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

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Date:	Fri, 5 Sep 2008 09:39:38 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jan Beulich <jbeulich@...ell.com>
cc:	linux@...dersweb.net, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <andi-suse@...stfloor.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] x86 kenel won't boot under Virtual PC



On Fri, 5 Sep 2008, Jan Beulich wrote:
> 
> I disagree here: If I configure a 686+ kernel, I expect these NOPs to be
> that way (and to work). If you want to run on something that's not
> compliant, you just shouldn't configure your kernel that way.

Well, if you actually do a

	git grep 'ASM_NOP[0-9]'

you'll find that just the _definitions_ of those things are the bulk of it 
BY FAR, and that there doesn't seem to be a single user that cares even 
remotely about performance.

So I actually think that the whole thing is a waste of time. We should 
probably 

 - pick a single set of NOP's per 32-bit/64-bit (since the good nops in 
   32-bit aren't 64-bit instructions at all, so we do want different nops 
   depending on _that_)

   The whole static choice by microarchitecture is pure garbage.

 - Probably also just declare that those default nops are single 
   instructions, just so that we never even have to think about it from a 
   dynamic replacement angle.

   Look at the uses again, and realize that it really is just pure garbage 
   to have this kind of complex and subtle stuff going on.

 - Move the optimized nop definitions (K7_NOPx etc) to the only place that 
   cares - asm/x86/kernel/alternative.c. When we do things _dynamically_, 
   it can actually make sense to pick a nop more precisely, but for this 
   whole static thing it's just a pain.

IOW, if it actually _worked_ reasonably, I wouldn't care. But clearly it 
doesn't. And once it's not working reasonably, it should be fixed.

		Linus
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