linux-kernel - Re: [PATCH perf/core 11/22] selftests/bpf: Use 5-byte nop for x86 usdt probes

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Message-ID: <CAEf4BzZvwH2GR6cr8EN8Up02tHBkGij_1v6UNPcKaVFATmKvUQ@mail.gmail.com>
Date: Thu, 24 Apr 2025 11:20:11 -0700
From: Andrii Nakryiko <andrii.nakryiko@...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@...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>, David Laight <David.Laight@...lab.com>, 
	Thomas Weißschuh <thomas@...ch.de>, 
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH perf/core 11/22] selftests/bpf: Use 5-byte nop for x86
 usdt probes

On Thu, Apr 24, 2025 at 9:29 AM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Thu, Apr 24, 2025 at 5:49 AM Jiri Olsa <olsajiri@...il.com> wrote:
> >
> > On Wed, Apr 23, 2025 at 10:33:18AM -0700, Andrii Nakryiko wrote:
> > > On Mon, Apr 21, 2025 at 2:46 PM Jiri Olsa <jolsa@...nel.org> wrote:
> > > >
> > > > Using 5-byte nop for x86 usdt probes so we can switch
> > > > to optimized uprobe them.
> > > >
> > > > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > > > ---
> > > >  tools/testing/selftests/bpf/sdt.h | 9 ++++++++-
> > > >  1 file changed, 8 insertions(+), 1 deletion(-)
> > > >
> > >
> > > So sdt.h is an exact copy/paste from systemtap-sdt sources. I'd prefer
> > > to not modify it unnecessarily.
> > >
> > > How about we copy/paste usdt.h ([0]) and use *that* for your
> > > benchmarks? I've already anticipated the need to change nop
> > > instruction, so you won't even need to modify the usdt.h file itself,
> > > just
> > >
> > > #define USDT_NOP .byte 0x0f, 0x1f, 0x44, 0x00, 0x00
> > >
> > > before #include "usdt.h"
> >
> >
> > sounds good, but it seems we need bit more changes for that,
> > so far I ended up with:
> >
> > -       __usdt_asm1(990:        USDT_NOP)                                                       \
> > +       __usdt_asm5(990:        USDT_NOP)                                                       \
> >
> > but it still won't compile, will need to spend more time on that,
> > unless you have better solution
> >
>
> Use
>
> #define USDT_NOP .ascii "\x0F\x1F\x44\x00\x00"
>
> for now, I'll need to improve macro magic to handle instructions with
> commas in them...

Ok, fixed in [0]. If you get the latest version, the .byte approach
will work (I have tests in CI now to validate this).

  [0] https://github.com/libbpf/usdt/pull/12

>
> > thanks,
> > jirka
> >
> > >
> > >
> > >   [0] https://github.com/libbpf/usdt/blob/main/usdt.h
> > >
> > > > diff --git a/tools/testing/selftests/bpf/sdt.h b/tools/testing/selftests/bpf/sdt.h
> > > > index 1fcfa5160231..1d62c06f5ddc 100644
> > > > --- a/tools/testing/selftests/bpf/sdt.h
> > > > +++ b/tools/testing/selftests/bpf/sdt.h
> > > > @@ -236,6 +236,13 @@ __extension__ extern unsigned long long __sdt_unsp;
> > > >  #define _SDT_NOP       nop
> > > >  #endif
> > > >
> > > > +/* Use 5 byte nop for x86_64 to allow optimizing uprobes. */
> > > > +#if defined(__x86_64__)
> > > > +# define _SDT_DEF_NOP _SDT_ASM_5(990:  .byte 0x0f, 0x1f, 0x44, 0x00, 0x00)
> > > > +#else
> > > > +# define _SDT_DEF_NOP _SDT_ASM_1(990:  _SDT_NOP)
> > > > +#endif
> > > > +
> > > >  #define _SDT_NOTE_NAME "stapsdt"
> > > >  #define _SDT_NOTE_TYPE 3
> > > >
> > > > @@ -288,7 +295,7 @@ __extension__ extern unsigned long long __sdt_unsp;
> > > >
> > > >  #define _SDT_ASM_BODY(provider, name, pack_args, args, ...)                  \
> > > >    _SDT_DEF_MACROS                                                            \
> > > > -  _SDT_ASM_1(990:      _SDT_NOP)                                             \
> > > > +  _SDT_DEF_NOP                                                               \
> > > >    _SDT_ASM_3(          .pushsection .note.stapsdt,_SDT_ASM_AUTOGROUP,"note") \
> > > >    _SDT_ASM_1(          .balign 4)                                            \
> > > >    _SDT_ASM_3(          .4byte 992f-991f, 994f-993f, _SDT_NOTE_TYPE)          \
> > > > --
> > > > 2.49.0
> > > >