linux-kernel - Re: [RFC] perf: Clear MSRs on kexec

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Date:	Tue, 4 Aug 2015 09:46:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
	Ingo Molnar <mingo@...nel.org>, dong.su@...csson.com,
	Vivek Goyal <vgoyal@...hat.com>,
	Haren Myneni <hbabu@...ibm.com>,
	Don Zickus <dzickus@...hat.com>
Subject: Re: [RFC] perf: Clear MSRs on kexec

On Tue, Aug 04, 2015 at 07:52:29AM +0200, Jiri Olsa wrote:
> On Mon, Aug 03, 2015 at 11:54:17PM +0200, Peter Zijlstra wrote:
> > On Mon, Aug 03, 2015 at 11:32:28PM +0200, Jiri Olsa wrote:
> > > hi,
> > > I'm getting following message on the kdump kernel start
> > > 
> > >   Broken BIOS detected, complain to your hardware vendor.\
> > >   [Firmware Bug]: the BIOS has corrupted hw-PMU resources (MSR 38d is b0)
> > > 
> > > it seems to be caused by NMI watchdog being configured
> > > and fixed counter values stays in MSRs, which triggers
> > > warning in check_hw_exists and disables perf support
> > > in kdump kernel.. which probably does not hurt ;-)
> > > 
> > > zeroing MSRs during kdump shutdown seems to work (attached)
> > > but I'm not sure thats correct place for kdump perf callback
> > 
> > Right, but why bother? All that kernel needs to do is write a memory
> > dump to someplace and reboot, right? The less you do, the less can go
> > wrong.
> 
> well, I was hunting that 'Broken BIOS..' message which is wrong

Not really. The previous kernel being the BIOS in this case did
leave the counters in a funky state.

> I wouldn't think anyone wants to use perf under kdump kernel,
> but you never know ;-)

Yeah, I think we knew about this back then (might've been 2010) and
chose to not 'fix' it.

http://lkml.iu.edu/hypermail/linux/kernel/1012.1/00380.html

Is what google finds me.
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