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Message-ID: <051df5c21cbf6d9ac396c22b3ab3c51d73c0def7.1765044697.git.jpoimboe@kernel.org>
Date: Sat, 6 Dec 2025 13:41:15 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
Linus Torvalds <torvalds@...ux-foundation.org>,
Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Borislav Petkov <bp@...en8.de>,
Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 8/8] x86/alternative: Convert alternatives to assembler macros
Improve code generation readability by converting the alternatives into
assembler macros which are created when alternative.h is included.
Before:
# ./arch/x86/include/asm/smap.h:47: alternative("", "stac", X86_FEATURE_SMAP);
# 47 "./arch/x86/include/asm/smap.h" 1
# ALT: oldinstr
771:
772:
# ALT: padding
.skip -(((775f-774f)-(772b-771b)) > 0) * ((775f-774f)-(772b-771b)),0x90
773:
.pushsection .altinstructions, "aM", @progbits, 14
.long 771b - .
.long 774f - .
.4byte ( 9*32+20)
.byte 773b-771b
.byte 775f-774f
.popsection
.pushsection .altinstr_replacement, "ax"
ANNOTATE_DATA_SPECIAL
# ALT: replacement
774:
stac
775:
.popsection
After:
# ./arch/x86/include/asm/smap.h:47: alternative("", "stac", X86_FEATURE_SMAP);
# 47 "./arch/x86/include/asm/smap.h" 1
ALTERNATIVE "", "stac", "( 9*32+20)"
Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
arch/x86/include/asm/alternative.h | 196 +++++++++++++++--------------
include/linux/annotate.h | 7 +-
2 files changed, 107 insertions(+), 96 deletions(-)
diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 03364510d5fe..89c69fe8f337 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -8,6 +8,90 @@
#include <asm/asm.h>
#include <asm/bug.h>
+/*
+ * CALC_MAX: assembler-compatible max based on the idea from:
+ * http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax
+ *
+ * The additional "-" is needed because gas uses a "true" value of -1.
+ */
+#define CALC_MAX(a, b) (a ^ ((a ^ b) & -(-(a < b))))
+
+/* Add padding after the original instructions so the replacements can fit. */
+#define __ALT_SKIP(orig_len, len1, len2, len3) \
+ .skip -((CALC_MAX(CALC_MAX(len1, len2), len3) - orig_len) > 0) *\
+ (CALC_MAX(CALC_MAX(len1, len2), len3) - orig_len), 0x90
+
+#define ALT_SKIP __ALT_SKIP((772b-771b), (775f-774f), (776f-775f), (777f-776f))
+
+#define ALT_REPLACE(newinstr1, newinstr2, newinstr3) \
+ .pushsection .altinstr_replacement, "ax"; \
+ 774: newinstr1; \
+ 775: newinstr2; \
+ 776: newinstr3; \
+ 777: \
+ .popsection
+
+#define ALT_ENTRY(orig_begin, orig_end, new_begin, new_end, ft_flags) \
+ __ANNOTATE_DATA_SPECIAL(new_begin); \
+ .pushsection .altinstructions, "aM", @progbits, ALT_INSTR_SIZE; \
+ .long orig_begin - .; \
+ .long new_begin - .; \
+ .4byte ft_flags; \
+ .byte orig_end - orig_begin; \
+ .byte new_end - new_begin; \
+ .popsection
+
+/*
+ * These ALTERNATIVE .macros get created when this file gets included:
+ */
+
+#define DEFINE_ALTERNATIVE \
+.macro ALTERNATIVE oldinstr, newinstr, ft_flags; \
+ 771: __ASM_C(\oldinstr, \\oldinstr); \
+ 772: ALT_SKIP; \
+ 773: \
+ ALT_REPLACE(__ASM_C(\newinstr, \\newinstr),,); \
+ ALT_ENTRY(771b, 773b, 774b, 775b, \
+ __ASM_C(\ft_flags, \\ft_flags)); \
+.endm
+
+#define DEFINE_ALTERNATIVE_2 \
+.macro ALTERNATIVE_2 oldinstr, newinstr1, ft_flags1, \
+ newinstr2, ft_flags2; \
+ 771: __ASM_C(\oldinstr, \\oldinstr); \
+ 772: ALT_SKIP; \
+ 773: \
+ ALT_REPLACE(__ASM_C(\newinstr1, \\newinstr1), \
+ __ASM_C(\newinstr2, \\newinstr2),); \
+ ALT_ENTRY(771b, 773b, 774b, 775b, \
