[PATCH 1/2] riscv: kprobes: simulate nop and c.nop instructions

[Xiaofeng Yuan]

nop and c.nop have no architectural effect, so allocating an
out-of-line instruction slot and single-stepping through them is
pure overhead. Simulate them directly in the breakpoint handler
by advancing the program counter, following the approach already
used on arm64.

This avoids slot allocation, text patching, IRQ flag save/restore
and the single-step exception for these instructions.

Compile tested on RISC-V, and verified with the RISC-V kprobes
KUnit test which now covers nop and c.nop.

Signed-off-by: Xiaofeng Yuan <yuanxiaofeng at eswincomputing.com>
---
 arch/riscv/include/asm/insn.h            | 11 +++++++++++
 arch/riscv/kernel/probes/decode-insn.c   |  6 ++++++
 arch/riscv/kernel/probes/simulate-insn.c | 14 ++++++++++++++
 arch/riscv/kernel/probes/simulate-insn.h |  2 ++
 4 files changed, 33 insertions(+)

diff --git a/arch/riscv/include/asm/insn.h b/arch/riscv/include/asm/insn.h
index c3005573e8..5b3944a5fa 100644
--- a/arch/riscv/include/asm/insn.h
+++ b/arch/riscv/include/asm/insn.h
@@ -228,6 +228,15 @@
 #define RVG_MASK_EBREAK		0xffffffff
 #define RVG_MASK_SRET		0xffffffff
 
+/*
+ * NOP and C.NOP have no variable fields, so all bits must match.
+ * NOP is encoded as ADDI x0, x0, 0 (0x00000013), C.NOP as C.ADDI x0, 0 (0x0001).
+ */
+#define RVG_MATCH_NOP		0x00000013
+#define RVG_MASK_NOP		0xffffffff
+#define RVC_MATCH_C_NOP		0x0001
+#define RVC_MASK_C_NOP		0xffff
+
 #define __INSN_LENGTH_MASK	_UL(0x3)
 #define __INSN_LENGTH_GE_32	_UL(0x3)
 #define __INSN_OPCODE_MASK	_UL(0x7F)
@@ -262,6 +271,8 @@ __RISCV_INSN_FUNCS(c_ebreak, RVC_MASK_C_EBREAK, RVC_MATCH_C_EBREAK)
 __RISCV_INSN_FUNCS(ebreak, RVG_MASK_EBREAK, RVG_MATCH_EBREAK)
 __RISCV_INSN_FUNCS(sret, RVG_MASK_SRET, RVG_MATCH_SRET)
 __RISCV_INSN_FUNCS(fence, RVG_MASK_FENCE, RVG_MATCH_FENCE);
+__RISCV_INSN_FUNCS(nop, RVG_MASK_NOP, RVG_MATCH_NOP)
+__RISCV_INSN_FUNCS(c_nop, RVC_MASK_C_NOP, RVC_MATCH_C_NOP)
 
 /* special case to catch _any_ system instruction */
 static __always_inline bool riscv_insn_is_system(u32 code)
diff --git a/arch/riscv/kernel/probes/decode-insn.c b/arch/riscv/kernel/probes/decode-insn.c
index 65d9590bfb..d28408f0b7 100644
--- a/arch/riscv/kernel/probes/decode-insn.c
+++ b/arch/riscv/kernel/probes/decode-insn.c
@@ -44,5 +44,11 @@ riscv_probe_decode_insn(probe_opcode_t *addr, struct arch_probe_insn *api)
 	RISCV_INSN_SET_SIMULATE(auipc,		insn);
 	RISCV_INSN_SET_SIMULATE(branch,		insn);
 
+	/* Simulate NOP for better performance */
+	RISCV_INSN_SET_SIMULATE(nop,		insn);
+#ifdef CONFIG_RISCV_ISA_C
+	RISCV_INSN_SET_SIMULATE(c_nop,		insn);
+#endif
+
 	return INSN_GOOD;
 }
diff --git a/arch/riscv/kernel/probes/simulate-insn.c b/arch/riscv/kernel/probes/simulate-insn.c
index fa581590c1..3b22d3ad53 100644
--- a/arch/riscv/kernel/probes/simulate-insn.c
+++ b/arch/riscv/kernel/probes/simulate-insn.c
@@ -237,3 +237,17 @@ bool __kprobes simulate_c_beqz(u32 opcode, unsigned long addr, struct pt_regs *r
 {
 	return simulate_c_bnez_beqz(opcode, addr, regs, false);
 }
+
+bool __kprobes simulate_nop(u32 opcode, unsigned long addr, struct pt_regs *regs)
+{
+	instruction_pointer_set(regs, addr + 4);
+
+	return true;
+}
+
+bool __kprobes simulate_c_nop(u32 opcode, unsigned long addr, struct pt_regs *regs)
+{
+	instruction_pointer_set(regs, addr + 2);
+
+	return true;
+}
diff --git a/arch/riscv/kernel/probes/simulate-insn.h b/arch/riscv/kernel/probes/simulate-insn.h
index 44ebbc444d..7e0936613f 100644
--- a/arch/riscv/kernel/probes/simulate-insn.h
+++ b/arch/riscv/kernel/probes/simulate-insn.h
@@ -29,5 +29,7 @@ bool simulate_c_jr(u32 opcode, unsigned long addr, struct pt_regs *regs);
 bool simulate_c_jalr(u32 opcode, unsigned long addr, struct pt_regs *regs);
 bool simulate_c_bnez(u32 opcode, unsigned long addr, struct pt_regs *regs);
 bool simulate_c_beqz(u32 opcode, unsigned long addr, struct pt_regs *regs);
+bool simulate_nop(u32 opcode, unsigned long addr, struct pt_regs *regs);
+bool simulate_c_nop(u32 opcode, unsigned long addr, struct pt_regs *regs);
 
 #endif /* _RISCV_KERNEL_PROBES_SIMULATE_INSN_H */
-- 
2.43.0