linux-kernel - Re: [PATCH] x86/cpufeature: drop *_MASK

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Date:   Wed, 28 Aug 2019 11:10:16 +0800
From:   Cao jin <caoj.fnst@...fujitsu.com>
To:     Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...el.com>
CC:     <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        <tglx@...utronix.de>, <mingo@...hat.com>, <hpa@...or.com>
Subject: Re: [PATCH] x86/cpufeature: drop *_MASK_CEHCK

On 8/28/19 1:20 AM, Borislav Petkov wrote:
> On Tue, Aug 27, 2019 at 09:33:11AM -0700, Dave Hansen wrote:
>> The point was that there are 5 files in the code that need to be changed
>> if you change NCAPINTS:
>>
>> 1. arch/x86/include/asm/required-features.h
>> 2. arch/x86/include/asm/disabled-features.h
>> 3. tools/arch/x86/include/asm/disabled-features.h
>> 4. tools/arch/x86/include/asm/required-features.h
>> 5. arch/x86/include/asm/cpufeature.h (2 sites)
>>
>> Each of those sites has a compile-time check for NCAPINTS.  The problem
>> is that the *-features.h code doesn't get compiled directly so a
>> BUILD_BUG_ON() doesn't work by itself.  So, for the sites there, we put
>> it somewhere that *will* get compiled: the macros that actually check
>> the bits.
>>
>> It looks weird, but the end effect is good: If you change NCAPINTS, you
>> get compile errors in 5 files and have to go edit those 5 files to fix
>> it.  Your patch makes it easier to introduce errors and miss one of
>> those sites.
> 
> ... and we wouldn't want to debug any strange bugs from missing a case.
> So, Cao, I wouldn't mind having the gist of that above somewhere around
> there in a comment explicitly.
> 

A subtle issue hard to detect by eyes.  Sure, on the way.
-- 
Sincerely,
Cao jin