linux-kernel - Re: [PATCH V4 1/2] perf ignore LBR and extra

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Date:	Thu, 10 Jul 2014 11:02:04 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Liang, Kan" <kan.liang@...el.com>
Cc:	"andi@...stfloor.org" <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH V4 1/2] perf ignore LBR and extra_regs.

/me reminds you of 78 char text wrap.

On Wed, Jul 09, 2014 at 07:32:09PM +0000, Liang, Kan wrote:
> > Sure; but what I meant was, check_msr() is broken when ran on such a
> > kernel. You need to fix check_msr() to return failure on these 'ignored'
> > MSRs, after all they don't function as expected, they're effectively broken.
> 
> The function is designed to check if the MSRs can be safely accessed
> (no #GP). It cannot guarantee the correctness of the MSRs.  If KVM
> applied patch 2 and guest applied patch 1, from the guest's
> perspective, the MSRs can be accessed (no #GP triggered). So return
> true is expected. It should not be a broken. 

You're not understanding. I know you wrote that function to do that. I'm
saying that's wrong.

Look at check_hw_exists() it explicitly checks for fake MSRs and reports
them broken.

These fake MSRs _ARE_ broken, they do not behave as expected. Not
crashing is not the right consideration here, we're interested in higher
order correct behaviour.

> The only unexpected
> thing for guest is that the counting/sampling result for LBR/extra reg
> is always 0. But the patch is a short term fix to stop things from
> crashing. I think it should be acceptable.

Patch 2 is fine, patch 1, in particular your check_msr() routine is not.
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