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

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Date:	Wed, 13 Jun 2012 18:11:39 +0200
From:	Borislav Petkov <bp@...64.org>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	Borislav Petkov <bp@...64.org>,
	Peter Zijlstra <peterz@...radead.org>,
	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 Wed, Jun 13, 2012 at 09:36:49AM -0300, Henrique de Moraes Holschuh wrote:
> Yes, please. I suggest we use core 0 for that. Using my Debian
> maintainer hat, I'd rather you got rid of the sysfs entries for every
> other core while at it, as it will make our life a lot simpler,
> distro-side.

Wouldn't we have some sort of ABI breakage if I remove the sysfs files?
Instead, I was thinking of having the rest of the files not on the BSP
return -EINVAL and only the BSP reload ucode on the whole system.

> This is still not the proper fix, which would be to add a new sysfs node
> to access the proper update-every-core functionality, but it is a damn
> good start in the right direction and required to make it safe without
> ripping out the old ABI entirely without a deprecation period.

Yes, I'd like to have the system-wide sysfs node somewhere under
/sys/devices/system/cpu/microcode/ but we'll see how that goes.

> Since Intel processors don't want the per-core behaviour either, you
> could fix it in the microcode core itself...

Yes.

> Ok. Please CC me in the patches, if the new ABI arrives in time, I'll
> even be able to get it supported on the next Debian stable (and push
> to get this stuff backported to the kernel 3.2 which we will ship,
> I consider this an important bug-fix to a pontentially very serious
> issue. We have *zero* chance of finding out what's wrong if an users'
> system start getting subtly crazy because it is running with skewed
> microcode among cores.

Ok, will do.

Btw, I have to think about whether we really want to backport this to
stable since it is not a regression fix but functionality change which
kinda fixes a some sort of bug. Hmm, the stable rules are kinda blurry
here. I could write a minimal fix with stable in mind though... we'll
see.

hpa, what is our take here, should we backport a minimal change
disabling reloading of ucode per-cpu for stable? It is the wrong thing
to do anyway.

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
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