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

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Message-ID: <alpine.LFD.1.10.0809080857450.3117@nehalem.linux-foundation.org>
Date:	Mon, 8 Sep 2008 09:07:41 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Andi Kleen <andi@...stfloor.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

On Mon, 8 Sep 2008, H. Peter Anvin wrote:
> 
> The help text is indeed out of date.  I did a patch yesterday to, among other
> things, update it; I also want to verify that we are disabling all options
> that can cause gcc or binutils to generate nopl's; I plan to push it today.

Peter.

The help text may be out of date because of changes to NOPL usage, but you 
should ask yourself whether the change is actually a _good_ change.

IOW, I really don't see why you are pushing changing the help-text, 
instead of just making the kernel work better.

The fact that some broken gcc/binutils versions may screw us over _anyway_ 
may well mean that we should just push back on _that_ change instead.

Quite frankly, from a user perspective, even a very _technical_ one, 
please tell me what the advantage of not being fairly generic by default 
is. Really.

Yes, there are some _big_ ISA issues where it is worth doing real static 
code selection (as opposed to just instruction selection and scheduling 
etc that still _works_ for everybody, but optimizes for certain 
archtiectures).

So things like cmpxchg/xadd (for atomics) and cmov (for compiler-generated 
code), and bswap (for networking) can really make a big difference, and 
are not really realistic to do dynamically. 

But NOPL? That's simply not _worth_ it being painful over.

And the fact is, the current help text describes

 (a) the historical meaning (optimize for a specific architecture, but 
     don't make extreme choices that are bad for others)
 (b) what people would generally _want_.

and I really don't think that changing the help text is the right solution 
here. It may be "technically correct", but it is simply not user-friendly 
or smart.

			Linus
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