Messages in this thread | | | Date | Mon, 12 Mar 2012 14:12:59 +0200 | From | Avi Kivity <> | Subject | Re: [PATCH v2] KVM: x86: add paging gcc optimization |
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On 03/08/2012 05:47 PM, Christian Borntraeger wrote: > 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.
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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