linux-kernel - Re: [PATCH 8/9] KVM, VMX: Add support for guest/host-only profiling

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Message-ID: <4E8AE9A0.2060706@redhat.com>
Date:	Tue, 04 Oct 2011 13:10:24 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	joerg.roedel@....com, mingo@...e.hu, a.p.zijlstra@...llo.nl
Subject: Re: [PATCH 8/9] KVM, VMX: Add support for guest/host-only profiling

On 10/04/2011 11:56 AM, Gleb Natapov wrote:
> On Tue, Oct 04, 2011 at 11:28:51AM +0200, Avi Kivity wrote:
> >  On 10/03/2011 05:36 PM, Gleb Natapov wrote:
> >  >On Mon, Oct 03, 2011 at 05:00:25PM +0200, Avi Kivity wrote:
> >  >>   On 10/03/2011 03:49 PM, Gleb Natapov wrote:
> >  >>   >Support guest/host-only profiling by switch perf msrs on
> >  >>   >a guest entry if needed.
> >  >>   >
> >  >>   >@@ -6052,6 +6056,26 @@ static void vmx_cancel_injection(struct kvm_vcpu *vcpu)
> >  >>   >    	vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, 0);
> >  >>   >    }
> >  >>   >
> >  >>   >+static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
> >  >>   >+{
> >  >>   >+#ifdef CONFIG_PERF_EVENTS
> >  >>
> >  >>   No need for #ifdef (if you also define perf_guest_get_msrs() when
> >  >>   !CONFIG_PERF_EVENTS).
> >  >>
> >  >Yes, but will compiler be smart enough to remove the code of the
> >  >function completely? It will have to figure that vmx->perf_msrs_cnt is
> >  >always 0 somehow.
> >
> >  It won't, but do we care?
> >
> Dead code, that likely to be inlined, on a hot path.

I mean, CONFIG_KVM && !CONFIG_PERF_EVENTS is an unlikely combination.  
If you're using kvm, you usually want PERF_EVENTS.

> >  >VMWRITE happens only when number of autoloaded MSRs changes (which is
> >  >rare), not on each call to add_atomic_switch_msr(). I thought about
> >  >optimizing this write too by doing
> >  >vmcs_write32(VM_(ENTRY|EXIT)_MSR_LOAD_COUNT, m->nr) only once by
> >  >checking that  m->nr changed during vmentry. Can be done later.
> >
> >  For EFER and PERF_CTRL, it's done unconditionally, no?
> For those yes. We do not cache their value currently. Can be added, but
> this is independent optimization.

Sure.  I note that on most cpus only these will ever be used.

-- 
error compiling committee.c: too many arguments to function

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