* [PATCH 1/1] alpha: Add support for HAVE_ARCH_JUMP_LABEL
2026-02-19 16:58 [PATCH 0/1] Add support for HAVE_ARCH_JUMP_LABEL Magnus Lindholm
@ 2026-02-19 16:58 ` Magnus Lindholm
0 siblings, 0 replies; 2+ messages in thread
From: Magnus Lindholm @ 2026-02-19 16:58 UTC (permalink / raw)
To: richard.henderson, mattst88, glaubitz, macro, macro, mcree, ink,
linux-kernel, linux-alpha
Cc: Magnus Lindholm
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 <linmag7@gmail.com>
---
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 <linmag7@gmail.com>
+ */
+
+
+#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 <linmag7@gmail.com>
+ */
+
+#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
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