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 Thu, Apr 25, 2024 at 3:14 AM Puranjay Mohan <puranjay@kernel.org> wrote:
>>
>> 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 nit: hmm, you are probably using a bit outdated bpftool, it should be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.
>
> Cool, would be nice to update the commit message with the right syntax
> for next revision, thanks!
>

Sure, will do.

>>
>> >
>> >>                                                 (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.
>
> Just add
>
> #else
> return -EFAULT;

I don't think we can return.

> #endif
>
> ?
>
> I'm trying to avoid this duplication of the defined(CONFIG_xxx) checks
> for supported architectures.

Does the following look correct?

I will do it like this:

                /* Implement bpf_get_smp_processor_id() inline. */
                if (insn->imm == BPF_FUNC_get_smp_processor_id &&
                    prog->jit_requested && bpf_jit_supports_percpu_insn()) {
                        /* BPF_FUNC_get_smp_processor_id inlining is an
                         * optimization, so if pcpu_hot.cpu_number is ever
                         * 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) };

                        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;
#else
                        goto next_insn;
#endif
                        new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
                        if (!new_prog)
                                return -ENOMEM;

                        delta    += cnt - 1;
                        env->prog = prog = new_prog;
                        insn      = new_prog->insnsi + i + delta;
                        goto next_insn;
                }


>>
>> >
>> >>                 /* 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.
>
> I see. It might make sense to turn x86-64 code into using MOV64_IMM as
> well to keep more of the logic common. Then it will be just the
> difference of an offset that's loaded. Give it a try?

I think MOV64_IMM would have more overhead than MOV32_IMM and if we can
use it in x86-64 we should keep doing it that way. Wdyt? 

>>
>> >
>> >> +                       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
>> >>