linux-kernel - Re: [BUG] x86 kenel won't boot under Virtual PC

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Message-ID: <48C54690.1060002@zytor.com>
Date:	Mon, 08 Sep 2008 08:36:48 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux@...dersweb.net, linux-kernel@...r.kernel.org,
	the arch/x86 maintainers <x86@...nel.org>,
	Andi Kleen <andi-suse@...stfloor.org>
Subject: Re: [BUG] x86 kenel won't boot under Virtual PC

Ingo Molnar wrote:
> 
> yes. X86_P6_NOPS is a totally insignificant optimization and if it makes 
> _any_ CPU not boot (be that virtual or real), then it's frankly not 
> worth it.
> 
> David, exactly how does the kernel fail to boot with latest -git? 
> (v2.6.27-rc5-313-g64f996f or later) Does detect_nopl() run? It really 
> should, and it should detect the non-working instructions.
> 

Okay, a few things here...

1. First, I wrote up a patch yesterday to update the CONFIG_X86_GENERIC 
description and to make it "default y".  It is currently on 
x86/defconfig, but I think it should be promoted to mainline immediately.

2. X86_P6_NOPS is not the only source of static NOPLs.  If gcc is set to 
optimize for specific architectures, then gcc/binutils will generate 
static NOPLs.  The only way we can prevent that is by not using specific 
-march options, as far as I can tell.

3. I'm not positive that CONFIG_X86_GENERIC currently avoid all cases of 
(2), but it obviously should.  I will verify that today and add a 
followup patch to the Makefiles if necessary.

Given all of this, I really think that putting this on 
CONFIG_X86_GENERIC, *AND* making CONFIG_X86_GENERIC the default is the 
right choice.

	-hpa

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