linux-kernel - Re: early microcode: how to disable at runtime?

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Message-ID: <20140831201126.GB12628@khazad-dum.debian.net>
Date:	Sun, 31 Aug 2014 17:11:26 -0300
From:	Henrique de Moraes Holschuh <hmh@....eng.br>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Borislav Petkov <bp@...en8.de>, Fenghua Yu <fenghua.yu@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: early microcode: how to disable at runtime?

On Sun, 31 Aug 2014, H. Peter Anvin wrote:
> On 08/31/2014 12:45 PM, Henrique de Moraes Holschuh wrote:
> > Currently, there is no way to disable early microcode update at runtime.  If
> > an early initramfs with microcode update data is available, it will always
> > be used.
> > 
> > This can be a very big deal when things go wrong: it is hard for the regular
> > user to recover from an initramfs image that crashes the system, and the
> > early initramfs has no "disable" trigger.
> > 
> > In the general case, booting from rescue media will be required to fix the
> > system.
> > 
> > Unfortunately, the BSP microcode update is handled before early_param() is
> > available, so a kernel command line parameter to disable early microcode
> > updates for the BSP isn't easily possible.
> > 
> > Other than duplicating much of the code in parse_early_param/parse_args to
> > search for a "noearlymcu" parameter, is there any other way we could add a
> > disable switch for the early microcode update system?
> > 
> 
> Probably more like something more like arch/x86/boot/cmdline.c.  Maybe
> we could even make it sharable.

Looks like that would work, yes.

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh
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