linux-kernel - Re: [PATCH v2 1/2] x86/cpufeature: Add facility to match microcode revisions

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Message-ID: <20181011114327.oo3at77hxucpfaon@khazad-dum.debian.net>
Date:   Thu, 11 Oct 2018 08:43:27 -0300
From:   Henrique de Moraes Holschuh <hmh@....eng.br>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     peterz@...radead.org, x86@...nel.org, eranian@...gle.com,
        kan.liang@...el.com, linux-kernel@...r.kernel.org,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v2 1/2] x86/cpufeature: Add facility to match microcode
 revisions

On Wed, 10 Oct 2018, Andi Kleen wrote:
> v2:
> Remove all CPU match, only check boot cpu

IMHO, since it looks like a v3 will be necessary anyway, it could
benefit from a comment reminding people about how to use it on older
systems where "mixed CPU stepping" configurations were common.

This is *not* a relevant limitation, and it is easy enough to handle.
But people writing quirks for very old Intel Xeon CPUs *today* (unlikely
as that might be) might well forget the mixed-stepping gotcha...

Note that while mixed-stepping SMP configurations are *not* current
practice, they *were* reasonably common practice more than a decade ago,
officially supported both by Intel (there are Intel documents detailing
the valid stepping combinations) and the server vendors.

Suggestion below.

> +/*
> + * Match specific microcodes
> + *
> + * vendor/family/model/stepping must be all set.
> + * min_ucode is optional and can be 0.

* only checks against the boot cpu.  When mixed-stepping configs are
  valid for a CPU model, add a quirk for every valid stepping and
  do the fine-tuning in the quirk handler.

-- 
  Henrique Holschuh