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Subject: Re: [PATCH] perf/x86: check ucode before disabling PEBS on
 SandyBridge
From: Peter Zijlstra <peterz@infradead.org>
To: Borislav Petkov <bp@amd64.org>
Cc: Stephane Eranian <eranian@google.com>,
        Robert Richter <robert.richter@amd.com>,
        Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
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        Andreas Herrmann <andreas.herrmann3@amd.com>,
        Dimitri Sivanich <sivanich@sgi.com>,
        Dmitry Adamushko <dmitry.adamushko@gmail.com>
Date: Tue, 12 Jun 2012 19:26:36 +0200
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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.