr Bueso <dave@....org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Christian Borntraeger <borntraeger@...ibm.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 Mon, 2012-03-12 at 14:12 +0200, Avi Kivity wrote:
> 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.

Avi, will you be taking this patch? I don't see it applied or for pull
in 3.4.

Thanks.


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