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

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Message-ID: <54037C07.8060703@zytor.com>
Date:	Sun, 31 Aug 2014 12:48:23 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Borislav Petkov <bp@...en8.de>,
	Fenghua Yu <fenghua.yu@...el.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: early microcode: how to disable at runtime?

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.

	-hpa


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