Re: [RFC PATCH bpf-next 2/3] bpf, arm64: Optimize AND,OR,XOR,JSET BPF_K using arm64 logical immediates

[Daniel Borkmann]

On 5/7/20 3:05 AM, Luke Nelson wrote:
> The current code for BPF_{AND,OR,XOR,JSET} BPF_K loads the immediate to
> a temporary register before use.
>
> This patch changes the code to avoid using a temporary register
> when the BPF immediate is encodable using an arm64 logical immediate
> instruction. If the encoding fails (due to the immediate not being
> encodable), it falls back to using a temporary register.
>
> Example of generated code for BPF_ALU32_IMM(BPF_AND, R0, 0x80000001):
>
> without optimization:
>
>    24: mov  w10, #0x8000ffff
>    28: movk w10, #0x1
>    2c: and  w7, w7, w10
>
> with optimization:
>
>    24: and  w7, w7, #0x80000001
>
> Since the encoding process is quite complex, the JIT reuses existing
> functionality in arch/arm64/kernel/insn.c for encoding logical immediates
> rather than duplicate it in the JIT.
>
> Co-developed-by: Xi Wang <xi.wang@xxxxxxxxx>
> Signed-off-by: Xi Wang <xi.wang@xxxxxxxxx>
> Signed-off-by: Luke Nelson <luke.r.nels@xxxxxxxxx>

Great find, thanks! Given Will wanted to queue them:

Acked-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>