Re: [PATCH bpf-next v2 2/2] bpf, arm64: inline bpf_get_smp_processor_id() helper

[Puranjay Mohan <puranjay@kernel.org>]

Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:

> On Wed, Apr 24, 2024 at 10:36 AM Puranjay Mohan <puranjay@kernel.org> wrote:
>>
>> 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.
>>
>> 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 = r0
>
> nit: hmm, you are probably using a bit outdated bpftool, it should be
> emitted as:
>
> (bf) r0 = &(void __percpu *)(r0)

Yes, I was using the bpftool shipped with the distro. I tried it again
with the latest bpftool and it emitted this as expected.

>
>>                                                 (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
>> arr-inc        :   23.253 ± 0.019M/s      arr-inc        :   23.742 ± 0.023M/s
>> hash-inc       :   12.258 ± 0.010M/s      hash-inc       :   12.625 ± 0.004M/s
>>
>> [1] https://github.com/anakryiko/linux/commit/8dec900975ef
>>
>> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
>> ---
>>  kernel/bpf/verifier.c | 11 ++++++++++-
>>  1 file changed, 10 insertions(+), 1 deletion(-)
>>
>
> Besides the nits, lgtm.
>
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 9715c88cc025..3373be261889 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -20205,7 +20205,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>>                         goto next_insn;
>>                 }
>>
>> -#ifdef CONFIG_X86_64
>> +#if defined(CONFIG_X86_64) || defined(CONFIG_ARM64)
>
> I think you can drop this, we are protected by
> bpf_jit_supports_percpu_insn() check and newly added inner #if/#elif
> checks?

If I remove this and later add support of percpu_insn on RISCV without
inlining bpf_get_smp_processor_id() then it will cause problems here
right? because then the last 5-6 lines inside this if(){} will be
executed for RISCV.

>
>>                 /* Implement bpf_get_smp_processor_id() inline. */
>>                 if (insn->imm == BPF_FUNC_get_smp_processor_id &&
>>                     prog->jit_requested && bpf_jit_supports_percpu_insn()) {
>> @@ -20214,11 +20214,20 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>>                          * changed in some incompatible and hard to support
>>                          * way, it's fine to back out this inlining logic
>>                          */
>> +#if defined(CONFIG_X86_64)
>>                         insn_buf[0] = BPF_MOV32_IMM(BPF_REG_0, (u32)(unsigned long)&pcpu_hot.cpu_number);
>>                         insn_buf[1] = BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0);
>>                         insn_buf[2] = BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0);
>>                         cnt = 3;
>> +#elif defined(CONFIG_ARM64)
>> +                       struct bpf_insn cpu_number_addr[2] = { BPF_LD_IMM64(BPF_REG_0, (u64)&cpu_number) };
>>
>
> this &cpu_number offset is not guaranteed to be within 4GB on arm64?

Unfortunately, the per-cpu section is not placed in the first 4GB and
therefore the per-cpu pointers are not 32-bit on ARM64.

>
>> +                       insn_buf[0] = cpu_number_addr[0];
>> +                       insn_buf[1] = cpu_number_addr[1];
>> +                       insn_buf[2] = BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0);
>> +                       insn_buf[3] = BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0);
>> +                       cnt = 4;
>> +#endif
>>                         new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
>>                         if (!new_prog)
>>                                 return -ENOMEM;
>> --
>> 2.40.1
>>