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Subject: Re: perf hw  in kexeced kernel broken in tip
From: Peter Zijlstra <peterz@infradead.org>
To: Yinghai Lu <yinghai@kernel.org>
Cc: Don Zickus <dzickus@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Vivek Goyal <vgoyal@redhat.com>, Ingo Molnar <mingo@elte.hu>,
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Date: Wed, 08 Dec 2010 20:17:35 +0100
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On Wed, 2010-12-08 at 11:05 -0800, Yinghai Lu wrote:
> On 12/08/2010 11:01 AM, Don Zickus wrote:
> > On Wed, Dec 08, 2010 at 10:43:14AM -0800, Yinghai Lu wrote:
> >>
> >> can you add sth force_... in command line to take over ownership of perf from BIOS or previous kernel ?
> >>
> >> then still can use perf etc after we kexec from RHEL or SLES kernel to later kernel ( from 2.6.37)
> > 
> > My understand is that you can't because the BIOS is actively using it
> > behind the scenes of the kernel (well during an SMI).  I have a machine
> > where I tried to force take it but it still stopped triggering interrupts.
> 
> how about second case: kexec from RHEL 6 stock kernel to upstream kernel ?

Its impossible to distinguish between a BIOS having claimed a counter
and a previous kernel not having shut things down properly.

The best we can do is allow a force parameter and let the user keep all
pieces when he uses it.