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

[H. Peter Anvin]

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.)

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.

	-hpa
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