linux-kernel - Re: [patch 0/4] x86: AMD microcode patch loading v2 fixes

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Date:	Wed, 30 Jul 2008 22:59:57 +0200
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Max Krasnyansky" <maxk@...lcomm.com>
Cc:	"Peter Oruba" <peter.oruba@....com>, "Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Tigran Aivazian" <tigran@...azian.fsnet.co.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch 0/4] x86: AMD microcode patch loading v2 fixes

2008/7/30 Max Krasnyansky <maxk@...lcomm.com>:
>
> Dmitry Adamushko wrote:
>> 2008/7/30 Peter Oruba <peter.oruba@....com>:
>>>> [ ... ]
>>> Since ucode updates may fix severe issues, it is supposed to happen as early
>>> as possible. If you re-plug your CPU into your socket, your BIOS also
>>> applies a ucode patch, but that won't necessarily be the latest and critical
>>> one.
> Sure. The question is would not workqueue be soon enough ?
> I'd say it is given the non-deterministic CPU hotplug callback sequence.

Max, cpu-hotplug callbacks might have been not the best choice in the
first place. So a comparison with them is not that relevant :-)

>
>> Hum, let's say we don't do it from cpu-hotplug handlers [1] but from
>> start_secondary() before calling cpu_idle()? [*]
>>
>> This way, we do it before any other task may have a chance to run on a
>> cpu which is not a case with cpu-hotplug handlers
>> (and we don't mess-up with cpu-hotplug events :-)
>>
>> [ the drawback is that 'microcode' subsystem is not local to
>> microcode.c anymore ]
>>
>> [1] if we need a sync. operation in cpu-hotplug handlers and IPI is
>> not ok (say, we need to run in a sleepablel context) then perhaps it's
>> workqueues + wait_on_cpu_work(). But then it's not a bit later than
>> could have been with [*].
> Why would not IPI be ok ? From looking at the code all we have to do is to
> factor request_firmware() out of the update path. So we'd do
> collect_cpu_info() in the IPI, then do request_firwmare() inplace and then do
> apply_microcode() in the IPI. ie The only thing that sleeps is request_firmware().

I think it's quite a complecated scheme. I still wonder whether e.g.
start_secondary() - cpu_idle() would be a better place or we just move
set_cpu(cpu, cpu_active_map) a bit :^)

But you know, at least short-term, it'd be nice if whoever might come
up with any working solution. It's already -rc1 and this thing is
still broken ;-)

btw., I've greped for "set_cpus_allowed_ptr()" and the following
scheme seems to be quite wide-spread (didn't check all of them so
maybe someone else does call it from cpu-hotplug notifications, heh)

        cpus_allowed = current->cpus_allowed;
        set_cpus_allowed_ptr(current, cpus);
        // do_something
        set_cpus_allowed_ptr(current, &cpus_allowed);

but _not_ safely used indeed. argh


>
> Max
>

-- 
Best regards,
Dmitry Adamushko
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