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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Sun, 28 Jan 2024 15:30:50 -0600
Jinghao Jia <jinghao7@illinois.edu> wrote:

> 
> 
> On 1/27/24 20:22, Masami Hiramatsu (Google) wrote:
> > On Fri, 26 Jan 2024 22:41:24 -0600
> > Jinghao Jia <jinghao7@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.
> >>
> > 
> > As far as we check the ModR/M byte, I think we can safely run these
> > instructions on trampoline buffer without adjusting results (this
> > means it can be "boosted").
> > I just have a minor comment, but basically this looks good to me.
> > 
> > Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > 
> >> Signed-off-by: Jinghao Jia <jinghao7@illinois.edu>
> >> ---
> >>  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 */
> > 
> > 		/* AMD uses nnn == 001 as TEST, but Intel makes it reserved. */
> > 
> 
> I will incorporate this into the v2. Since nnn == 001 is still considered
> reserved by Intel, we still need to prevent it from being boosted, don't
> we?
> 
> --Jinghao
> 
> >> +		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
> >>
> > 
> > Thamnk you,
> > 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>