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SubjectRe: [PATCH v2 bpf-next 12/13] bpf: Add tests for new BPF atomic operations
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On 12/1/20 4:56 AM, Brendan Jackman wrote:
> On Mon, Nov 30, 2020 at 07:55:02PM -0800, Yonghong Song wrote:
>> On 11/27/20 9:57 AM, Brendan Jackman wrote:
> [...]
>>> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
>>> index 3d5940cd110d..5eadfd09037d 100644
>>> --- a/tools/testing/selftests/bpf/Makefile
>>> +++ b/tools/testing/selftests/bpf/Makefile
>>> @@ -228,6 +228,12 @@ IS_LITTLE_ENDIAN = $(shell $(CC) -dM -E - </dev/null | \
>>> grep 'define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__')
>>> MENDIAN=$(if $(IS_LITTLE_ENDIAN),-mlittle-endian,-mbig-endian)
>>> +# Determine if Clang supports BPF arch v4, and therefore atomics.
>>> +CLANG_SUPPORTS_V4=$(if $(findstring v4,$(shell $(CLANG) --target=bpf -mcpu=? 2>&1)),true,)
>>> +ifeq ($(CLANG_SUPPORTS_V4),true)
>>> + CFLAGS += -DENABLE_ATOMICS_TESTS
>>> +endif
>>> +
>>> CLANG_SYS_INCLUDES = $(call get_sys_includes,$(CLANG))
>>> BPF_CFLAGS = -g -D__TARGET_ARCH_$(SRCARCH) $(MENDIAN) \
>>> -I$(INCLUDE_DIR) -I$(CURDIR) -I$(APIDIR) \
>>> @@ -250,7 +256,9 @@ define CLANG_BPF_BUILD_RULE
>>> $(call msg,CLNG-LLC,$(TRUNNER_BINARY),$2)
>>> $(Q)($(CLANG) $3 -O2 -target bpf -emit-llvm \
>>> -c $1 -o - || echo "BPF obj compilation failed") | \
>>> - $(LLC) -mattr=dwarfris -march=bpf -mcpu=v3 $4 -filetype=obj -o $2
>>> + $(LLC) -mattr=dwarfris -march=bpf \
>>> + -mcpu=$(if $(CLANG_SUPPORTS_V4),v4,v3) \
>>> + $4 -filetype=obj -o $2
>>> endef
>>> # Similar to CLANG_BPF_BUILD_RULE, but with disabled alu32
>>> define CLANG_NOALU32_BPF_BUILD_RULE
>>> @@ -391,7 +399,7 @@ TRUNNER_EXTRA_SOURCES := test_progs.c cgroup_helpers.c trace_helpers.c \
>>> TRUNNER_EXTRA_FILES := $(OUTPUT)/urandom_read \
>>> $(wildcard progs/btf_dump_test_case_*.c)
>>> TRUNNER_BPF_BUILD_RULE := CLANG_BPF_BUILD_RULE
>>> -TRUNNER_BPF_CFLAGS := $(BPF_CFLAGS) $(CLANG_CFLAGS)
>>> +TRUNNER_BPF_CFLAGS := $(BPF_CFLAGS) $(CLANG_CFLAGS) $(if $(CLANG_SUPPORTS_V4),-DENABLE_ATOMICS_TESTS,)
>>
>> If the compiler indeed supports cpu v4 (i.e., atomic insns),
>> -DENABLE_ATOMICS_TESTS will be added to TRUNNER_BPF_FLAGS and
>> eventually -DENABLE_ATOMICS_TESTS is also available for
>> no-alu32 test and this will cause compilation error.
>>
>> I did the following hack to workaround the issue, i.e., only adds
>> the definition to default (alu32) test run.
>>
>> index 5eadfd09037d..3d1320fd93eb 100644
>> --- a/tools/testing/selftests/bpf/Makefile
>> +++ b/tools/testing/selftests/bpf/Makefile
>> @@ -230,9 +230,6 @@ MENDIAN=$(if
>> $(IS_LITTLE_ENDIAN),-mlittle-endian,-mbig-endian)
>>
>> # Determine if Clang supports BPF arch v4, and therefore atomics.
>> CLANG_SUPPORTS_V4=$(if $(findstring v4,$(shell $(CLANG) --target=bpf
>> -mcpu=? 2>&1)),true,)
>> -ifeq ($(CLANG_SUPPORTS_V4),true)
>> - CFLAGS += -DENABLE_ATOMICS_TESTS
>> -endif
>>
>> CLANG_SYS_INCLUDES = $(call get_sys_includes,$(CLANG))
>> BPF_CFLAGS = -g -D__TARGET_ARCH_$(SRCARCH) $(MENDIAN) \
>> @@ -255,6 +252,7 @@ $(OUTPUT)/flow_dissector_load.o: flow_dissector_load.h
>> define CLANG_BPF_BUILD_RULE
>> $(call msg,CLNG-LLC,$(TRUNNER_BINARY),$2)
>> $(Q)($(CLANG) $3 -O2 -target bpf -emit-llvm \
>> + $(if $(CLANG_SUPPORTS_V4),-DENABLE_ATOMICS_TESTS,) \
>> -c $1 -o - || echo "BPF obj compilation failed") | \
>> $(LLC) -mattr=dwarfris -march=bpf \
>> -mcpu=$(if $(CLANG_SUPPORTS_V4),v4,v3) \
>> @@ -399,7 +397,7 @@ TRUNNER_EXTRA_SOURCES := test_progs.c cgroup_helpers.c
>> trace_helpers.c \
>> TRUNNER_EXTRA_FILES := $(OUTPUT)/urandom_read \
>> $(wildcard progs/btf_dump_test_case_*.c)
>> TRUNNER_BPF_BUILD_RULE := CLANG_BPF_BUILD_RULE
>> -TRUNNER_BPF_CFLAGS := $(BPF_CFLAGS) $(CLANG_CFLAGS) $(if
>> $(CLANG_SUPPORTS_V4),-DENABLE_ATOMICS_TESTS,)
>> +TRUNNER_BPF_CFLAGS := $(BPF_CFLAGS) $(CLANG_CFLAGS)
>> TRUNNER_BPF_LDFLAGS := -mattr=+alu32
>> $(eval $(call DEFINE_TEST_RUNNER,test_progs))
>
> Ah, good point. I think your "hack" actually improves the overall result
> anyway since it avoids the akward global mutation of CFLAGS. Thanks!
>
> I wonder if we should actually have Clang define a built-in macro to say
> that the atomics are supported?

