linux-kernel - Re: Bug on 2.6.26 - x86 VIA Nehemiah CentaurHauls processor cannot boot

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Message-ID: <87prp07b0y.fsf@basil.nowhere.org>
Date:	Sat, 26 Jul 2008 20:31:09 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"Luis R. Rodriguez" <mcgrof@...il.com>,
	linux kernel <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...nel.org>,
	Ivan Seskar <Seskar@...lab.rutgers.edu>,
	jfm3 <jfm3@...lab.rutgers.edu>, Sujith <m.sujith@...il.com>
Subject: Re: Bug on 2.6.26 - x86 VIA Nehemiah CentaurHauls processor cannot boot

"H. Peter Anvin" <hpa@...or.com> writes:

> Ingo Molnar wrote:
>>>
>>> Not a bug.
>> it would still be nice to get a nice printk and panic during bootup
>> instead of some obscure crash, hm?
>>
>
> Yes.  The fundamental problem is that Centaur has a set of CPUs which
> report family == 6 but don't have the long NOP instructions.  We would
> need an exact CPUID criterion for these CPUs in order to be able to
> report it as an error.  An alternative would be to attempt trapping in
> the real-mode code (#UD is one of the *very* few CPU exceptions which
> can be reliably captured in real mode on a BIOS system), but doing so
> would probably mean breaking Loadlin at the very least.
>
> We can't "printk and panic" because we never get that far in the
> kernel proper, for obvious reasons: the code is quite littered with
> these buggers.

This was originally supposed to be handled in the early real mode
head.S code. That is why I put the CPUID checking code in there
to error out early when you can still print to the console
using the BIOS functions.

I suspect this regressed when that code was moved to C, because
now the C compiler generates CMOV early.

How about always building the real mode C code with -march=i386?
It is not performance critical so that is ok.

-Andi
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