On Sun, 4 Feb 2024 22:39:32 -0600 Jinghao Jia <jingh...@illinois.edu> wrote:
> On 2/4/24 06:09, Masami Hiramatsu (Google) wrote: > > On Sat, 3 Feb 2024 21:13:00 -0600 > > Jinghao Jia <jingh...@illinois.edu> wrote: > > > >> 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. > >> > > > > This looks good to me. And can you check how many instructions in the > > vmlinux will be covered by this change typically? > > > > I collected the stats from the LLVM CodeGen backend on kernel version 6.7.3 > using Gentoo's dist-kernel config (with a mod2yesconfig to make modules > builtin) and here are the number of Grp 2/3/4/5 instructions that are newly > covered by this patch: > > Kernel total # of insns: 28552017 (from objdump) > Grp2 insns: 286249 (from LLVM) > Grp3 insns: 286556 (from LLVM) > Grp4 insns: 5832 (from LLVM) > Grp5 insns: 146314 (from LLVM) > > Note that using LLVM means we miss the stats from inline assembly and > assembly source files. Thanks for checking! so it increases the coverage ~2.5% :) Thank you, > > --Jinghao > > > Thank you, > > > >> 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 > >> > > > > -- Masami Hiramatsu (Google) <mhira...@kernel.org>
