On Sun, Feb 4, 2024 at 11:20 AM Yonghong Song <yonghong.song@xxxxxxxxx> wrote: > > > On 2/2/24 2:18 PM, Andrii Nakryiko wrote: > > On Wed, Jan 31, 2024 at 7:56 AM Leon Hwang <hffilwlqm@xxxxxxxxx> wrote: > >> This patchset introduces a new generic kfunc bpf_ffs64(). This kfunc > >> allows bpf to reuse kernel's __ffs64() function to improve ffs > >> performance in bpf. > >> > > The downside of using kfunc for this is that the compiler will assume > > that R1-R5 have to be spilled/filled, because that's function call > > convention in BPF. > > > > If this was an instruction, though, it would be much more efficient > > and would avoid this problem. But I see how something like ffs64 is > > useful. I think it would be good to also have popcnt instruction and a > > few other fast bit manipulation operations as well. > > > > Perhaps we should think about another BPF ISA extension to add fast > > bit manipulation instructions? > > Sounds a good idea to start the conversion. Besides popcnt, lzcnt > is also a candidate. From llvm perspective, it would be hard to > generate ffs64/popcnt/lzcnt etc. from source generic implementation. I'm curious why? I assumed that if a user used __builtin_popcount() Clang could just generate BPF's popcnt instruction (assuming the right BPF cpu version is enabled, of course). > So most likely, inline asm will be used. libbpf could define > some macros to make adoption easier. Verifier and JIT will do > proper thing, either using corresponding arch insns directly or > verifier will rewrite so JIT won't be aware of these insns. > > > > >> In patch "bpf: Add generic kfunc bpf_ffs64()", there is some data to > >> confirm that this kfunc is able to save around 10ns for every time on > >> "Intel(R) Xeon(R) Silver 4116 CPU @ 2.10GHz" CPU server, by comparing > >> with bpf-implemented __ffs64(). > >> > >> However, it will be better when convert this kfunc to "rep bsf" in > >> JIT on x86, which is able to avoid a call. But, I haven't figure out the > >> way. > >> > >> Leon Hwang (2): > >> bpf: Add generic kfunc bpf_ffs64() > >> selftests/bpf: Add testcases for generic kfunc bpf_ffs64() > >> > >> kernel/bpf/helpers.c | 7 +++ > >> .../testing/selftests/bpf/prog_tests/bitops.c | 54 +++++++++++++++++++ > >> tools/testing/selftests/bpf/progs/bitops.c | 21 ++++++++ > >> 3 files changed, 82 insertions(+) > >> create mode 100644 tools/testing/selftests/bpf/prog_tests/bitops.c > >> create mode 100644 tools/testing/selftests/bpf/progs/bitops.c > >> > >> > >> base-commit: c5809f0c308111adbcdbf95462a72fa79eb267d1 > >> -- > >> 2.42.1 > >>
