[RFC PATCH 2/2] x86/kprobes: boost more instructions from grp2/3/4/5

[Jinghao Jia]

With the instruction decoder, we are now able to decode and recognize
instructions with opcode extensions. There are more instructions in
these groups that can be boosted:

Group 2: ROL, ROR, RCL, RCR, SHL/SAL, SHR, SAR
Group 3: TEST, NOT, NEG, MUL, IMUL, DIV, IDIV
Group 4: INC, DEC (byte operation)
Group 5: INC, DEC (word/doubleword/quadword operation)

These instructions are not boosted previously because there are reserved
opcodes within the groups, e.g., group 2 with ModR/M.nnn == 110 is
unmapped. As a result, kprobes attached to them requires two int3 traps
as being non-boostable also prevents jump-optimization.

Some simple tests on QEMU show that after boosting and jump-optimization
a single kprobe on these instructions with an empty pre-handler runs 10x
faster (~1000 cycles vs. ~100 cycles).

Since these instructions are mostly ALU operations and do not touch
special registers like RIP, let's boost them so that we get the
performance benefit.

Signed-off-by: Jinghao Jia <jinghao7@xxxxxxxxxxxx>
---
 arch/x86/kernel/kprobes/core.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 792b38d22126..f847bd9cc91b 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -169,22 +169,31 @@ int can_boost(struct insn *insn, void *addr)
 	case 0x62:		/* bound */
 	case 0x70 ... 0x7f:	/* Conditional jumps */
 	case 0x9a:		/* Call far */
-	case 0xc0 ... 0xc1:	/* Grp2 */
 	case 0xcc ... 0xce:	/* software exceptions */
-	case 0xd0 ... 0xd3:	/* Grp2 */
 	case 0xd6:		/* (UD) */
 	case 0xd8 ... 0xdf:	/* ESC */
 	case 0xe0 ... 0xe3:	/* LOOP*, JCXZ */
 	case 0xe8 ... 0xe9:	/* near Call, JMP */
 	case 0xeb:		/* Short JMP */
 	case 0xf0 ... 0xf4:	/* LOCK/REP, HLT */
-	case 0xf6 ... 0xf7:	/* Grp3 */
-	case 0xfe:		/* Grp4 */
 		/* ... are not boostable */
 		return 0;
+	case 0xc0 ... 0xc1:	/* Grp2 */
+	case 0xd0 ... 0xd3:	/* Grp2 */
+		/* ModR/M nnn == 110 is reserved */
+		return X86_MODRM_REG(insn->modrm.bytes[0]) != 6;
+	case 0xf6 ... 0xf7:	/* Grp3 */
+		/* ModR/M nnn == 001 is reserved */
+		return X86_MODRM_REG(insn->modrm.bytes[0]) != 1;
+	case 0xfe:		/* Grp4 */
+		/* Only inc and dec are boostable */
+		return X86_MODRM_REG(insn->modrm.bytes[0]) == 0 ||
+		       X86_MODRM_REG(insn->modrm.bytes[0]) == 1;
 	case 0xff:		/* Grp5 */
-		/* Only indirect jmp is boostable */
-		return X86_MODRM_REG(insn->modrm.bytes[0]) == 4;
+		/* Only inc, dec, and indirect jmp are boostable */
+		return X86_MODRM_REG(insn->modrm.bytes[0]) == 0 ||
+		       X86_MODRM_REG(insn->modrm.bytes[0]) == 1 ||
+		       X86_MODRM_REG(insn->modrm.bytes[0]) == 4;
 	default:
 		return 1;
 	}
-- 
2.43.0