linux-kernel - Re: [PATCH 2/3] x86 perf: Protect LBR msrs accessing against potential #GP

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Date:	Fri, 1 Aug 2014 09:44:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	kan.liang@...el.com
Cc:	andi@...stfloor.org, alexander.shishkin@...ux.intel.com,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/3] x86 perf: Protect LBR msrs accessing against
 potential #GP

On Fri, Aug 01, 2014 at 09:38:25AM +0200, Peter Zijlstra wrote:
> On Thu, Jul 31, 2014 at 02:41:02AM -0700, kan.liang@...el.com wrote:
> > From: Kan Liang <kan.liang@...el.com>
> > 
> > Intel PT will take over LBR hardware. If RTIT_CTL.TraceEn=1, any attempt to
> > read or write the LBR or LER MSRs, including LBR_TOS, will result in a #GP.
> > Intel PT can be enabled/disabled at runtime by hardware/BIOS, so it's better
> > LBR MSRs can be protected at runtime.
> > 
> > The {rd,wr}msrl_goto can protect LBR accessing against the potential #GP.
> > Furthermore, it will not impact the "fast" path's performance.
> 
> NAK!
> 
> I already said this isn't going to ever happen.
> 
> Both PT and LBR are arbitrated through the kernel, therefore we can (and
> must) deny PT when there's existing LBR usage and vice versa.
> 
> We will not hijack resources like this full stop end of story.
> 
> Fuck hardware/BIOS, they should _NOT_ be touching this.
> 
> The 3 people in the world with access to an x86 hardware debugger had
> better be competent and know WTF they're doing and the BIOS can just
> piss off right now, they should not be touching this _EVER_.

I yelled at BIOS engineers over their PMU usage and $vendor added a BIOS
knob to disable that, I'll yell at BIOS engineers again, just give me
their number.

Really, say NO already.

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