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List:       kvm
Subject:    Re: [PATCH] call vmx_load_host_state() only if msr is cached
From:       Gleb Natapov <gleb () redhat ! com>
Date:       2009-08-31 14:09:26
Message-ID: 20090831140926.GW30093 () redhat ! com
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On Mon, Aug 31, 2009 at 11:04:27AM -0300, Marcelo Tosatti wrote:
> On Thu, Aug 27, 2009 at 03:07:30PM +0300, Gleb Natapov wrote:
> > No need to call it before each kvm_(set|get)_msr_common()
> > 
> > Signed-off-by: Gleb Natapov <gleb@redhat.com>
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > index 6b57eed..7494ef1 100644
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -1005,9 +1005,9 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 *pdata)
> >  		data = vmcs_readl(GUEST_SYSENTER_ESP);
> >  		break;
> >  	default:
> > -		vmx_load_host_state(to_vmx(vcpu));
> >  		msr = find_msr_entry(to_vmx(vcpu), msr_index);
> >  		if (msr) {
> > +			vmx_load_host_state(to_vmx(vcpu));
> >  			data = msr->data;
> >  			break;
> >  		}
> > @@ -1064,9 +1064,9 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 data)
> >  		}
> >  		/* Otherwise falls through to kvm_set_msr_common */
> >  	default:
> > -		vmx_load_host_state(vmx);
> >  		msr = find_msr_entry(vmx, msr_index);
> >  		if (msr) {
> > +			vmx_load_host_state(vmx);
> 
> All is needed here save_msrs(vmx->guest_msrs), right?
> 
Yes. Otherwise we return wrong value for cached MSR.

> Looks fine to me. Do you have any numbers or just notice it out of code
> review?
We saw this while profiling IPI ping-pong with x2apic (which uses MSR).
This patch improves numbers by ~5us IIRC.

--
			Gleb.
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