Re: [PATCH bpf-next 09/13] selftests/bpf: Use 5-byte nop for x86 usdt probes

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Dec 16, 2024 at 12:32 AM Jiri Olsa <olsajiri@gmail.com> wrote:
>
> On Fri, Dec 13, 2024 at 01:58:33PM -0800, Andrii Nakryiko wrote:
> > On Wed, Dec 11, 2024 at 5:35 AM 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(-)
> > >
> >
> > This change will make it impossible to run latest selftests on older
> > kernels. Let's do what we did with -DENABLE_ATOMICS_TESTS and allow to
> > disable this through Makefile, ok?
>
> ok, I wanted to start addressing this after this version
>
> so the problem is using this macro with nop5 in application running
> on older kernels that do not have nop5 emulation and uprobe syscall
> optimization, because uprobe/usdt on top of nop5 (single-stepped)
> will be slower than on top of current nop1 (emulated)
>
> AFAICS selftests should still work, just bit slower due to nop5 emulation

ah, fair enough, it won't break anything, true

>
> one part of the solution would be to backport [1] to stable kernels
> which is an easy fix (even though it needs changes now)
>
> if that's not enough we'd need to come up with that nop1/nop5 macro
> solution, where tooling (libbpf with extra data in usdt note) would
> install uprobe on top of nop1 on older kernels and on top of nop5 on
> new ones.. but that'd need more work of course

yes, I think that's the way we should go, but as you said, that's
outside of kernel and we should work on that as a follow up to this
series

>
> jirka
>
>
> [1] patch#7 - uprobes/x86: Add support to emulate nop5 instruction
>
>
> >
> >
> > > diff --git a/tools/testing/selftests/bpf/sdt.h b/tools/testing/selftests/bpf/sdt.h
> > > index ca0162b4dc57..7ac9291f45f1 100644
> > > --- a/tools/testing/selftests/bpf/sdt.h
> > > +++ b/tools/testing/selftests/bpf/sdt.h
> > > @@ -234,6 +234,13 @@ __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
> > >
> > > @@ -286,7 +293,7 @@ __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)          \
> > > --
> > > 2.47.0
> > >