[tip: x86/cleanups] x86/cpufeature: Explain the macro duplication

[tip-bot2 for Cao Jin]

The following commit has been merged into the x86/cleanups branch of tip:

Commit-ID:     cbb1133b563a63901cf778220eb17e3ff1425aed
Gitweb:        https://git.kernel.org/tip/cbb1133b563a63901cf778220eb17e3ff1425aed
Author:        Cao Jin <caoj.fnst@xxxxxxxxxxxxxx>
AuthorDate:    Wed, 28 Aug 2019 14:11:00 +08:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Wed, 28 Aug 2019 08:38:39 +02:00

x86/cpufeature: Explain the macro duplication

Explain the intent behind the duplication of the

  BUILD_BUG_ON_ZERO(NCAPINTS != n)

check in *_MASK_CHECK and its immediate use in the *MASK_BIT_SET macros
too.

 [ bp: Massage. ]

Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Cao Jin <caoj.fnst@xxxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Nadav Amit <namit@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: x86-ml <x86@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20190828061100.27032-1-caoj.fnst@xxxxxxxxxxxxxx
---
 arch/x86/include/asm/cpufeature.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 58acda5..59bf91c 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -61,6 +61,13 @@ extern const char * const x86_bug_flags[NBUGINTS*32];
 #define CHECK_BIT_IN_MASK_WORD(maskname, word, bit)	\
 	(((bit)>>5)==(word) && (1UL<<((bit)&31) & maskname##word ))
 
+/*
+ * {REQUIRED,DISABLED}_MASK_CHECK below may seem duplicated with the
+ * following BUILD_BUG_ON_ZERO() check but when NCAPINTS gets changed, all
+ * header macros which use NCAPINTS need to be changed. The duplicated macro
+ * use causes the compiler to issue errors for all headers so that all usage
+ * sites can be corrected.
+ */
 #define REQUIRED_MASK_BIT_SET(feature_bit)		\
 	 ( CHECK_BIT_IN_MASK_WORD(REQUIRED_MASK,  0, feature_bit) ||	\
 	   CHECK_BIT_IN_MASK_WORD(REQUIRED_MASK,  1, feature_bit) ||	\