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

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

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. */

> +		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>