Re: Bug on 2.6.26 - x86 VIA Nehemiah CentaurHauls processor cannotboot

[Jeff Garzik]

H. Peter Anvin wrote:
> Jeff Garzik wrote:
> > > We're only referring specifically to the family == 6 VIA processors 
> > > here.
> >
> > To be specific, I was merely saying that VIA processors where 
> > c->x86_model==6 may lack CMOV.
> >
> > I have not kept track of what current Kconfig options will set, but in 
> > the past it was quite easy to build a "generic 686 kernel" that 
> > required CMOV and thus excluded these VIA processors.
> >
> > Distros in the past often wound up intentionally -not- supporting some 
> > of these VIA processors, because they did not want to create a 
> > non-CMOV kernel.  (This policy obviously excluded older x86 as well)
> >
> > If these things have been addressed recently (< 12-18 months) then all 
> > good.
>
> I am pretty sure CONFIG_X86_GENERIC doesn't disable CMOV, and since CMOV 
> is a separate CPUID flag it's all good (if the chip doesn't have it, 
> it'll trap.)

It's generally more an issue of making sure the compiler is not 
instructed to issue cmov (-march=i686).


> Unfortunately Intel didn't assign a CPUID flag for the long NOPs, and 
> then didn't document them (I think partially because they were a 
> retcon), but yet it reflected a serious hole in Centaur's 
> characterization effort that they bumped family to 6 without following 
> P6 behaviour for a massive range of opcodes.
>
> The main reason for disabling P6 NOPs for CONFIG_X86_GENERIC is that the 
> win is so small, and that a number of vendors got it wrong.

Yeah.

	Jeff



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