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

[Jiri Olsa <olsajiri@gmail.com>]

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

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
> >