* [PATCH v2] KVM: x86: add paging gcc optimization
@ 2012-03-08 11:45 Davidlohr Bueso
2012-03-08 15:47 ` Christian Borntraeger
0 siblings, 1 reply; 6+ messages in thread
From: Davidlohr Bueso @ 2012-03-08 11:45 UTC (permalink / raw)
To: Avi Kivity, Marcelo Tosatti; +Cc: KVM, lkml
From: Davidlohr Bueso <dave@gnu.org>
Since most guests will have paging enabled for memory management, add likely() optimization
around CR0.PG checks.
Signed-off-by: Davidlohr Bueso <dave@gnu.org>
---
arch/x86/kvm/x86.h | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index cb80c29..3d1134d 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -64,7 +64,7 @@ static inline int is_pse(struct kvm_vcpu *vcpu)
static inline int is_paging(struct kvm_vcpu *vcpu)
{
- return kvm_read_cr0_bits(vcpu, X86_CR0_PG);
+ return likely(kvm_read_cr0_bits(vcpu, X86_CR0_PG));
}
static inline u32 bit(int bitno)
--
1.7.8.3
^ permalink raw reply related [flat|nested] 6+ messages in thread* Re: [PATCH v2] KVM: x86: add paging gcc optimization
2012-03-08 11:45 [PATCH v2] KVM: x86: add paging gcc optimization Davidlohr Bueso
@ 2012-03-08 15:47 ` Christian Borntraeger
2012-03-08 21:27 ` Davidlohr Bueso
2012-03-12 12:12 ` Avi Kivity
0 siblings, 2 replies; 6+ messages in thread
From: Christian Borntraeger @ 2012-03-08 15:47 UTC (permalink / raw)
To: Davidlohr Bueso; +Cc: Avi Kivity, Marcelo Tosatti, KVM, lkml
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.
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
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH v2] KVM: x86: add paging gcc optimization
2012-03-08 15:47 ` Christian Borntraeger
@ 2012-03-08 21:27 ` Davidlohr Bueso
2012-03-12 12:12 ` Avi Kivity
1 sibling, 0 replies; 6+ messages in thread
From: Davidlohr Bueso @ 2012-03-08 21:27 UTC (permalink / raw)
To: Christian Borntraeger; +Cc: Avi Kivity, Marcelo Tosatti, KVM, lkml
On Thu, 2012-03-08 at 16:47 +0100, 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.
> Is that the case here? This patch might cause a mis-prediction for non-paging guests all
> the time.
Branch predictions are all about probability of occurrence, and I cannot
imagine guests having paging disabled and mapping gva->gpa directly, so
for most of the cases it would be beneficial. For those peculiar users
that actually don't use paging, then yes, mispredictions would occur.
- Davidlohr
>
> 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
>
>
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH v2] KVM: x86: add paging gcc optimization
2012-03-08 15:47 ` Christian Borntraeger
2012-03-08 21:27 ` Davidlohr Bueso
@ 2012-03-12 12:12 ` Avi Kivity
2012-03-23 21:51 ` Davidlohr Bueso
1 sibling, 1 reply; 6+ messages in thread
From: Avi Kivity @ 2012-03-12 12:12 UTC (permalink / raw)
To: Christian Borntraeger; +Cc: Davidlohr Bueso, Marcelo Tosatti, KVM, lkml
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
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH v2] KVM: x86: add paging gcc optimization
2012-03-12 12:12 ` Avi Kivity
@ 2012-03-23 21:51 ` Davidlohr Bueso
2012-03-28 16:36 ` Avi Kivity
0 siblings, 1 reply; 6+ messages in thread
From: Davidlohr Bueso @ 2012-03-23 21:51 UTC (permalink / raw)
To: Avi Kivity; +Cc: Christian Borntraeger, Marcelo Tosatti, KVM, lkml
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@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.
Avi, will you be taking this patch? I don't see it applied or for pull
in 3.4.
Thanks.
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2012-03-28 16:36 UTC | newest]
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2012-03-08 11:45 [PATCH v2] KVM: x86: add paging gcc optimization Davidlohr Bueso
2012-03-08 15:47 ` Christian Borntraeger
2012-03-08 21:27 ` Davidlohr Bueso
2012-03-12 12:12 ` Avi Kivity
2012-03-23 21:51 ` Davidlohr Bueso
2012-03-28 16:36 ` Avi Kivity
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