Re: nop5-optimized USDTs WAS: Re: [PATCHv6 perf/core 09/22] uprobes/x86: Add uprobe syscall to speed up uprobe

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Sep 05, 2025 at 10:24:47AM +0200, Peter Zijlstra wrote:
> +bool insn_is_nop(struct insn *insn)
> +{
> +	u8 rex, rex_b = 0, rex_x = 0, rex_r = 0, rex_w = 0;
> +	u8 modrm, modrm_mod, modrm_reg, modrm_rm;
> +	u8 sib = 0, sib_scale, sib_index, sib_base;
> +
> +	if (insn->rex_prefix.nbytes) {
> +		rex = insn->rex_prefix.bytes[0];
> +		rex_w = !!X86_REX_W(rex);
> +		rex_r = !!X86_REX_R(rex);
> +		rex_x = !!X86_REX_X(rex);
> +		rex_b = !!X86_REX_B(rex);
> +	}
> +
> +	if (insn->modrm.nbytes) {
> +		modrm = insn->modrm.bytes[0];
> +		modrm_mod = X86_MODRM_MOD(modrm);
> +		modrm_reg = X86_MODRM_REG(modrm) + 8*rex_r;
> +		modrm_rm  = X86_MODRM_RM(modrm)  + 8*rex_b;
> +	}
> +
> +	if (insn->sib.nbytes) {
> +		sib = insn->sib.bytes[0];
> +		sib_scale = X86_SIB_SCALE(sib);
> +		sib_index = X86_SIB_INDEX(sib) + 8*rex_x;
> +		sib_base  = X86_SIB_BASE(sib)  + 8*rex_b;
> +
> +		modrm_rm = sib_base;
> +	}
> +
> +	switch (insn->opcode.bytes[0]) {
> +	case 0x0f: /* 2nd byte */
> +		break;
> +
> +	case 0x89: /* MOV */
> +		if (modrm_mod != 3) /* register-direct */
> +			return false;
> +
> +		if (insn->x86_64 && !rex_w) /* native size */
> +			return false;
> +
> +		for (int i = 0; i < insn->prefixes.nbytes; i++) {
> +			if (insn->prefixes.bytes[i] == 0x66) /* OSP */
> +				return false;
> +		}
> +
> +		return modrm_reg == modrm_rm; /* MOV %reg, %reg */
> +
> +	case 0x8d: /* LEA */
> +		if (modrm_mod == 0 || modrm_mod == 3) /* register-indirect with disp */
> +			return false;
> +
> +		if (insn->x86_64 && !rex_w) /* native size */
> +			return false;
> +
> +		if (insn->displacement.value != 0)
> +			return false;
> +
> +		if (sib & (sib_scale != 0 || sib_index != 4)) /* (%reg, %eiz, 1) */

Argh, that should obviously be: &&

> +			return false;
> +
> +		for (int i = 0; i < insn->prefixes.nbytes; i++) {
> +			if (insn->prefixes.bytes[i] != 0x3e) /* DS */
> +				return false;
> +		}
> +
> +		return modrm_reg == modrm_rm; /* LEA 0(%reg), %reg */
> +
> +	case 0x90: /* NOP */
> +		for (int i = 0; i < insn->prefixes.nbytes; i++) {
> +			if (insn->prefixes.bytes[i] == 0xf3) /* REP */
> +				return false; /* REP NOP -- PAUSE */
> +		}
> +		return true;
> +
> +	case 0xe9: /* JMP.d32 */
> +	case 0xeb: /* JMP.d8 */
> +		return insn->immediate.value == 0; /* JMP +0 */
> +
> +	default:
> +		return false;
> +	}
> +
> +	switch (insn->opcode.bytes[1]) {
> +	case 0x1f:
> +		return modrm_reg == 0; /* 0f 1f /0 -- NOPL */
> +
> +	default:
> +		return false;
> +	}
> +}