will xxh64 work in ebpf?

[kolinfluence]

great work! will xxh64 work in ebpf?

otherwise, which version will work 100% in ebpf?

[easyaspi314]

xxHash is portable with the basic C90+long long ISA, so it will always work.

However, whether it is performant is a different story. eBPF seems to have a lack of unaligned access so there may be a significant overhead on unaligned loads. All xxHash functions will work fine, but XXH3 will run rather inefficiently since there is no aligned path (and a lack of SIMD). XXH32 and XXH64 will run at full speed if the pointer is sufficiently aligned though.

[easyaspi314]

I take that back, Clang eBPF doesn't support struct returns.

Therefore XXH3 will not work, but XXH32 and XXH64 should compile with -DXXH_NO_XXH3.

@Cyan4973 perhaps should we try to work on this? Perhaps having XXH3_NO_STRUCT_RETURN as an alternative ABI?

[easyaspi314]

Update: Clang supports returning structs with static inline __attribute__((always_inline)):

extern void XXH3_128bits_noStructReturn(XXH128_hash_t *XXH_RESTRICT output, const void *XXH_RESTRICT ptr, size_t n);

__attribute__((always_inline, unused))
static __inline__ XXH128_hash_t XXH3_128bits(const void *ptr, size_t n)
{
    XXH128_hash_t ret;
    XXH3_128bits_noStructReturn(&ret, ptr, n);
    return ret;
}

[easyaspi314]

it was at this moment I realized this was my own repo. I'm dumb.

XXH64 will definitely work, but I would recommend using Cyan4973/xxHash as that has a lot of low level optimizations.

[kolinfluence]

@easyaspi314 thx for the prompt feedback can you take a look at this issue and the pdf link below to confirm your comment?

https://github.com/Cyan4973/xxHash/issues/808

trying to make sure ebpf will be portable. so not very sure if you can confirm this. thx there's an article online comparing jhash and spookyhash with ebpf https://fosdem.org/2023/schedule/event/bpf_hashing/attachments/slides/5812/export/events/attachments/bpf_hashing/slides/5812/the_talk

do hope u can come up with an ebpf compatible one. performance-wise is secondary. just need it working in ebpf

[kolinfluence]

@easyaspi314 do you know if there's a version of yours that's available in ebpf, a lot of people will use it? i'm checking out the license now. can you update it? it's not really one of those mainstream ones. i mean it's not like apache / bsd / mit.

waiting for your feedback!

p.s. : i have working cityhash using ebpf but it's extremely exciting if one of the xxhash line is available for use in ebpf. pls help verify this. appreciate it.