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

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Message-ID: <532CB54C.8020003@zytor.com>
Date:	Fri, 21 Mar 2014 14:55:24 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>
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 02:48 PM, Andi Kleen wrote:
> "H. Peter Anvin" <hpa@...or.com> writes:
>>
>> 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.
> 
> That means we could just throw rdmsr() away and it would be completely
> replaced with rdmsr_safe(). But then that will likely cause all kinds
> of problems with how to handle these errors and where and how to handle
> these exceptions.
> 
> I much prefer just to fix KVM. I cannot think of any case
> where 0 would cause a major issue.
> 
> After all it's virtualization not "rewrite complete kernel for it"
> 

Actually, Ingo, Borislav and I have been discussing making rdmsr_safe()
more of the default, especially for things like this where the error
handling is obvious (doesn't work?  Disable the PMU.)

	-hpa


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