We are using gcc builtin's and they are all supported by clang, so
"#if __has_builtin(__sync_fetch_and_or)" is always true so it
won't work here.

We could add a macro like __BPF_ATOMICS_SUPPORTED__ in clang.
But you still need a checking to decide whether to use -mcpu=v4. If
you have that information, it will be trivial to add your
own macros if it is -mcpu=v4.

>
>>> diff --git a/tools/testing/selftests/bpf/prog_tests/atomics_test.c b/tools/testing/selftests/bpf/prog_tests/atomics_test.c
>>> new file mode 100644
>>> index 000000000000..8ecc0392fdf9
>>> --- /dev/null
>>> +++ b/tools/testing/selftests/bpf/prog_tests/atomics_test.c
>>> @@ -0,0 +1,329 @@
>>> +// SPDX-License-Identifier: GPL-2.0
>>> +
>>> +#include <test_progs.h>
>>> +
>>> +#ifdef ENABLE_ATOMICS_TESTS
>>> +
>>> +#include "atomics_test.skel.h"
>>> +
>>> +static void test_add(void)
>> [...]
>>> +
>>> +#endif /* ENABLE_ATOMICS_TESTS */
>>> diff --git a/tools/testing/selftests/bpf/progs/atomics_test.c b/tools/testing/selftests/bpf/progs/atomics_test.c
> [...]
>>> +__u64 xor64_value = (0x110ull << 32);
>>> +__u64 xor64_result = 0;
>>> +__u32 xor32_value = 0x110;
>>> +__u32 xor32_result = 0;
>>> +SEC("fentry/bpf_fentry_test1")
>>> +int BPF_PROG(xor, int a)
>>> +{
>>> + xor64_result = __sync_fetch_and_xor(&xor64_value, 0x011ull << 32);
>>> + xor32_result = __sync_fetch_and_xor(&xor32_value, 0x011);
>>> +
>>> + return 0;
>>> +}
>>
>> All above __sync_fetch_and_{add, sub, and, or, xor} produces a return
>> value used later. To test atomic_<op> instructions, it will be good if
>> you can add some tests which ignores the return value.
>
> Good idea - adding an extra case to each prog. This won't assert that
> LLVM is generating "optimal" code (without BPF_FETCH) but we can at
> least get some confidence we aren't generating total garbage.
>

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