Re: RFC: [2.6 patch] better i386 CPU selection

[Adrian Bunk <bunk@fs.tum.de>]

On Sun, Sep 07, 2003 at 06:47:51PM +0200, Mikael Pettersson wrote:
> On Sun, 7 Sep 2003 13:28:13 +0200, Adrian Bunk wrote:
> >The patch below tries to implement a better i386 CPU selection.
> >
> >In 2.4 selecting e.g. M486 has the semantics to get a kernel that runs 
> >on a 486 and above.
> >
> >In 2.6 selecting M486 means that only the 486 is supported.
> 
> Can you prove your claim about 2.6?
> 
> There is to the best of my knowledge nothing in 2.6.0-test4
> that prevents a kernel compiled for CPU type N from working
> with CPU types >N. Just to prove it, I built a CONFIG_M486
> 2.6.0-test4 and booted it w/o problems on P4, PIII, and K6-III.

Look at X86_L1_CACHE_SHIFT in arch/i386/Kconfig.

> >There are two different needs:
> >1. the installation kernel of a distribution should support all CPUs 
> >   this distribution supports (perhaps starting with the 386)
> >2. a sysadmin might e.g. want a kernel that support both a Pentium-III
> >   and a Pentium 4, but doesn't need to support a 386
> 
> How are 1 and 2 different? Both need support for CPU type N
> or higher. Since a kernel configured for a lower CPU type
> still works on higher CPU types, where is the problem?
> (In case 2 configure for PIII and use that on PIII and P4.)
> 
> Maybe I'm missing something but I don't see any problem here.

In current 2.6 this is only legal with X86_GENERIC enabled.

> /Mikael

cu
Adrian

-- 

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