linux-kernel - [PATCH v7 26/31] x86/jump_label: Add ASM support for static_branch

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20251114151428.1064524-6-vschneid@redhat.com>
Date: Fri, 14 Nov 2025 16:14:23 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	rcu@...r.kernel.org,
	x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org,
	loongarch@...ts.linux.dev,
	linux-riscv@...ts.infradead.org,
	linux-arch@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org
Cc: Frederic Weisbecker <frederic@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Arnd Bergmann <arnd@...db.de>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Jason Baron <jbaron@...mai.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ard Biesheuvel <ardb@...nel.org>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Han Shen <shenhan@...gle.com>,
	Rik van Riel <riel@...riel.com>,
	Jann Horn <jannh@...gle.com>,
	Dan Carpenter <dan.carpenter@...aro.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Clark Williams <williams@...hat.com>,
	Yair Podemsky <ypodemsk@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Daniel Wagner <dwagner@...e.de>,
	Petr Tesarik <ptesarik@...e.com>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>
Subject: [PATCH v7 26/31] x86/jump_label: Add ASM support for static_branch_likely()

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@...nel.org>
Signed-off-by: Valentin Schneider <vschneid@...hat.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 61dd1dee7812e..3c9ba3948e225 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>
-- 
2.51.0