linux-kernel - Re: x86, microcode: BUG: microcode update that changes x86

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Message-ID: <20140919150042.GC5318@nazgul.tnic>
Date:	Fri, 19 Sep 2014 17:00:42 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Chuck Ebbert <cebbert.lkml@...il.com>
Cc:	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: x86, microcode: BUG: microcode update that changes x86_capability

On Fri, Sep 19, 2014 at 07:54:14AM -0500, Chuck Ebbert wrote:
> Assuming we can identify all the affected models and steppings, maybe
> something like this would work:
> 
> 1) Refuse to finish booting if a microcode update that disables TSX
> isn't applied before userspace starts running on those CPUs.

Well, I think when we're booting, we would have already applied the
early microcode, no? Because then it is a non-issue.

> 2) Don't allow a late update if TSX is still enabled on those
> processors.

Yeah, so the use case I have in mind is when a long-running machine
wants to apply microcode and this microcode disables CPUID bits and
instructions. And the machine cannot be rebooted.

I guess in that case we would have to issue a warning only on the
affected processors that a rebooted is mandatory and fail the update...
Maybe something like that.

> (1) could be overridden by a command line option for people who want
> to develop TSX code.

The way I understand it, those people shouldn't apply the microcode
patch at all.

-- 
Regards/Gruss,
    Boris.
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