[PATCH bpf-next v4 2/2] bpf, arm64: inline bpf_get_smp_processor_id() helper

[Puranjay Mohan]

Puranjay Mohan <puranjay at kernel.org> writes:

> As ARM64 JIT now implements BPF_MOV64_PERCPU_REG instruction, inline
> bpf_get_smp_processor_id().
>
> ARM64 uses the per-cpu variable cpu_number to store the cpu id.

While implementing this on the RISC-V JIT[1], I realized that reading the
cpu_number from the per-cpu variable is not the best way to do this on
arm64.

arm64 now has the cpu number in the thread_info and reading that would
be more efficient.

Implementation in this patch is emitting:

; int cpu = bpf_get_smp_processor_id();
mov     x7, #0xffff8000ffffffff
movk    x7, #0x8207, lsl #16   
movk    x7, #0x2008            
mrs     x10, tpidr_el1         
add     x7, x7, x10            
ldr     w7, [x7]               

If we do this in the JIT like I did for RISC-V[1]
We can emit:

; int cpu = bpf_get_smp_processor_id();
mrs     x10, sp_el0
ldr     w7, [x10, #24]

This gives ~ 10% improvement on glob-arr-inc and hash-inc compared to
3-4 % which this patch is providing. 

I will send v5 using this approach.

[1] https://lore.kernel.org/all/20240430175834.33152-3-puranjay@kernel.org/

> Here is how the BPF and ARM64 JITed assembly changes after this commit:
>
>                                          BPF
>          		                =====
>               BEFORE                                       AFTER
>              --------                                     -------
>
> int cpu = bpf_get_smp_processor_id();           int cpu = bpf_get_smp_processor_id();
> (85) call bpf_get_smp_processor_id#229032       (18) r0 = 0xffff800082072008
>                                                 (bf) r0 = &(void __percpu *)(r0)
>                                                 (61) r0 = *(u32 *)(r0 +0)
>
> 				      ARM64 JIT
> 				     ===========
>
>               BEFORE                                       AFTER
>              --------                                     -------
>
> int cpu = bpf_get_smp_processor_id();           int cpu = bpf_get_smp_processor_id();
> mov     x10, #0xfffffffffffff4d0                mov     x7, #0xffff8000ffffffff
> movk    x10, #0x802b, lsl #16                   movk    x7, #0x8207, lsl #16
> movk    x10, #0x8000, lsl #32                   movk    x7, #0x2008
> blr     x10                                     mrs     x10, tpidr_el1
> add     x7, x0, #0x0                            add     x7, x7, x10
>                                                 ldr     w7, [x7]
>
> Performance improvement using benchmark[1]
>
>              BEFORE                                       AFTER
>             --------                                     -------
>
> glob-arr-inc   :   23.817 ± 0.019M/s      glob-arr-inc   :   24.631 ± 0.027M/s [+ 3.41%]
> arr-inc        :   23.253 ± 0.019M/s      arr-inc        :   23.742 ± 0.023M/s [+ 2.10%]
> hash-inc       :   12.258 ± 0.010M/s      hash-inc       :   12.625 ± 0.004M/s [+ 3.00%]
>
> [1] https://github.com/anakryiko/linux/commit/8dec900975ef


Thanks,
Puranjay