[PATCH -next 2/2] riscv: implemented branch simulate instructions

[Chen Lifu <chenlifu@huawei.com>]

To test the kprobe-based event tracing, we prepare
a kernel module 'kprobe_test.ko' to add the probes.
The assembly codes (partially) of the module are as follows:
...
0000000000000000 <kprobe_test_branch>:
...

0000000000000080 <.L7>:
  80:	414986bb          	subw	a3,s3,s4
  84:	4785                	li	a5,1
  86:	00d7c363          	blt	a5,a3,8c <.L8>
  8a:	2485                	addiw	s1,s1,1

000000000000008c <.L8>:
  8c:	012987bb          	addw	a5,s3,s2
  90:	4711                	li	a4,4
  92:	00f75363          	bge	a4,a5,98 <.L9>
  96:	2485                	addiw	s1,s1,1
...

00000000000000aa <.L11>:
  aa:	01299363          	bne	s3,s2,b0 <.L12>
  ae:	2485                	addiw	s1,s1,1

00000000000000b0 <.L12>:
  b0:	012a0363          	beq	s4,s2,b6 <.L13>
  b4:	2485                	addiw	s1,s1,1
...

00000000000000c2 <.L14>:
  c2:	0009861b          	sext.w	a2,s3
  c6:	013a7363          	bgeu	s4,s3,cc <.L15>
  ca:	2485                	addiw	s1,s1,1
...

00000000000000d2 <.L16>:
  d2:	00e7e363          	bltu	a5,a4,d8 <.L17>
  d6:	2485                	addiw	s1,s1,1
...

Test the kprobe-based event tracing in qemu-system-riscv64:
First, install the kprobe test module:
insmod /root/kprobe_test.ko

Then, add probes as new events at the branch instructions,
i.e. blt, bge, bne, beq, bgeu and bltu.
The following errors occur due to the instructions not allowed to probe yet:
echo "p:blt kprobe_test:kprobe_test_branch+0x86 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
sh: write error: Invalid argument
echo "p:bge kprobe_test:kprobe_test_branch+0x92 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
sh: write error: Invalid argument
echo "p:bne kprobe_test:kprobe_test_branch+0xaa epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
sh: write error: Invalid argument
echo "p:beq kprobe_test:kprobe_test_branch+0xb0 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
sh: write error: Invalid argument
echo "p:bgeu kprobe_test:kprobe_test_branch+0xc6 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
sh: write error: Invalid argument
echo "p:bltu kprobe_test:kprobe_test_branch+0xd2 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
sh: write error: Invalid argument

This patch implemented the branch simulate instructions and allowed to probe them.
Merge this patch and perform the test again, the test results are as follows:
First, add probes at the branch instructions:
echo "p:blt kprobe_test:kprobe_test_branch+0x86 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
echo "p:bge kprobe_test:kprobe_test_branch+0x92 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
echo "p:bne kprobe_test:kprobe_test_branch+0xaa epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
echo "p:beq kprobe_test:kprobe_test_branch+0xb0 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
echo "p:bgeu kprobe_test:kprobe_test_branch+0xc6 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
echo "p:bltu kprobe_test:kprobe_test_branch+0xd2 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
echo 1 > /sys/kernel/debug/tracing/events/kprobes/blt/enable
echo 1 > /sys/kernel/debug/tracing/events/kprobes/bge/enable
echo 1 > /sys/kernel/debug/tracing/events/kprobes/bne/enable
echo 1 > /sys/kernel/debug/tracing/events/kprobes/beq/enable
echo 1 > /sys/kernel/debug/tracing/events/kprobes/bgeu/enable
echo 1 > /sys/kernel/debug/tracing/events/kprobes/bltu/enable

Then, do something to run to the probes.
After that, see the traced information:
cat /sys/kernel/debug/tracing/trace
sysctl-63      [001] d...  2505.263969: blt: (kprobe_test_branch+0x86/0x10e [kprobe_test]) epc=0xffffffff016122f8 opcode=0x100073
sysctl-63      [001] d...  2505.263991: bge: (kprobe_test_branch+0x92/0x10e [kprobe_test]) epc=0xffffffff01612304 opcode=0x100073
sysctl-63      [001] d...  2505.264001: bne: (kprobe_test_branch+0xaa/0x10e [kprobe_test]) epc=0xffffffff0161231c opcode=0x100073
sysctl-63      [001] d...  2505.264011: beq: (kprobe_test_branch+0xb0/0x10e [kprobe_test]) epc=0xffffffff01612322 opcode=0x100073
sysctl-63      [001] d...  2505.264019: bgeu: (kprobe_test_branch+0xc6/0x10e [kprobe_test]) epc=0xffffffff01612338 opcode=0x100073
sysctl-63      [001] d...  2505.264027: bltu: (kprobe_test_branch+0xd2/0x10e [kprobe_test]) epc=0xffffffff01612344 opcode=0x100073

Now we can see the traced information.
The actual address of the symbol 'kprobe_test_branch' is as follows:
cat /proc/kallsyms | grep kprobe_test_branch
ffffffff01612272 t kprobe_test_branch   [kprobe_test]

Based on the traced information and the actual address of the symbol
'kprobe_test_branch', we can also see that the branch instructions
have been replaced by 'ebreak(0x100073)' instructions.

