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

Borislav Petkov
2012-06-13T06:51:02Z

On Tue, Jun 12, 2012 at 10:04:13PM -0300, Henrique de Moraes Holschuh wrote:
> On Tue, 12 Jun 2012, Borislav Petkov wrote:
> > I'd guess this is still there to support mixed ucode revisions for some
>
> It is still there because of the stable ABI rules. As far as I can tell,
> it was a crap interface when it got in, and it still is a crap interface
> now because there simply isn't any sane usercase that requires it, it is
> dangerous as implemented right now (at least in the Intel case), and even
> if we fixed the kernel to do the right thing, userspace would not be able
> to know that and would still need to request 1 microcode refresh per core.

Well, I'm disabling it on AMD.

We could make it iterate over _all_ cores and do the update on each one
of them even though the user does a sysfs write only for a single core..

> As far as I know, we always want to refresh the microcode on every core,
> use the firmware interface to pick up a copy of the newest version of the
> microcode matching the signature of each core, and leave no core behind
> without an update.

Yep.

> And preferably, we want to request_firmware() only once per microcode,
> which is rather easy to do: cache every microcode that will be needed,
> check cache first before doing request_firmware() in the per-core
> worker threads, and invalidate the cache when the user requests
> "refresh_all_microcode". So, the cache speeds up multi-core updates,
> and is also usable when restoring the system from suspend/hibernation,
> but doesn't get in the way of userspace trying to apply a microcode
> update.

Yes, agreed too.

> This would make my pathetic system do one request_firmware instead of
> eight. And even the old junk with mixed-stepping SMP at work would
> only require two, instead of four (or eight? I don't recall how many
> cores per die it has) request_firmware calls.

Ok.

> > described above. Maybe this interface should be behind a family, model
> > check or so, so that users don't shoot themselves in the foot but it is
> > root-only anyway.
>
> This interface should _DIE_.

Yeppers! :-)

> Perhaps we could make it work only for CPU 0 and return EBUSY or whatever
> for all others (or just don't publish the sysfs attribute for the others),
> and change the CPU0 refresh firmware request into a refresh all cores call.

That could also work, it sounds like a sane thing to do having in mind
the current sysfs layout. I will cook up something soonish.

> We could then add a new sysfs attribute to cleanly do the
> update-all-cores request.

That's what Fenghua is doing.

--
Regards/Gruss,
Boris.

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