linux-kernel - Re: [PATCH v6 03/10] x86, mpx: add macro cpu_has

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Date:	Thu, 19 Jun 2014 11:02:54 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Dave Hansen <dave.hansen@...el.com>,
	Borislav Petkov <bp@...en8.de>,
	Qiaowei Ren <qiaowei.ren@...el.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 03/10] x86, mpx: add macro cpu_has_mpx

On 06/18/2014 09:25 AM, Dave Hansen wrote:
> On 06/18/2014 07:59 AM, H. Peter Anvin wrote:
>> On 06/18/2014 07:35 AM, Dave Hansen wrote:
>>> It looks like static_cpu_has() is the right thing to use instead of
>>> boot_cpu_has().  But, this doesn't just obfuscate things.
>>>
>>> We actually _want_ the compiler to cull code out when the config option
>>> is off.  Things like do_bounds() will see code savings with _some_ kind
>>> of #ifdef rather than using static_cpu_has().
>>>
>>> So, we can either use the well worn, consistent with other features in
>>> x86, cpu_has_$foo approach.  Or, we can roll our own macros.
>>
>> We could do something like:
>>
>> #define MPX_ENABLED (IS_ENABLED(CONFIG_X86_MPX) &&
>> static_cpu_has(X86_FEATURE_MPX))
> 
> How about something like the attached patch?
> 
> This lets us use static_cpu_has() for the checks, and allows us to
> easily add new checks for other features that might be compile-time
> disabled.
> 

Hmm... I would like something similar to required-features.h which
reflect features which *cannot* be enabled or will always be ignored; we
actually already have a handful of those.

	-hpa

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