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

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Date:	Tue, 4 Oct 2011 11:56:54 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Avi Kivity <avi@...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 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.

> >>  >
> >>  >+
> >>  >+	perf_guest_get_msrs(vmx->perf_msrs_cnt, vmx->perf_msrs);
> >>  >+	for (i = 0; i<   vmx->perf_msrs_cnt; i++) {
> >>  >+		struct perf_guest_switch_msr *msr =&vmx->perf_msrs[i];
> >>  >+		if (msr->host == msr->guest)
> >>  >+			clear_atomic_switch_msr(vmx, msr->msr);
> >>  >+		else
> >>  >+			add_atomic_switch_msr(vmx, msr->msr, msr->guest,
> >>  >+					msr->host);
> >>
> >>  This generates a lot of VMWRITEs even if nothing changes, just to
> >>  re-set bits in the VMCS to their existing values.  Need to add
> >>  something like this:
> >>
> >>     if (loaded_vmcs->msr[i].host == msr->host
> >>  &&  loaded_vmcs->msr[i].guest == msr->guest)
> >>            continue;
> >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. 

> 
> >>
> >>  btw, shouldn't the msr autoload list be part of loaded_vmcs as well?
> >>
> >Why?
> 
> Any caching is only relative to the vmcs (unless we invalidate the
> cache on vmcs switch).
Ah, you are talking about not yet existent cache. Then I see why it
should be in loaded_vmcs for EFER and PERF_CTRL.

> 
> >>
> >>  Do we really need a private buffer?  Perhaps perf_guest_get_msrs()
> >>  can return a perf-internal buffer (but then, we will need to copy it
> >>  for the optimization above, but that's a separate issue).
> >>
> >The buffer will be small, so IMHO private one is not an issue. We can
> >make it perf internal per cpu buffer I think.
> >
> 
> I think the API is nicer with perf returning a read-only internal
> buffer; this way there is no kmalloc involved since perf knows its
> internal limits.
> 
Yeah, I am trying it now it it looks nicer.

--
			Gleb.
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