linux-kernel - Re: [PATCHv3 perf/core 1/2] uprobes/x86: Add support to emulate nop instructions

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Message-ID: <CAEf4BzajT9VHS5erhS0NgVN4hot9m2tW8SijZk1tyyHe9ArFLg@mail.gmail.com>
Date: Mon, 14 Apr 2025 09:13:12 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jiri Olsa <jolsa@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	x86@...nel.org, Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>, 
	John Fastabend <john.fastabend@...il.com>, Hao Luo <haoluo@...gle.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Alan Maguire <alan.maguire@...cle.com>
Subject: Re: [PATCHv3 perf/core 1/2] uprobes/x86: Add support to emulate nop instructions

On Mon, Apr 14, 2025 at 1:36 AM Jiri Olsa <jolsa@...nel.org> wrote:
>
> Adding support to emulate all nop instructions as the original uprobe
> instruction.
>
> This change speeds up uprobe on top of all nop instructions and is a
> preparation for usdt probe optimization, that will be done on top of
> nop5 instruction.
>
> With this change the usdt probe on top of nop5 won't take the performance
> hit compared to usdt probe on top of standard nop instruction.
>
> Suggested-by: Oleg Nesterov <oleg@...hat.com>
> Suggested-by: Andrii Nakryiko <andrii@...nel.org>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
> v3 changes:
>  - use insn->length as index to x86_nops [Andrii]
>
>  arch/x86/kernel/uprobes.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index 9194695662b2..6d383839e839 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -840,6 +840,11 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
>         insn_byte_t p;
>         int i;
>
> +       /* x86_nops[insn->length]; same as jmp with .offs = 0 */
> +       if (insn->length <= ASM_NOP_MAX &&
> +           !memcmp(insn->kaddr, x86_nops[insn->length], insn->length))
> +               goto setup;
> +

LGTM, thanks!

Acked-by: Andrii Nakryiko <andrii@...nel.org>


>         switch (opc1) {
>         case 0xeb:      /* jmp 8 */
>         case 0xe9:      /* jmp 32 */
> --
> 2.49.0
>