Re: [PATCH] perf/x86: check ucode before disabling PEBS onSandyBridge

[Borislav Petkov]

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.

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Boris.

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