Re: Bug on 2.6.26 - x86 VIA Nehemiah CentaurHauls processor cannotboot
From: H. Peter Anvin
Date: Tue Jul 22 2008 - 14:42:21 EST
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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