linux-kernel - Re: Bug on 2.6.26 - x86 VIA Nehemiah CentaurHauls processor cannot boot

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Message-ID: <488627E2.2060101@garzik.org>
Date:	Tue, 22 Jul 2008 14:33:06 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	"Luis R. Rodriguez" <mcgrof@...il.com>,
	linux kernel <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...nel.org>,
	Ivan Seskar <Seskar@...lab.rutgers.edu>,
	jfm3 <jfm3@...lab.rutgers.edu>, Sujith <m.sujith@...il.com>
Subject: Re: Bug on 2.6.26 - x86 VIA Nehemiah CentaurHauls processor cannot
 boot

H. Peter Anvin wrote:
> Jeff Garzik wrote:
>>>
>>> Thanks for taking a  look at this. So well, it would be a
>>> misconfiguration bug by the distribution then to try to support a
>>> generic 686 kernel wihtout GENERIC then.
>>
>> Well, it may be intentional -- some distros simply exclude support for 
>> the lower-volume VIA processors, since that might imply building their 
>> "generic 686 kernel" sans CMOV and some other instructions, and 
>> changing the compiler's instruction scheduling to something less 
>> optimal for the majority.  :/
>>
> 
> X86_GENERIC shouldn't disable CMOV?

I said "generic 686 kernel" not a specific Kconfig option (for reasons 
stated below), which is a bit different.

> 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.

	Jeff

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