Re: [tip: perf/core] uprobes/x86: Add support to emulate NOP5 instruction

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 09, 2025 at 11:43:19AM -0000, tip-bot2 for Jiri Olsa wrote:
> The following commit has been merged into the perf/core branch of tip:
> 
> Commit-ID:     38440aebd4acc7bb3721eea77829bdb724d2551a
> Gitweb:        https://git.kernel.org/tip/38440aebd4acc7bb3721eea77829bdb724d2551a
> Author:        Jiri Olsa <jolsa@kernel.org>
> AuthorDate:    Tue, 08 Apr 2025 23:13:09 +02:00
> Committer:     Ingo Molnar <mingo@kernel.org>
> CommitterDate: Wed, 09 Apr 2025 12:35:17 +02:00
> 
> uprobes/x86: Add support to emulate NOP5 instruction
> 
> Adding support to emulate NOP5 as the original uprobe instruction.
> 
> This change speeds up uprobe on top of NOP5 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.
> 
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Brian Gerst <brgerst@gmail.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Link: https://lore.kernel.org/r/20250408211310.51491-1-jolsa@kernel.org
> ---
>  arch/x86/kernel/uprobes.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index 9194695..63cc68e 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -608,6 +608,16 @@ static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
>  		*sr = utask->autask.saved_scratch_register;
>  	}
>  }
> +
> +static int is_nop5_insn(uprobe_opcode_t *insn)
> +{
> +	return !memcmp(insn, x86_nops[5], 5);
> +}
> +
> +static bool emulate_nop5_insn(struct arch_uprobe *auprobe)
> +{
> +	return is_nop5_insn((uprobe_opcode_t *) &auprobe->insn);
> +}
>  #else /* 32-bit: */
>  /*
>   * No RIP-relative addressing on 32-bit
> @@ -621,6 +631,10 @@ static void riprel_pre_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
>  static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
>  {
>  }
> +static bool emulate_nop5_insn(struct arch_uprobe *auprobe)
> +{
> +	return false;
> +}
>  #endif /* CONFIG_X86_64 */
>  
>  struct uprobe_xol_ops {
> @@ -852,6 +866,8 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
>  		break;
>  
>  	case 0x0f:
> +		if (emulate_nop5_insn(auprobe))
> +			goto setup;
>  		if (insn->opcode.nbytes != 2)
>  			return -ENOSYS;
>  		/*

This is still very weird code. See the comment here:

https://lkml.kernel.org/r/20241213104536.GZ35539@noisy.programming.kicks-ass.net