The pesudoinstructions of the branch instructions,
i.e. bnez, beqz, blez, bgez, bltz, bgtz, bleu, bgtu and ble
are allowed to probe as well.

--------

Signed-off-by: Chen Lifu <chenlifu@huawei.com>
---
 arch/riscv/kernel/probes/decode-insn.c   |  3 +-
 arch/riscv/kernel/probes/simulate-insn.c | 78 ++++++++++++++++++++++++
 2 files changed, 79 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/kernel/probes/decode-insn.c b/arch/riscv/kernel/probes/decode-insn.c
index 5eb03fb61450..64f6183b4717 100644
--- a/arch/riscv/kernel/probes/decode-insn.c
+++ b/arch/riscv/kernel/probes/decode-insn.c
@@ -38,11 +38,10 @@ riscv_probe_decode_insn(probe_opcode_t *addr, struct arch_probe_insn *api)
 	RISCV_INSN_REJECTED(c_ebreak,		insn);
 #endif
 
-	RISCV_INSN_REJECTED(branch,		insn);
-
 	RISCV_INSN_SET_SIMULATE(jal,		insn);
 	RISCV_INSN_SET_SIMULATE(jalr,		insn);
 	RISCV_INSN_SET_SIMULATE(auipc,		insn);
+	RISCV_INSN_SET_SIMULATE(branch,		insn);
 
 	return INSN_GOOD;
 }
diff --git a/arch/riscv/kernel/probes/simulate-insn.c b/arch/riscv/kernel/probes/simulate-insn.c
index b81719522d5c..efc989b28859 100644
--- a/arch/riscv/kernel/probes/simulate-insn.c
+++ b/arch/riscv/kernel/probes/simulate-insn.c
@@ -117,3 +117,81 @@ bool __kprobes simulate_auipc(u32 opcode, unsigned long addr, struct pt_regs *re
 
 	return true;
 }
+
+#define branch_rs1_idx(opcode) \
+	(((opcode) >> 15) & 0x1f)
+
+#define branch_rs2_idx(opcode) \
+	(((opcode) >> 20) & 0x1f)
+
+#define branch_funct3(opcode) \
+	(((opcode) >> 12) & 0x7)
+
+#define branch_imm(opcode) \
+	(((((opcode) >> 8 ) & 0xf ) << 1 ) | \
+	 ((((opcode) >> 25) & 0x3f) << 5 ) | \
+	 ((((opcode) >> 7 ) & 0x1 ) << 11) | \
+	 ((((opcode) >> 31) & 0x1 ) << 12))
+
+#define branch_offset(opcode) \
+	sign_extend32((branch_imm(opcode)), 12)
+
+#define BRANCH_BEQ	0x0
+#define BRANCH_BNE	0x1
+#define BRANCH_BLT	0x4
+#define BRANCH_BGE	0x5
+#define BRANCH_BLTU	0x6
+#define BRANCH_BGEU	0x7
+
+bool __kprobes simulate_branch(u32 opcode, unsigned long addr, struct pt_regs *regs)
+{
+	/*
+	 * branch instructions:
+	 *      31    30       25 24 20 19 15 14    12 11       8    7      6      0
+	 * | imm[12] | imm[10:5] | rs2 | rs1 | funct3 | imm[4:1] | imm[11] | opcode |
+	 *     1           6        5     5      3         4         1         7
+	 *     imm[12|10:5]        rs2   rs1    000       imm[4:1|11]       1100011  BEQ
+	 *     imm[12|10:5]        rs2   rs1    001       imm[4:1|11]       1100011  BNE
+	 *     imm[12|10:5]        rs2   rs1    100       imm[4:1|11]       1100011  BLT
+	 *     imm[12|10:5]        rs2   rs1    101       imm[4:1|11]       1100011  BGE
+	 *     imm[12|10:5]        rs2   rs1    110       imm[4:1|11]       1100011  BLTU
+	 *     imm[12|10:5]        rs2   rs1    111       imm[4:1|11]       1100011  BGEU
+	 */
+
+	s32 offset;
+	s32 offset_tmp;
+	unsigned long rs1_val;
+	unsigned long rs2_val;
+
+	if (!rv_insn_reg_get_val(regs, branch_rs1_idx(opcode), &rs1_val) ||
+	    !rv_insn_reg_get_val(regs, branch_rs2_idx(opcode), &rs2_val))
+		return false;
+
+	offset_tmp = branch_offset(opcode);
+	switch (branch_funct3(opcode)) {
+	case BRANCH_BEQ:
+		offset = (rs1_val == rs2_val) ? offset_tmp : 4;
+		break;
+	case BRANCH_BNE:
+		offset = (rs1_val != rs2_val) ? offset_tmp : 4;
+		break;
+	case BRANCH_BLT:
+		offset = ((long)rs1_val < (long)rs2_val) ? offset_tmp : 4;
+		break;
+	case BRANCH_BGE:
+		offset = ((long)rs1_val >= (long)rs2_val) ? offset_tmp : 4;
+		break;
+	case BRANCH_BLTU:
+		offset = (rs1_val < rs2_val) ? offset_tmp : 4;
+		break;
+	case BRANCH_BGEU:
+		offset = (rs1_val >= rs2_val) ? offset_tmp : 4;
+		break;
+	default:
+		return false;
+	}
+
+	instruction_pointer_set(regs, addr + offset);
+
+	return true;
+}
-- 
2.32.0