linux-kernel - Re: [PATCH RFCv2 08/18] uprobes/x86: Add uprobe syscall to speed up uprobe

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Message-ID: <CAADnVQ+eKFOAM5PWpBJAy4pKokwbCOMgEghxLu92sw+HApBUYA@mail.gmail.com>
Date: Tue, 25 Feb 2025 18:36:54 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Jiri Olsa <olsajiri@...il.com>
Cc: Oleg Nesterov <oleg@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>, X86 ML <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>, David Laight <David.Laight@...lab.com>, 
	Thomas Weißschuh <thomas@...ch.de>
Subject: Re: [PATCH RFCv2 08/18] uprobes/x86: Add uprobe syscall to speed up uprobe

On Tue, Feb 25, 2025 at 10:06 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Feb 25, 2025 at 5:35 AM Jiri Olsa <olsajiri@...il.com> wrote:
> >
> > > In later patches I see nop5 is replaced with a call to
> > > uprobe_trampoline_entry, but which part saves
> > > rdi and other regs?
> > > Compiler doesn't automatically spill/fill around USDT's nop/nop5.
> > > Selftest is doing:
> > > +__naked noinline void uprobe_test(void)
> > > so just lucky ?
> >
> > if you mean registers that would carry usdt arguments, ebpf programs
> > access those based on assembler operand string stored in usdt record:
>
> No. I'm talking about all normal registers that trap-style uprobe
> preserves, but this nop5->call will scratch.
> Instead of void uprobe_test(void)
> add some arguments to it, and read them before and after nop5 uprobe.
> They must remain the same.

Ignore me. It's a syscall insn. All fine.