linux-kernel - Re: [PATCH] perf, x86: Add workaround for MEM_*

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Message-ID: <20130506190024.GB17723@dyad.programming.kicks-ass.net>
Date:	Mon, 6 May 2013 21:00:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Andi Kleen <andi@...stfloor.org>, "mingo@...e.hu" <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf, x86: Add workaround for MEM_*_RETIRED errata BV98

On Mon, May 06, 2013 at 07:41:54PM +0200, Stephane Eranian wrote:

> Problem is, those events are really important. We need to be able to
> measure them.

At what cost? If the code we need to make it work is horrendous we need to
maintain that too -- possibly quite a lot longer than Intel thinks IVB is
relevant.

Anyway; since these events are immediately dangerous (they corrupt state)
blocking them is the right thing; we can always merge a patch that undoes that
once we've figured out a sane way.

Currently I'm still waiting for someone to explain (in complete detail) how the
corruption manifests. Until that is clear I can't even begin formulating what
might be needed to make it work again.

One mode that makes sense to me is where CNTx on SMTi corrupts CNTx on SMTj for
i != j. However there's also been said that the only case that doesn't corrupt
is for SMTj to have all counters disabled -- which contradicts the previous
case.

The former case we could maybe make work; the latter I can't see how we could
possible make work at all.
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