linux-kernel - Re: [PATCH] ix86: improve determination of X86_MINIMUM_CPU

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Date:	Fri, 07 Sep 2012 08:31:35 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	<mingo@...e.hu>, <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ix86: improve determination of
 X86_MINIMUM_CPU_FAMILY

>>> On 07.09.12 at 09:16, "H. Peter Anvin" <hpa@...or.com> wrote:
> On 09/06/2012 11:48 PM, Jan Beulich wrote:
>>
>> --- 3.6-rc4/arch/x86/Kconfig.cpu
>> +++ 3.6-rc4-x86-minimum-cpu-family/arch/x86/Kconfig.cpu
>> @@ -409,9 +409,9 @@ config X86_CMOV
>>   config X86_MINIMUM_CPU_FAMILY
>>   	int
>>   	default "64" if X86_64
>> -	default "6" if X86_32 && X86_P6_NOP
>> -	default "5" if X86_32 && X86_CMPXCHG64
>> -	default "4" if X86_32 && (X86_XADD || X86_CMPXCHG || X86_BSWAP || X86_WP_WORKS_OK)
>> +	default "6" if X86_P6_NOP || X86_CMOV
>> +	default "5" if X86_CMPXCHG64 || X86_TSC
>> +	default "4" if X86_XADD || X86_CMPXCHG || X86_BSWAP || X86_WP_WORKS_OK
>>   	default "3"
>>
> 
> Erk, this isn't right either.  We're not supposed to include 
> CPUID-enumerable features here, so X86_CMOV, X86_CMPXCHG64 and X86_TSC 
> don't belong, and should instead be done via required-features.h.

Ah, so I got misguided by there already being a CPUID-detectable
feature here (X86_CMPXCHG64). And by the fact that e.g. having
M686 (or better) selected is then in fact not expected to have a
relationship to the value chosen here (worth a comment perhaps).

Further, the "6" case is then completely pointless - X86_P6_NOP
depends on X86_64. And with all features the kernel cares about
that got added with Pentium and later being CPUID-enumerable,
the "5" case would then be superfluous too.

Jan

Jan

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