+ __ASM_C(\ft_flags1, \\ft_flags1)); \
+ ALT_ENTRY(771b, 773b, 775b, 776b, \
+ __ASM_C(\ft_flags2, \\ft_flags2)); \
+.endm
+
+#define DEFINE_ALTERNATIVE_3 \
+.macro ALTERNATIVE_3 oldinstr, newinstr1, ft_flags1, \
+ newinstr2, ft_flags2, \
+ newinstr3, ft_flags3; \
+ 771: __ASM_C(\oldinstr, \\oldinstr); \
+ 772: ALT_SKIP; \
+ 773: \
+ ALT_REPLACE(__ASM_C(\newinstr1, \\newinstr1), \
+ __ASM_C(\newinstr2, \\newinstr2), \
+ __ASM_C(\newinstr3, \\newinstr3)); \
+ ALT_ENTRY(771b, 773b, 774b, 775b, \
+ __ASM_C(\ft_flags1, \\ft_flags1)); \
+ ALT_ENTRY(771b, 773b, 775b, 776b, \
+ __ASM_C(\ft_flags2, \\ft_flags2)); \
+ ALT_ENTRY(771b, 773b, 776b, 777b, \
+ __ASM_C(\ft_flags3, \\ft_flags3)); \
+.endm
+
+DEFINE_MACRO(ALTERNATIVE);
+DEFINE_MACRO(ALTERNATIVE_2);
+DEFINE_MACRO(ALTERNATIVE_3);
+
+
#define ALT_FLAGS_SHIFT 16
#define ALT_FLAG_NOT (1 << 0)
@@ -184,53 +268,30 @@ static inline int alternatives_text_reserved(void *start, void *end)
#define ALT_CALL_INSTR "call BUG_func"
-#define alt_slen "772b-771b"
-#define alt_total_slen "773b-771b"
-#define alt_rlen "775f-774f"
+#define ALTERNATIVE(oldinstr, newinstr, ft_flags) \
+ "ALTERNATIVE \"" oldinstr "\", " \
+ "\"" newinstr "\", \"" __stringify(ft_flags) "\"\n"
-#define OLDINSTR(oldinstr) \
- "# ALT: oldinstr\n" \
- "771:\n\t" oldinstr "\n772:\n" \
- "# ALT: padding\n" \
- ".skip -(((" alt_rlen ")-(" alt_slen ")) > 0) * " \
- "((" alt_rlen ")-(" alt_slen ")),0x90\n" \
- "773:\n"
+#define ALTERNATIVE_2(oldinstr, \
+ newinstr1, ft_flags1, \
+ newinstr2, ft_flags2) \
+ "ALTERNATIVE_2 \"" oldinstr "\", " \
+ "\"" newinstr1 "\", \"" __stringify(ft_flags1) "\", " \
+ "\"" newinstr2 "\", \"" __stringify(ft_flags2) "\"\n"
-#define ALTINSTR_ENTRY(ft_flags) \
- ".pushsection .altinstructions, \"aM\", @progbits, " \
- __stringify(ALT_INSTR_SIZE) "\n" \
- " .long 771b - .\n" /* label */ \
- " .long 774f - .\n" /* new instruction */ \
- " .4byte " __stringify(ft_flags) "\n" /* feature + flags */ \
- " .byte " alt_total_slen "\n" /* source len */ \
- " .byte " alt_rlen "\n" /* replacement len */ \
- ".popsection\n"
-
-#define ALTINSTR_REPLACEMENT(newinstr) /* replacement */ \
- ".pushsection .altinstr_replacement, \"ax\"\n" \
- ANNOTATE_DATA_SPECIAL "\n" \
- "# ALT: replacement\n" \
- "774:\n\t" newinstr "\n775:\n" \
- ".popsection\n"
-
-/* alternative assembly primitive: */
-#define ALTERNATIVE(oldinstr, newinstr, ft_flags) \
- OLDINSTR(oldinstr) \
- ALTINSTR_ENTRY(ft_flags) \
- ALTINSTR_REPLACEMENT(newinstr)
-
-#define ALTERNATIVE_2(oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2) \
- ALTERNATIVE(ALTERNATIVE(oldinstr, newinstr1, ft_flags1), newinstr2, ft_flags2)
+#define ALTERNATIVE_3(oldinstr, \
+ newinstr1, ft_flags1, \
+ newinstr2, ft_flags2, \
+ newinstr3, ft_flags3) \
+ "ALTERNATIVE_3 \"" oldinstr "\", " \
+ "\"" newinstr1 "\", \"" __stringify(ft_flags1) "\", " \
+ "\"" newinstr2 "\", \"" __stringify(ft_flags2) "\", " \
+ "\"" newinstr3 "\", \"" __stringify(ft_flags3) "\"\n"
/* If @feature is set, patch in @newinstr_yes, otherwise @newinstr_no. */
#define ALTERNATIVE_TERNARY(oldinstr, ft_flags, newinstr_yes, newinstr_no) \
ALTERNATIVE_2(oldinstr, newinstr_no, X86_FEATURE_ALWAYS, newinstr_yes, ft_flags)
-#define ALTERNATIVE_3(oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2, \
- newinstr3, ft_flags3) \
- ALTERNATIVE(ALTERNATIVE_2(oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2), \
- newinstr3, ft_flags3)
-
/*
* Alternative instructions for different CPU types or capabilities.
