linux-kernel - Re: [PATCH] x86, cpu: X86_FEATURE_NOPL should be disabled on 32-bit only

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Date:	Sun, 03 Oct 2010 15:22:13 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...en8.de>
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86, cpu: X86_FEATURE_NOPL should be disabled on 32-bit only

Sort of... only if the input is a constant.

"Borislav Petkov" <bp@...en8.de> wrote:

>From: "H. Peter Anvin" <hpa@...or.com>
>Date: Sun, Oct 03, 2010 at 11:19:18AM -0700
>
>> I think we actually need to set it on 64 bits.
>
>Ok, correct me if I'm wrong but I think it is already "indirectly" set
>on 64-bits through REQUIRED_MASK3:
>
>#define REQUIRED_MASK3  (NEED_NOPL)
>
>which is defined as
>
>#define NEED_NOPL      (1<<(X86_FEATURE_NOPL & 31))
>
>when either CONFIG_X86_P6_NOP or CONFIG_X86_64 are selected.
>
>and cpu_has() does
>
>#define cpu_has(c, bit)                                                 \
>        (__builtin_constant_p(bit) &&                                   \
>	...
>           (((bit)>>5)==3 && (1UL<<((bit)&31) & REQUIRED_MASK3)) ||     \
>	...
>	? 1 : test_cpu_cap(c, bit))
>
>which returns 1 when testing for X86_FEATURE_NOPL, no?
>
>-- 
>Regards/Gruss,
>    Boris.

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