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

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Date:	Mon, 8 Sep 2008 17:23:30 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, 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


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Sun, 7 Sep 2008, H. Peter Anvin wrote:
> > 
> > Under that logic we shouldn't even have CPU configurables, since you want it
> > to "just work" whatever crap you're running on.  That is EXACTLY what
> > CONFIG_X86_GENERIC means
> 
> I dunno.. Event he help-text doesn't actually agree with that:
> 
>   config X86_GENERIC
>         bool "Generic x86 support"
>         depends on X86_32
>         help
>           Instead of just including optimizations for the selected
>           x86 variant (e.g. PII, Crusoe or Athlon), include some more
>           generic optimizations as well. This will make the kernel
>           perform better on x86 CPUs other than that selected.
>           
>           This is really intended for distributors who need more
>           generic optimizations.
> 
> Also, quite frankly, while the CPU processor type message says
> 
>           The kernel will not necessarily run on earlier architectures than
>           the one you have chosen, e.g. a Pentium optimized kernel will run on
>           a PPro, but not necessarily on a i486.
> 
> I thought you agreed that CPU virtualization can be a problem? That 
> was the whole excuse for why the dynamic code was changed. Why would 
> it not be true for the static code?
> 
> The fact is, if you want to run on a Core2 or other modern CPU, then 
> "Virtual PC" is apparently buggy in this respect. You worked around it 
> for the dynamic choice - but that's totally _pointless_ if you then 
> don't want to work around it for the static one.

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.

	Ingo
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