[RFC] riscv: Add jump-label implementation

[Emil Renner Berthing]

Add basic jump-label implementation heavily based
on the ARM64 version.

Tested on the HiFive Unleashed.

Signed-off-by: Emil Renner Berthing <kernel at esmil.dk>
---

This seems to work on my HiFive Unleashed. At least boots, runs fine
and the static key self-tests doesn't complain, but I'm sure I've missed
something, so I'm sending this as an RFC.

/Emil

 .../core/jump-labels/arch-support.txt         |  2 +-
 arch/riscv/Kconfig                            |  2 +
 arch/riscv/include/asm/jump_label.h           | 59 +++++++++++++++++++
 arch/riscv/kernel/Makefile                    |  2 +
 arch/riscv/kernel/jump_label.c                | 44 ++++++++++++++
 5 files changed, 108 insertions(+), 1 deletion(-)
 create mode 100644 arch/riscv/include/asm/jump_label.h
 create mode 100644 arch/riscv/kernel/jump_label.c

diff --git a/Documentation/features/core/jump-labels/arch-support.txt b/Documentation/features/core/jump-labels/arch-support.txt
index 632a1c7aefa2..760243d18ed7 100644
--- a/Documentation/features/core/jump-labels/arch-support.txt
+++ b/Documentation/features/core/jump-labels/arch-support.txt
@@ -23,7 +23,7 @@
     |    openrisc: | TODO |
     |      parisc: |  ok  |
     |     powerpc: |  ok  |
-    |       riscv: | TODO |
+    |       riscv: |  ok  |
     |        s390: |  ok  |
     |          sh: | TODO |
     |       sparc: |  ok  |
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index fd639937e251..d2f5c53fdc19 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -46,6 +46,8 @@ config RISCV
 	select GENERIC_TIME_VSYSCALL if MMU && 64BIT
 	select HANDLE_DOMAIN_IRQ
 	select HAVE_ARCH_AUDITSYSCALL
+	select HAVE_ARCH_JUMP_LABEL
+	select HAVE_ARCH_JUMP_LABEL_RELATIVE
 	select HAVE_ARCH_KASAN if MMU && 64BIT
 	select HAVE_ARCH_KGDB
 	select HAVE_ARCH_KGDB_QXFER_PKT
diff --git a/arch/riscv/include/asm/jump_label.h b/arch/riscv/include/asm/jump_label.h
new file mode 100644
index 000000000000..29be6d351866
--- /dev/null
+++ b/arch/riscv/include/asm/jump_label.h
@@ -0,0 +1,59 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2020 Emil Renner Berthing
+ *
+ * Based on arch/arm64/include/asm/jump_label.h
+ */
+#ifndef __ASM_JUMP_LABEL_H
+#define __ASM_JUMP_LABEL_H
+
+#ifndef __ASSEMBLY__
+
+#include <linux/types.h>
+
+#define JUMP_LABEL_NOP_SIZE 4
+
+static __always_inline bool arch_static_branch(struct static_key *key,
+					       bool branch)
+{
+	asm_volatile_goto(
+		"	.option push				\n\n"
+		"	.option norelax				\n\n"
+		"	.option norvc				\n\n"
+		"1:	nop					\n\t"
+		"	.option pop				\n\n"
+		"	.pushsection	__jump_table, \"aw\"	\n\t"
+		"	.align		3			\n\t"
+		"	.long		1b - ., %l[l_yes] - .	\n\t"
+		"	.quad		%0 - .			\n\t"
+		"	.popsection				\n\t"
+		:  :  "i"(&((char *)key)[branch ? 1 : 0]) :  : l_yes);
+
+	return false;
+l_yes:
+	return true;
+}
+
+static __always_inline bool arch_static_branch_jump(struct static_key *key,
+						    bool branch)
+{
+	asm_volatile_goto(
+		"	.option push				\n\n"
+		"	.option norelax				\n\n"
+		"	.option norvc				\n\n"
+		"1:	jal		zero, %l[l_yes]		\n\t"
+		"	.option pop				\n\n"
+		"	.pushsection	__jump_table, \"aw\"	\n\t"
+		"	.align		3			\n\t"
+		"	.long		1b - ., %l[l_yes] - .	\n\t"
+		"	.quad		%0 - .			\n\t"
+		"	.popsection				\n\t"
+		:  :  "i"(&((char *)key)[branch ? 1 : 0]) :  : l_yes);
+
+	return false;
+l_yes:
+	return true;
+}
+
+#endif  /* __ASSEMBLY__ */
+#endif	/* __ASM_JUMP_LABEL_H */
diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
index b355cf485671..a5287ab9f7f2 100644
--- a/arch/riscv/kernel/Makefile
+++ b/arch/riscv/kernel/Makefile
@@ -53,4 +53,6 @@ endif
 obj-$(CONFIG_HOTPLUG_CPU)	+= cpu-hotplug.o
 obj-$(CONFIG_KGDB)		+= kgdb.o
 
+obj-$(CONFIG_JUMP_LABEL)	+= jump_label.o
+
 clean:
diff --git a/arch/riscv/kernel/jump_label.c b/arch/riscv/kernel/jump_label.c
new file mode 100644
index 000000000000..55b2d742efe1
--- /dev/null
+++ b/arch/riscv/kernel/jump_label.c
@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2020 Emil Renner Berthing
+ *
+ * Based on arch/arm64/kernel/jump_label.c
+ */
+#include <linux/kernel.h>
+#include <linux/jump_label.h>
+#include <asm/patch.h>
+
+#define RISCV_INSN_NOP 0x00000013
+#define RISCV_INSN_JAL 0x0000006f
+
+void arch_jump_label_transform(struct jump_entry *entry,
+			       enum jump_label_type type)
+{
+	void *addr = (void *)jump_entry_code(entry);
+	u32 insn;
+
+	if (type == JUMP_LABEL_JMP) {
+		u32 offset = jump_entry_target(entry) - jump_entry_code(entry);
+
+		insn = RISCV_INSN_JAL |
+			((offset & GENMASK(19, 12)) << (12 - 12)) |
+			((offset & GENMASK(11, 11)) << (20 - 11)) |
+			((offset & GENMASK(10,  1)) << (21 -  1)) |
+			((offset & GENMASK(20, 20)) << (31 - 20));
+	} else
+		insn = RISCV_INSN_NOP;
+
+	patch_text_nosync(addr, &insn, sizeof(insn));
+}
+
+void arch_jump_label_transform_static(struct jump_entry *entry,
+				      enum jump_label_type type)
+{
+	/*
+	 * We use the same instructions in the arch_static_branch and
+	 * arch_static_branch_jump inline functions, so there's no
+	 * need to patch them up here.
+	 * The core will call arch_jump_label_transform  when those
+	 * instructions need to be replaced.
+	 */
+}
-- 
2.27.0