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

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Date:	Fri, 15 Jun 2012 14:42:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Borislav Petkov <bp@...64.org>
Cc:	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, 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?

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