On 9/3/24 6:10 AM, Yonghong Song wrote:
Hi, Suggested by Alexei, I put a llvm20 diff to make cpu=v3 as the default cpu version: https://github.com/llvm/llvm-project/pull/107008 cpu=v3 has been introduced in llvm9 (2019 H2) and the kernel cpu=v3 support should be available around the same time although I cannot remember the exact kernel version. There are two motivation to move cpu version default from v1 to v3. First, to resolve correct usage of code like (void)__sync_fetch_and_add(&ptr, value); In cpu v1/v2, the above insn generates locked add insn, and for cpu >= v3, the above insn generates atomic_fetch_add insn. The atomic_fetch_add insn is the correct way for the eventual insn for arm64. Otherwise, with locked add insn in arm64, proper barrier will be missing and incorrect results may be generated. Second, cpu=v3 should have better performance than cpu=v1 in most cases. In Meta, several years ago, we have conducted performance evaluation to compare v1 and v3 for major bpf programs running in our platform and we concluded v3 is better than v1 in most cases and in other rare cases v1 and v3 have the same performance. So moving to v3 can help performance too. If in rare cases, e.g. really old kernels, v1/v2 is the only option, then users can set -mcpu=v1 explicitly. Please let us know if you still have some concerns in your setup w.r.t. cpu v1->v3 transition.
Sounds good to me! Is there a place somewhere in LLVM where this can be documented for the BPF backend (along with the various extensions), so that developers can find sth in the official LLVM docs if they search the web? I see that riscv and some other archs have documentation under [0] which seems to get deployed under [1]. [0] https://github.com/llvm/llvm-project/tree/main/llvm/docs [1] https://llvm.org/docs/RISCV/RISCVVectorExtension.html Thanks, Daniel
