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 <jingh...@illinois.edu>
---
arch/x86/kernel/kprobes/core.c | 23 +++++++++++++++++------
1 file changed, 17 insertions(+), 6 deletions(-)
diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 7a08d6a486c8..530f6d4b34f4 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -169,22 +169,33 @@ bool 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 false;
+ case 0xc0 ... 0xc1: /* Grp2 */
+ case 0xd0 ... 0xd3: /* Grp2 */
+ /*
+ * AMD uses nnn == 110 as SHL/SAL, but Intel makes it reserved.
+ */
+ return X86_MODRM_REG(insn->modrm.bytes[0]) != 0b110;
+ case 0xf6 ... 0xf7: /* Grp3 */
+ /* AMD uses nnn == 001 as TEST, but Intel makes it reserved. */
+ return X86_MODRM_REG(insn->modrm.bytes[0]) != 0b001;
+ case 0xfe: /* Grp4 */
+ /* Only INC and DEC are boostable */
+ return X86_MODRM_REG(insn->modrm.bytes[0]) == 0b000 ||
+ X86_MODRM_REG(insn->modrm.bytes[0]) == 0b001;
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]) == 0b000 ||
+ X86_MODRM_REG(insn->modrm.bytes[0]) == 0b001 ||
+ X86_MODRM_REG(insn->modrm.bytes[0]) == 0b100;
default:
return true;
}
--
2.43.0
