[PATCH 1/1] alpha: Add support for HAVE_ARCH_JUMP_LABEL

Implement static key (jump label) support for Alpha.

Provide arch_static_branch() helpers and implement
arch_jump_label_transform() to patch a single instruction site
between NOP and BR, with appropriate I-cache synchronization.

Tested on Alpha UP2000+ SMP. Static key sites toggle correctly
under tracepoint enable/disable and repeated stress testing
without faults.

Signed-off-by: Magnus Lindholm <[email protected]>
---
 arch/alpha/Kconfig                  |  1 +
 arch/alpha/include/asm/jump_label.h | 66 +++++++++++++++++++++++
 arch/alpha/kernel/Makefile          |  1 +
 arch/alpha/kernel/jump_label.c      | 81 +++++++++++++++++++++++++++++
 4 files changed, 149 insertions(+)
 create mode 100644 arch/alpha/include/asm/jump_label.h
 create mode 100644 arch/alpha/kernel/jump_label.c

diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index 6c7dbf0adad6..71db2cc7b3c4 100644
--- a/arch/alpha/Kconfig
+++ b/arch/alpha/Kconfig
@@ -41,6 +41,7 @@ config ALPHA
 	select MMU_GATHER_RCU_TABLE_FREE
 	select SPARSEMEM_EXTREME if SPARSEMEM
 	select ZONE_DMA
+	select HAVE_ARCH_JUMP_LABEL
 	help
 	  The Alpha is a 64-bit general-purpose processor designed and
 	  marketed by the Digital Equipment Corporation of blessed memory,
diff --git a/arch/alpha/include/asm/jump_label.h b/arch/alpha/include/asm/jump_label.h
new file mode 100644
index 000000000000..b570a7cef4c9
--- /dev/null
+++ b/arch/alpha/include/asm/jump_label.h
@@ -0,0 +1,66 @@
+/* SPDX-License-Identifier: GPL-2.0
+ *
+ * Alpha architecture jump label (static key) definitions
+ *
+ * Defines patch site emission and jump table layout for
+ * Alpha static key support.
+ *
+ * Copyright (C) 2026 Magnus Lindholm <[email protected]>
+ */
+
+
+#ifndef _ASM_ALPHA_JUMP_LABEL_H
+#define _ASM_ALPHA_JUMP_LABEL_H
+
+#ifndef __ASSEMBLER__
+
+#include <linux/types.h>
+
+#define JUMP_LABEL_NOP_SIZE 4
+
+typedef u64 jump_label_t;
+
+struct jump_entry {
+	jump_label_t code;
+	jump_label_t target;
+	jump_label_t key;
+};
+
+static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
+{
+	asm goto("1:\n\t"
+		 "nop\n\t"
+		 ".pushsection __jump_table, \"aw\"\n\t"
+		 ".align 3\n\t"
+		 ".quad 1b, %l[l_yes], %0\n\t"
+		 ".popsection\n\t"
+		 :
+		 : "i"(&((char *)key)[branch])
+		 :
+		 : l_yes);
+
+	return false;
+l_yes:
+	return true;
+}
+
+static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
+{
+	asm goto("1:\n\t"
+		 "br $31, %l[l_yes]\n\t"
+		 ".pushsection __jump_table, \"aw\"\n\t"
+		 ".align 3\n\t"
+		 ".quad 1b, %l[l_yes], %0\n\t"
+		 ".popsection\n\t"
+		 :
+		 : "i"(&((char *)key)[branch])
+		 :
+		 : l_yes);
+
+	return false;
+l_yes:
+	return true;
+}
+
+#endif /* __ASSEMBLER__ */
+#endif /* _ASM_ALPHA_JUMP_LABEL_H */
diff --git a/arch/alpha/kernel/Makefile b/arch/alpha/kernel/Makefile
index 187cd8df2faf..8a357b78eab1 100644
--- a/arch/alpha/kernel/Makefile
+++ b/arch/alpha/kernel/Makefile
@@ -19,6 +19,7 @@ obj-$(CONFIG_MODULES)	+= module.o
 obj-$(CONFIG_PERF_EVENTS) += perf_event.o
 obj-$(CONFIG_RTC_DRV_ALPHA) += rtc.o
 obj-$(CONFIG_AUDIT)	+= audit.o
+obj-$(CONFIG_JUMP_LABEL) += jump_label.o
 
 ifdef CONFIG_ALPHA_GENERIC
 
diff --git a/arch/alpha/kernel/jump_label.c b/arch/alpha/kernel/jump_label.c
new file mode 100644
index 000000000000..ff061a09d813
--- /dev/null
+++ b/arch/alpha/kernel/jump_label.c
@@ -0,0 +1,81 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Alpha architecture jump label (static key) support
+ *
+ * Implements runtime patching of static key sites by replacing
+ * a NOP instruction with an unconditional branch and vice versa.
+ *
+ * Copyright (C) 2026 Magnus Lindholm <[email protected]>
+ */
+
+#include <linux/jump_label.h>
+#include <linux/kernel.h>
+#include <linux/mutex.h>
+#include <asm/cacheflush.h>
+
+/*
+ * Alpha instruction encoding helpers.
+ *
+ * Branch format:
+ *   [31:26] opcode
+ *   [25:21] Ra
+ *   [20:0 ] disp (signed, in instructions; hardware multiplies by 4)
+ *
+ * Unconditional branch:
+ *   BR opcode is 0x30.  We use Ra=r31 so no link register is written.
+ *
+ * Updated PC semantics:
+ *   Target = (pc + 4) + (disp << 2)
+ * so disp = (target - (pc + 4)) >> 2.
+ */
+#define ALPHA_OP_BR	0x30
+#define ALPHA_RA_R31	31
+#define ALPHA_BR_DISP_MASK	((1u << 21) - 1)
+
+#define ALPHA_INSN_NOP	0x47FF041Fu /* BIS r31,r31,r31 */ /* common Alpha NOP */
+
+static inline u32 alpha_br_insn(unsigned long pc, unsigned long target)
+{
+	long off_bytes = (long)target - (long)(pc + 4);
+	long disp = off_bytes >> 2;
+
+	/*
+	 * 21-bit signed displacement: range is [-2^20, 2^20-1] instructions.
+	 * If this trips, the site/target are too far apart for a BR.
+	 */
+	if (disp < -(1L << 20) || disp > ((1L << 20) - 1)) {
+		/*
+		 * Most arches WARN and fall back to something else (or BUG),
+		 * but jump-label sites are expected to be in range.
+		 */
+		WARN_ON_ONCE(1);
+		disp = 0;
+	}
+
+	return (ALPHA_OP_BR << 26) |
+	       (ALPHA_RA_R31 << 21) |
+	       ((u32)disp & ALPHA_BR_DISP_MASK);
+}
+
+static inline void alpha_patch_text(u32 *site, u32 insn)
+{
+	WRITE_ONCE(*site, insn);
+	/*
+	 * Alpha needs an I-cache sync after patching executable text.
+	 */
+	flush_icache_range((unsigned long)site, (unsigned long)site + sizeof(*site));
+}
+
+void arch_jump_label_transform(struct jump_entry *entry,
+			       enum jump_label_type type)
+{
+	u32 *site = (u32 *)jump_entry_code(entry);
+	u32 insn;
+
+	if (type == JUMP_LABEL_JMP)
+		insn = alpha_br_insn((unsigned long)site, jump_entry_target(entry));
+	else
+		insn = ALPHA_INSN_NOP;
+
+	alpha_patch_text(site, insn);
+}
-- 
2.52.0