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

Andrii Nakryiko
2025-04-24T16:29:49Z

On Thu, Apr 24, 2025 at 5:49 AM Jiri Olsa <olsajiri@gmail.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@kernel.org> wrote:
> > >
> > > Using 5-byte nop for x86 usdt probes so we can switch
> > > to optimized uprobe them.
> > >
> > > Signed-off-by: Jiri Olsa <jolsa@kernel.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...

> 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
> > >