Messages in this thread | | | Subject | Re: [PATCH] x86/cpufeature: drop *_MASK_CEHCK | From | Dave Hansen <> | Date | Tue, 27 Aug 2019 09:33:11 -0700 |
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On 8/27/19 12:41 AM, Borislav Petkov wrote: >> #define DISABLED_MASK_BIT_SET(feature_bit) \ >> @@ -104,7 +103,6 @@ extern const char * const x86_bug_flags[NBUGINTS*32]; >> CHECK_BIT_IN_MASK_WORD(DISABLED_MASK, 16, feature_bit) || \ >> CHECK_BIT_IN_MASK_WORD(DISABLED_MASK, 17, feature_bit) || \ >> CHECK_BIT_IN_MASK_WORD(DISABLED_MASK, 18, feature_bit) || \ >> - DISABLED_MASK_CHECK || \ >> BUILD_BUG_ON_ZERO(NCAPINTS != 19)) >> >> #define cpu_has(c, bit) \ > If you do a little bit of git archeology: > > $ git annotate arch/x86/include/asm/cpufeature.h > > after a while, you'll see that this was added in: > > 1e61f78baf89 ("x86/cpufeature: Make sure DISABLED/REQUIRED macros are updated") > > and then you could Cc Dave and ask what he was thinking then? > > Leaving in the rest for reference.
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.
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