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[PATCH v2] linux-gen: added wfe and wfet library functions #2256

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[PATCH v2] linux-gen: added wfe and wfet library functions #2256

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a7b0c8e
linux-gen: added wfe and wfet library functions
sanvai01 Jun 30, 2025
02befb5
Merge branch 'master' into wfe_and_wfet_instructions
sanvai01 Sep 12, 2025
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7 changes: 7 additions & 0 deletions platform/linux-generic/arch/aarch64/cpu_flags.c
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Original file line number Diff line number Diff line change
Expand Up @@ -992,6 +992,13 @@ static void _odp_sys_info_print_acle_flags(void)
_ODP_PRINT("%s\n", ndef);
#endif

_ODP_PRINT(" __ARM_FEATURE_WFXT ");
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@JannePeltonen JannePeltonen Sep 17, 2025

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See my comment in PR 2257 about commit message and merge commits.

#ifdef __ARM_FEATURE_WFXT
_ODP_PRINT("%i\n", __ARM_FEATURE_WFXT);
#else
_ODP_PRINT("%s\n", ndef);
#endif

_ODP_PRINT("\n");
}

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26 changes: 26 additions & 0 deletions platform/linux-generic/arch/aarch64/odp/api/abi/wait_until.h
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Expand Up @@ -33,6 +33,32 @@ _odp_wait_until_equal_acq_u32(odp_atomic_u32_t *addr, uint32_t expected)
} while (expected != value);
}

static inline void
_odp_wait_until_equal_rel_u32(odp_atomic_u32_t *addr, uint32_t expected)
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@JannePeltonen JannePeltonen Sep 17, 2025

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How is this function supposed/allowed to be different from _odp_wait_until_equal_32()? What does the rel part of this function indicate? In ODP, rel in a function name implies release memory ordering, but I cannot see what release ordering would mean in a function that does not stores. Or does ldxr synchronize with acquire loads in other threads? Even if it does, I am not sure providing a release type function would be useful. It could also be cumbersome for a generic C implementation, but since such is not provided in this PR, then why is this even a separate function in this aarch64 specific implementation?

{
uint32_t value;
uint32_t *var = &addr->v;

__asm__ volatile("sevl" : : : "memory");
do {
#ifdef __ARM_FEATURE_WFXT
__asm__ volatile("wfet %x0" : : "r"(100) : "memory");
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@JannePeltonen JannePeltonen Sep 17, 2025

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Why would WFET be useful here and better than WFE? If it is useful, then why not make the same change in the existing _odp_wait_until_equal_acq_u32() function?

#else
__asm__ volatile("wfe" : : : "memory");
#endif
__asm__ volatile("ldxr %w0, [%1]"
: "=&r" (value)
: "r" (var)
: "memory");
} while (expected != value);
}

static inline void
_odp_wait_until_equal_32(odp_atomic_u32_t *addr, uint32_t expected)
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@JannePeltonen JannePeltonen Sep 17, 2025

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Do we even need this function with a relaxed memory order? Aren't most (practically all?) concievable uses such that acquire is required? Maybe there is not need to introduce this new function until there is an actual user of it.

{
_odp_wait_until_equal_rel_u32(addr, expected);
}

#ifdef __cplusplus
}
#endif
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6 changes: 6 additions & 0 deletions platform/linux-generic/arch/default/odp/api/abi/wait_until_generic.h
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Expand Up @@ -18,6 +18,12 @@ _odp_wait_until_equal_acq_u32(odp_atomic_u32_t *addr, uint32_t expected)
odp_cpu_pause();
}

static inline void
_odp_wait_until_equal_u32(odp_atomic_u32_t *addr, uint32_t expected)
{
while (odp_atomic_load_u32(addr) != expected)
odp_cpu_pause();
}
#ifdef __cplusplus
}
#endif
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