On 9/3/24 4:50 PM, Yonghong Song wrote:
On 9/2/24 11:52 PM, Daniel Borkmann wrote:
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].
Thanks Daniel.
Trying to have llvm doc for BPF backend has been discussed
before. IIRC, Fangrui Song suggested this when we tried to
add BPF reloc documents in Documentation/bpf/llvm_reloc.rst.
Eventually we added llvm_reloc.rst to kernel since this doc
is mostly interesting for kernel/bpf folks. We should add
an entry in bpf_devel_QA.rst to mention that default cpu
version change from v1 to v3.
Not sure whether we should have the same doc in
llvm.org/docs/. Let me discuss with other folks on this.
I was mostly thinking that not everyone might be looking into
kernel docs (say, eBPF for Windows folks using LLVM), and at
least on gcc docs/wiki you'll find information & quirks about
gcc-bpf backend [2].
[2] https://gcc.gnu.org/wiki/BPFBackEnd
[0] https://github.com/llvm/llvm-project/tree/main/llvm/docs
[1] https://llvm.org/docs/RISCV/RISCVVectorExtension.html
Thanks,
Daniel
