linux-kernel - Re: KVM_GUEST support breaks page fault tracing

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Message-ID: <alpine.DEB.2.02.1405090024050.6261@ionos.tec.linutronix.de>
Date:	Fri, 9 May 2014 00:24:33 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dave Hansen <dave.hansen@...el.com>
cc:	x86@...nel.org, Peter Zijlstra <peterz@...radead.org>,
	Seiji Aguchi <seiji.aguchi@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Gleb Natapov <gleb@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: KVM_GUEST support breaks page fault tracing

On Thu, 8 May 2014, Dave Hansen wrote:

You forgot to cc Steven :)

> I noticed on some of my systems that page fault tracing doesn't work:
> 
> 	cd /sys/kernel/debug/tracing
> 	echo 1 > events/exceptions/enable
> 	cat trace;
> 	# nothing shows up
> 
> I eventually traced it down to CONFIG_KVM_GUEST.  At least in a KVM VM,
> enabling that option breaks page fault tracing, and disabling fixes it.
>  I tried on some old kernels and this does not appear to be a
> regression: it never worked.
> 
> Anybody have any theories about what is going on?
> 
> I'm also having problems with the page fault tracing on bare metal (same
> symptom of no trace output).  I'm unsure if it's related.
> 
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