Re: [PATCH v2 3/3] x86/kprobes: Boost more instructions from grp2/3/4/5

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

On Sun, 4 Feb 2024 22:39:32 -0600
Jinghao Jia <jinghao7@illinois.edu> wrote:

> On 2/4/24 06:09, Masami Hiramatsu (Google) wrote:
> > On Sat,  3 Feb 2024 21:13:00 -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.
> >>
> > 
> > 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 <jinghao7@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) <mhiramat@kernel.org>