linux-kernel - Re: GPF in intel_pmu_lbr_reset() with qemu -cpu host

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Message-ID: <532CB21B.4040502@zytor.com>
Date:	Fri, 21 Mar 2014 14:41:47 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <ak@...ux.intel.com>
CC:	Peter Wu <peter@...ensteyn.nl>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: GPF in intel_pmu_lbr_reset() with qemu -cpu host

On 03/21/2014 02:37 PM, Andi Kleen wrote:
> On Fri, Mar 21, 2014 at 01:46:04PM -0700, H. Peter Anvin wrote:
>> Not really.  That is equally braindamaged.  The problem is that KVM is telling the host that our is something it simply cannot be.
> 
> Well it has to pick something. It's unlikely it will ever implement 100% of that particular CPU.
> 0 is the best you can get in many cases.
> 
> Also I thought Xen did return 0.
> 

That's why at least to some extent The Right Thing is not to try to
pretend to be a CPU you don't even know how to emulate.

But again, that has its own issues, too, mostly with userspace
optimization, and making the Linux code more resilient wouldn't hurt.
In that sense #GP(0) is *much* better than 0: it unambiguously gives an
error to work with.

However, labeling it a bug in Linux is silly.

	-hpa

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