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

Andrii Nakryiko
2025-04-23T17:33:35Z

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"

[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
>