linux-kernel - Re: [RFC perf/core 05/11] uprobes: Add mapping for optimized uprobe trampolines

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Message-ID: <Zzyrnvnw1I8HfAYN@krava>
Date: Tue, 19 Nov 2024 16:15:42 +0100
From: Jiri Olsa <olsajiri@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Jiri Olsa <olsajiri@...il.com>, Oleg Nesterov <oleg@...hat.com>,
	Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.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>,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [RFC perf/core 05/11] uprobes: Add mapping for optimized uprobe
 trampolines

On Tue, Nov 19, 2024 at 10:13:48AM +0100, Peter Zijlstra wrote:
> On Mon, Nov 18, 2024 at 10:06:51PM -0800, Andrii Nakryiko wrote:
> 
> > > > Jiri, we could also have an option to support 64-bit call, right? We'd
> > > > need nop9 for that, but it's an option as well to future-proofing this
> > > > approach, no?
> > >
> > > hm, I don't think there's call with relative 64bit offset
> > 
> > why do you need a relative, when you have 64 bits? ;) there is a call
> > to absolute address, no?
> 
> No, there is not :/ You get to use an indirect call, which means
> multiple instructions and all the speculation joy.
> 
> IFF USDT thingies have AX clobbered (I couldn't find in a hurry) then
> patching the multi instruction thing is relatively straight forward, if
> they don't, its going to be a pain.

I don't follow, what's the reason for that?

jirka