linux-kernel - Re: perf hw in kexeced kernel broken in tip

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Date:	Thu, 9 Dec 2010 15:20:08 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>, gorcunov@...il.com
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Jason Wessel <jason.wessel@...driver.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Haren Myneni <hbabu@...ibm.com>
Subject: Re: perf hw  in kexeced kernel broken in tip

On Wed, Dec 08, 2010 at 03:59:16PM +0100, Peter Zijlstra wrote:
> On Wed, 2010-12-08 at 15:20 +0100, Peter Zijlstra wrote:
> 
> > > I wonder if you should reverse these checks.  If the bios has the perf
> > > counter enabled, there might be a high chance that it fails the first
> > > check and never gets to the actually bios checks.
> > 
> > Ah, good point. 
> 
> Something like so..

Getting closer...

Pentium4s are special they need the double write, so...

> +	/*
> +	 * Now write a value and read it back to see if it matches,
> +	 * this is needed to detect certain hardware emulators (qemu/kvm)
> +	 * that don't trap on the MSR access and always return 0s.
> +	 */
>  	val = 0xabcdUL;
> -	ret |= checking_wrmsrl(x86_pmu.perfctr, val);
> +	ret = checking_wrmsrl(x86_pmu.perfctr, val);

	if (x86_pmu.perfctr_second_write)
		ret |= checking_wrmsrl(x86_pmu.perfctr, val);


solved my p4 problems for kexec.

>  	ret |= rdmsrl_safe(x86_pmu.perfctr, &val_new);
>  	if (ret || val != val_new)
> -		return false;
> +		goto msr_fail;
>  
>  	return true;

Cheers,
Don
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