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@xxxxxxx>
> > >
> > > 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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