*
@@ -332,65 +393,10 @@ void nop_func(void);
.endm
#endif
-/*
- * Issue one struct alt_instr descriptor entry (need to put it into
- * the section .altinstructions, see below). This entry contains
- * enough information for the alternatives patching code to patch an
- * instruction. See apply_alternatives().
- */
-.macro altinstr_entry orig alt ft_flags orig_len alt_len
- .long \orig - .
- .long \alt - .
- .4byte \ft_flags
- .byte \orig_len
- .byte \alt_len
-.endm
-
.macro ALT_CALL_INSTR
call BUG_func
.endm
-/*
- * Define an alternative between two instructions. If @feature is
- * present, early code in apply_alternatives() replaces @oldinstr with
- * @newinstr. ".skip" directive takes care of proper instruction padding
- * in case @newinstr is longer than @oldinstr.
- */
-#define __ALTERNATIVE(oldinst, newinst, flag) \
-740: \
- oldinst ; \
-741: \
- .skip -(((744f-743f)-(741b-740b)) > 0) * ((744f-743f)-(741b-740b)),0x90 ;\
-742: \
- .pushsection .altinstructions, "aM", @progbits, ALT_INSTR_SIZE ;\
- altinstr_entry 740b,743f,flag,742b-740b,744f-743f ; \
- .popsection ; \
- .pushsection .altinstr_replacement,"ax" ; \
-743: \
- ANNOTATE_DATA_SPECIAL ; \
- newinst ; \
-744: \
- .popsection ;
-
-.macro ALTERNATIVE oldinstr, newinstr, ft_flags
- __ALTERNATIVE(\oldinstr, \newinstr, \ft_flags)
-.endm
-
-/*
- * Same as ALTERNATIVE macro above but for two alternatives. If CPU
- * has @feature1, it replaces @oldinstr with @newinstr1. If CPU has
- * @feature2, it replaces @oldinstr with @feature2.
- */
-.macro ALTERNATIVE_2 oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2
- __ALTERNATIVE(__ALTERNATIVE(\oldinstr, \newinstr1, \ft_flags1),
- \newinstr2, \ft_flags2)
-.endm
-
-.macro ALTERNATIVE_3 oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2, newinstr3, ft_flags3
- __ALTERNATIVE(ALTERNATIVE_2(\oldinstr, \newinstr1, \ft_flags1, \newinstr2, \ft_flags2),
- \newinstr3, \ft_flags3)
-.endm
-
/* If @feature is set, patch in @newinstr_yes, otherwise @newinstr_no. */
#define ALTERNATIVE_TERNARY(oldinstr, ft_flags, newinstr_yes, newinstr_no) \
ALTERNATIVE_2 oldinstr, newinstr_no, X86_FEATURE_ALWAYS, \
diff --git a/include/linux/annotate.h b/include/linux/annotate.h
index 8b28f1a81ec4..a80fef36dc92 100644
--- a/include/linux/annotate.h
+++ b/include/linux/annotate.h
@@ -100,8 +100,13 @@ DEFINE_MACRO(ANNOTATE_DATA(DATA_SPECIAL));
* to find and extract individual special section entries as needed.
*/
#define ANNOTATE_DATA_SPECIAL "ANNOTATE_DATA_SPECIAL"
+#define __ANNOTATE_DATA_SPECIAL(label) __ANNOTATE .discard.annotate_data, ANNOTYPE_DATA_SPECIAL, label
-#endif /* !__ASSEMBLY__ */
+#else /* __ASSEMBLY__ */
+
+#define __ANNOTATE_DATA_SPECIAL(label) ANNOTATE_DATA_SPECIAL loc=label
+
+#endif /* __ASSEMBLY__ */
#else /* !OBJTOOL */
--
2.52.0
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