linux-kernel - Re: [PATCH] perf/x86: check ucode before disabling PEBS on SandyBridge

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Date:	Fri, 15 Jun 2012 14:52:40 +0200
From:	Borislav Petkov <bp@...64.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Borislav Petkov <bp@...64.org>,
	LKML <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Dimitri Sivanich <sivanich@....com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Greg Kroah-Hartman <greg@...ah.com>
Subject: Re: [PATCH] perf/x86: check ucode before disabling PEBS on
 SandyBridge

On Fri, Jun 15, 2012 at 02:42:41PM +0200, Peter Zijlstra wrote:
> On Fri, 2012-06-15 at 14:37 +0200, Borislav Petkov wrote:
> > This is a minimal fix which we'd like to see in stable too thus the
> > more-or-less arbitrary decision to allow system-wide reloading only on
> > the BSP:
> > 
> > $ echo 1 > /sys/devices/system/cpu/cpu0/microcode/reload
> > ...
> > 
> > and disable the interface on the other cores:
> > 
> > $ echo 1 > /sys/devices/system/cpu/cpu23/microcode/reload
> > -bash: echo: write error: Invalid argument 
> 
> One could allow the reload from all cpus and simply do all cpus, but I
> guess the rationale for not doing that and restricting it to cpu0 is to
> avoid the O(n^2) thing in case userspace issues a reload on all cpus?

That's a good point.

Actually, the BSP thing was more-or-less an arbitrary deal. What we want as an
endresult is to do:

$ echo 1 > /sys/devices/system/cpu/microcode/reload

(notice this is in the toplevel "cpu" directory in sysfs)

and do the system-wide reload there.

Obviously, this needs a couple of more patches and we wanted to have a
simplest stable fix first.

> If so it would be good to mention in the Changelog so people see its not
> a 'random' choice.

Yes, will do, thanks!

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
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