[RFC PATCH v8 07/10] x86/jump_label: Add ASM support for static_branch_likely()

[Valentin Schneider <vschneid@redhat.com>]

A later commit will add some early entry code that only needs to be
executed if nohz_full is present on the cmdline, not just if
CONFIG_NO_HZ_FULL is compiled in. Add an ASM-callable static branch macro.

Note that I haven't found a way to express unlikely (i.e. out-of-line)
static branches in ASM macros without using extra jumps, which kind of
defeats the purpose. Consider:

  .macro FOOBAR
	  // Key enabled:  JMP .Ldostuff_\@
	  // Key disabled: NOP
	  STATIC_BRANCH_UNLIKELY key, .Ldostuff_\@ // Patched to JMP if enabled
	  jmp .Lend_\@
  .Ldostuff_\@:
	  <dostuff>
  .Lend_\@:
  .endm

Instead, this should be expressed as a likely (i.e. in-line) static key:

  .macro FOOBAR
	  // Key enabled:  NOP
	  // Key disabled: JMP .Lend_\@
	  STATIC_BRANCH_LIKELY key, .Lend\@ // Patched to NOP if enabled
	  <dostuff>
  .Lend_\@:
  .endm

Suggested-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
 arch/x86/include/asm/jump_label.h | 33 ++++++++++++++++++++++++++++++-
 1 file changed, 32 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index 05b16299588d5..ea587598abe7c 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -7,7 +7,38 @@
 #include <asm/asm.h>
 #include <asm/nops.h>
 
-#ifndef __ASSEMBLER__
+#ifdef __ASSEMBLER__
+
+/*
+ * There isn't a neat way to craft unlikely static branches in ASM, so they
+ * all have to be expressed as likely (inline) static branches. This macro
+ * thus assumes a "likely" usage.
+ */
+.macro ARCH_STATIC_BRANCH_LIKELY_ASM key, label, jump, hack
+1:
+.if \jump || \hack
+	jmp \label
+.else
+	.byte BYTES_NOP5
+.endif
+	.pushsection __jump_table, "aw"
+	_ASM_ALIGN
+	.long 1b - .
+	.long \label - .
+	/* LIKELY so bit0=1, bit1=hack */
+	_ASM_PTR \key + 1 + (\hack << 1) - .
+	.popsection
+.endm
+
+.macro STATIC_BRANCH_TRUE_LIKELY key, label
+	ARCH_STATIC_BRANCH_LIKELY_ASM \key, \label, 0, IS_ENABLED(CONFIG_HAVE_JUMP_LABEL_HACK)
+.endm
+
+.macro STATIC_BRANCH_FALSE_LIKELY key, label
+	ARCH_STATIC_BRANCH_LIKELY_ASM \key, \label, 1, 0
+.endm
+
+#else /* !__ASSEMBLER__ */
 
 #include <linux/stringify.h>
 #include <linux/types.h>
-- 
2.52.0