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Date: Thu, 25 Apr 2024 10:14:26 +0000
From: Puranjay Mohan <puranjay@...nel.org>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon
<will@...nel.org>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann
<daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, Martin KaFai
Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu
<song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, John Fastabend
<john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>, Stanislav
Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa
<jolsa@...nel.org>, Zi Shen Lim <zlim.lnx@...il.com>, Xu Kuohai
<xukuohai@...wei.com>, Florent Revest <revest@...omium.org>,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 2/2] bpf, arm64: inline
bpf_get_smp_processor_id() helper
Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
> On Wed, Apr 24, 2024 at 10:36 AM Puranjay Mohan <puranjay@...nel.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 nit: hmm, you are probably using a bit outdated bpftool, it should be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@...nel.org>
>> ---
>> kernel/bpf/verifier.c | 11 ++++++++++-
>> 1 file changed, 10 insertions(+), 1 deletion(-)
>>
>
> Besides the nits, lgtm.
>
> Acked-by: Andrii Nakryiko <andrii@...nel.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
>>
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