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Date: Thu, 4 Sep 2025 22:52:10 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Jiri Olsa <olsajiri@gmail.com>, Oleg Nesterov <oleg@redhat.com>,
	Andrii Nakryiko <andrii@kernel.org>, bpf <bpf@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	Linux trace kernel <linux-trace-kernel@vger.kernel.org>,
	X86 ML <x86@kernel.org>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	Hao Luo <haoluo@google.com>, Steven Rostedt <rostedt@goodmis.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Alan Maguire <alan.maguire@oracle.com>,
	David Laight <David.Laight@aculab.com>,
	Thomas =?iso-8859-1?Q?Wei=DFschuh?= <thomas@t-8ch.de>,
	Ingo Molnar <mingo@kernel.org>
Subject: Re: nop5-optimized USDTs WAS: Re: [PATCHv6 perf/core 09/22]
 uprobes/x86: Add uprobe syscall to speed up uprobe
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On Thu, Sep 04, 2025 at 01:49:49PM -0700, Andrii Nakryiko wrote:
> On Thu, Sep 4, 2025 at 1:35 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Thu, Sep 04, 2025 at 11:27:45AM -0700, Andrii Nakryiko wrote:
> >
> > > > > So I've been thinking what's the simplest and most reliable way to
> > > > > feature-detect support for this sys_uprobe (e.g., for libbpf to know
> > > > > whether we should attach at nop5 vs nop1), and clearly that would be
> > > >
> > > > wrt nop5/nop1.. so the idea is to have USDT macro emit both nop1,nop5
> > > > and store some info about that in the usdt's elf note, right?
> >
> > Wait, what? You're doing to emit 6 bytes and two nops? Why? Surely the
> > old kernel can INT3 on top of a NOP5?
> >
> 
> Yes it can, but it's 2x slower in terms of uprobe triggering compared
> to nop1. 

Why? That doesn't really make sense.

I realize its probably to late to fix the old kernel not to be stupid --
this must be something stupid, right? But now I need to know.