Re: [PATCH bpf-next v3 2/2] bpf, arm64: inline bpf_get_smp_processor_id() helper

[Puranjay Mohan <puranjay@kernel.org>]

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

> On Fri, Apr 26, 2024 at 5:14 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 = &(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
>> 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>
>> Acked-by: Andrii Nakryiko <andrii@kernel.org>
>> ---
>>  kernel/bpf/verifier.c | 24 +++++++++++++++++-------
>>  1 file changed, 17 insertions(+), 7 deletions(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 4e474ef44e9c..6ff4e63b2ef2 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -20273,20 +20273,31 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>>                         goto next_insn;
>>                 }
>>
>> -#ifdef CONFIG_X86_64
>>                 /* 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
>> +                        * optimization, so if cpu_number_addr is ever
>>                          * changed in some incompatible and hard to support
>>                          * way, it's fine to back out this inlining logic
>>                          */
>> -                       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;
>> +                       u64 cpu_number_addr;
>>
>> +#if defined(CONFIG_X86_64)
>> +                       cpu_number_addr = (u64)&pcpu_hot.cpu_number;
>> +#elif defined(CONFIG_ARM64)
>> +                       cpu_number_addr = (u64)&cpu_number;
>> +#else
>> +                       goto next_insn;
>> +#endif
>> +                       struct bpf_insn ld_cpu_number_addr[2] = {
>> +                               BPF_LD_IMM64(BPF_REG_0, cpu_number_addr)
>> +                       };
>
> here we are violating C89 requirement to have a single block of
> variable declarations by mixing variables and statements. I'm
> surprised this is not triggering any build errors on !arm64 &&
> !x86_64.
>
> I think we can declare this BPF_LD_IMM64 instruction with zero "addr".
> And then update
>
> ld_cpu_number_addr[0].imm = (u32)cpu_number_addr;
> ld_cpu_number_addr[1].imm = (u32)(cpu_number_addr >> 32);
>
> WDYT?
>
> nit: I'd rename ld_cpu_number_addr to ld_insn or something short like that

I agree with you,
What do you think about the following diff:

--- 8< ---

-#ifdef CONFIG_X86_64
                /* 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
+                        * optimization, so if cpu_number_addr is ever
                         * changed in some incompatible and hard to support
                         * way, it's fine to back out this inlining logic
                         */
-                       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;
+                       u64 cpu_number_addr;
+                       struct bpf_insn ld_insn[2] = {
+                               BPF_LD_IMM64(BPF_REG_0, 0)
+                       };
+
+#if defined(CONFIG_X86_64)
+                       cpu_number_addr = (u64)&pcpu_hot.cpu_number;
+#elif defined(CONFIG_ARM64)
+                       cpu_number_addr = (u64)&cpu_number;
+#else
+                       goto next_insn;
+#endif
+                       ld_insn[0].imm = (u32)cpu_number_addr;
+                       ld_insn[1].imm = (u32)(cpu_number_addr >> 32);
+                       insn_buf[0] = ld_insn[0];
+                       insn_buf[1] = ld_insn[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;

                        new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
                        if (!new_prog)
@@ -20296,7 +20310,6 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
                        insn      = new_prog->insnsi + i + delta;
                        goto next_insn;
                }
-#endif
                /* Implement bpf_get_func_arg inline. */

--- >8---

Thanks,
Puranjay