[PATCH v3 1/2] riscv: kprobes: simulate nop and c.nop instructions

[Xiaofeng Yuan <yuanxiaofeng@eswincomputing.com>]

A kprobe or uprobe placed on a nop currently replays the instruction
out-of-line: the breakpoint trap runs the probe handler and redirects
execution to a copy of the instruction sitting in an XOL slot, and a
second breakpoint appended after the copy traps again to finish the
hit.  That costs an extra exception round-trip per hit; for uprobes
the slot runs in user mode, so the re-trap is a full
kernel<->userspace round-trip.

nop and c.nop have no architectural effect, so the replay can be
replaced by simply advancing the program counter when handling the
initial breakpoint, completing every hit within a single trap.
arm64 does the same in its probe-decode path, which is also shared
between kprobes and uprobes; see commit ac4ad5c09b34 ("arm64: insn:
Simulate nop instruction for better uprobe performance").

riscv_probe_decode_insn() is shared by kprobes and uprobes, so the
simulation applies to both.  It primarily matters for uprobes on
USDT probe sites: under the SystemTap SDT ABI (sys/sdt.h, parsed by
libbpf), the recorded probe location is by construction a plain nop
or c.nop, the same case that motivated the arm64 series.

Measured on QEMU (RISC-V virt, emulated): for a uprobe on a USDT
style nop site the per-hit cost drops by roughly 80 percent; for a
kprobe on a nop by roughly 40 percent.  On real arm64 hardware the
referenced commit measured ~2x.

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@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


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