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

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Message-ID: <1339521996.31548.95.camel@twins>
Date:	Tue, 12 Jun 2012 19:26:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Borislav Petkov <bp@...64.org>
Cc:	Stephane Eranian <eranian@...gle.com>,
	Robert Richter <robert.richter@....com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	andi@...stfloor.org, mingo@...e.hu, ming.m.lin@...el.com,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Dimitri Sivanich <sivanich@....com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: Re: [PATCH] perf/x86: check ucode before disabling PEBS on
 SandyBridge

On Tue, 2012-06-12 at 19:23 +0200, Borislav Petkov wrote:
> On Tue, Jun 12, 2012 at 07:18:13PM +0200, Peter Zijlstra wrote:
> > But the ucode loader can be (ab)used to only update one of the many
> > CPUs.
> 
> Not if you don't support mixed ucode revisions. In that case, you want
> the same ucode patch applied to _all_ cores on the system. Otherwise,
> random freezes, explosions and other cataclysmic events can happen.

You want, yes, but afaict the stuff in
arch/x86/kernel/microcode_core.c:reload_store means you can force a
single cpu to update while leaving the others alone.

Afaict this is not a sane thing to do, but quite possible.
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