Re: [PATCH bpf-next v3 1/2] arm64, bpf: add internal-only MOV instruction to resolve per-CPU addrs

[Andrii Nakryiko]

On Fri, Apr 26, 2024 at 5:14 AM Puranjay Mohan <puranjay@xxxxxxxxxx> wrote:
>
> From: Puranjay Mohan <puranjay12@xxxxxxxxx>
>
> Support an instruction for resolving absolute addresses of per-CPU
> data from their per-CPU offsets. This instruction is internal-only and
> users are not allowed to use them directly. They will only be used for
> internal inlining optimizations for now between BPF verifier and BPF
> JITs.
>
> Since commit 7158627686f0 ("arm64: percpu: implement optimised pcpu
> access using tpidr_el1"), the per-cpu offset for the CPU is stored in
> the tpidr_el1/2 register of that CPU.
>
> To support this BPF instruction in the ARM64 JIT, the following ARM64
> instructions are emitted:
>
> mov dst, src            // Move src to dst, if src != dst
> mrs tmp, tpidr_el1/2    // Move per-cpu offset of the current cpu in tmp.
> add dst, dst, tmp       // Add the per cpu offset to the dst.
>
> To measure the performance improvement provided by this change, the
> benchmark in [1] was used:
>
> Before:
> glob-arr-inc   :   23.597 ± 0.012M/s
> arr-inc        :   23.173 ± 0.019M/s
> hash-inc       :   12.186 ± 0.028M/s
>
> After:
> glob-arr-inc   :   23.819 ± 0.034M/s
> arr-inc        :   23.285 ± 0.017M/s

I still expected a better improvement (global-arr-inc's results
improved more than arr-inc, which is completely different from
x86-64), but it's still a good thing to support this for arm64, of
course.

ack for generic parts I can understand:

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

> hash-inc       :   12.419 ± 0.011M/s
>
> [1] https://github.com/anakryiko/linux/commit/8dec900975ef
>
> Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
> ---
>  arch/arm64/include/asm/insn.h |  7 +++++++
>  arch/arm64/lib/insn.c         | 11 +++++++++++
>  arch/arm64/net/bpf_jit.h      |  6 ++++++
>  arch/arm64/net/bpf_jit_comp.c | 14 ++++++++++++++
>  4 files changed, 38 insertions(+)
>

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