linux-kernel - Re: [PATCH bpf-next v8 2/2] selftests/bpf: Add an usdt_o2 test case in selftests to cover SIB handling logic

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Message-ID: <CAEf4BzbQ00YtbSqRotMEN4eBJC7aYNy9fFRO-Q_=Z0uS6O06mg@mail.gmail.com>
Date: Wed, 13 Aug 2025 17:04:35 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jiawei Zhao <phoenix500526@....com>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org, 
	yonghong.song@...ux.dev, bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v8 2/2] selftests/bpf: Add an usdt_o2 test case
 in selftests to cover SIB handling logic

On Wed, Aug 6, 2025 at 7:35 PM Jiawei Zhao <phoenix500526@....com> wrote:
>
> When using GCC on x86-64 to compile an usdt prog with -O1 or higher
> optimization, the compiler will generate SIB addressing mode for global
> array and PC-relative addressing mode for global variable,
> e.g. "1@-96(%rbp,%rax,8)" and "-1@...1(%rip)".
>
> In this patch:
> - add usdt_o2 test case to cover SIB addressing usdt argument spec
>   handling logic
>
> Signed-off-by: Jiawei Zhao <phoenix500526@....com>
> ---
>  tools/testing/selftests/bpf/Makefile          |  8 +++
>  .../selftests/bpf/prog_tests/usdt_o2.c        | 71 +++++++++++++++++++
>  .../selftests/bpf/progs/test_usdt_o2.c        | 37 ++++++++++
>  3 files changed, 116 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/usdt_o2.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_usdt_o2.c
>
> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> index 910d8d6402ef..68cf6a9cf05f 100644
> --- a/tools/testing/selftests/bpf/Makefile
> +++ b/tools/testing/selftests/bpf/Makefile
> @@ -759,6 +759,14 @@ TRUNNER_BPF_BUILD_RULE := $$(error no BPF objects should be built)
>  TRUNNER_BPF_CFLAGS :=
>  $(eval $(call DEFINE_TEST_RUNNER,test_maps))
>
> +# Use -O2 optimization to generate SIB addressing usdt argument spec
> +# Only apply on x86 architecture where SIB addressing is relevant
> +ifeq ($(ARCH), x86)
> +$(OUTPUT)/usdt_o2.test.o: CFLAGS:=$(subst O0,O2,$(CFLAGS))
> +$(OUTPUT)/cpuv4/usdt_o2.test.o: CFLAGS:=$(subst O0,O2,$(CFLAGS))
> +$(OUTPUT)/no_alu32/usdt_o2.test.o: CFLAGS:=$(subst O0,O2,$(CFLAGS))
> +endif
> +

Have you considered using GCC's __attribute__((optimize("O2")))
attribute. It seems like Clang doesn't have support for something like
that, but we'll still have this covered in BPF CI for GCC-built
selftests. Then I'd just add this as another subtest to existing usdt
tests.

Can you please try that?

>  # Define test_verifier test runner.
>  # It is much simpler than test_maps/test_progs and sufficiently different from
>  # them (e.g., test.h is using completely pattern), that it's worth just
> diff --git a/tools/testing/selftests/bpf/prog_tests/usdt_o2.c b/tools/testing/selftests/bpf/prog_tests/usdt_o2.c
> new file mode 100644
> index 000000000000..f04b756b3640
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/usdt_o2.c
> @@ -0,0 +1,71 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2025 Jiawei Zhao <phoenix500526@....com>. */
> +#include <test_progs.h>
> +
> +#define _SDT_HAS_SEMAPHORES 1
> +#include "../sdt.h"
> +#include "test_usdt_o2.skel.h"
> +
> +int lets_test_this(int);
> +
> +#define test_value 0xFEDCBA9876543210ULL
> +#define SEC(name) __attribute__((section(name), used))
> +
> +
> +static volatile __u64 array[1] = {test_value};
> +unsigned short test_usdt1_semaphore SEC(".probes");
> +

Is semaphore essential to this test?

> +static __always_inline void trigger_func(void)
> +{
> +       /* Base address + offset + (index * scale) */
> +       if (test_usdt1_semaphore) {
> +               for (volatile int i = 0; i <= 0; i++)
> +                       STAP_PROBE1(test, usdt1, array[i]);
> +       }
> +}
> +

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