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

[Andrii Nakryiko]

On Thu, May 2, 2024 at 6:16 AM Puranjay Mohan <puranjay@xxxxxxxxxx> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes:
>
> > On Tue, Apr 30, 2024 at 10:59 AM Puranjay Mohan <puranjay@xxxxxxxxxx> wrote:
> >>
> >> Inline the calls to bpf_get_smp_processor_id() in the riscv bpf jit.
> >>
> >> RISCV saves the pointer to the CPU's task_struct in the TP (thread
> >> pointer) register. This makes it trivial to get the CPU's processor id.
> >> As thread_info is the first member of task_struct, we can read the
> >> processor id from TP + offsetof(struct thread_info, cpu).
> >>
> >>           RISCV64 JIT output for `call bpf_get_smp_processor_id`
> >>           ======================================================
> >>
> >>                 Before                           After
> >>                --------                         -------
> >>
> >>          auipc   t1,0x848c                  ld    a5,32(tp)
> >>          jalr    604(t1)
> >>          mv      a5,a0
> >>
> >
> > Nice, great find! Would you be able to do similar inlining for x86-64
> > as well? Disassembling bpf_get_smp_processor_id for x86-64 shows this:
> >
> > Dump of assembler code for function bpf_get_smp_processor_id:
> >    0xffffffff810f91a0 <+0>:     0f 1f 44 00 00  nopl   0x0(%rax,%rax,1)
> >    0xffffffff810f91a5 <+5>:     65 8b 05 60 79 f3 7e    mov
> > %gs:0x7ef37960(%rip),%eax        # 0x30b0c <pcpu_hot+12>
> >    0xffffffff810f91ac <+12>:    48 98   cltq
> >    0xffffffff810f91ae <+14>:    c3      ret
> > End of assembler dump.
> > We should be able to do the same in x86-64 BPF JIT. (it's actually how
> > I started initially, I had a dedicated instruction reading per-cpu
> > memory, but ended up with more general "calculate per-cpu address").
>
> I feel in x86-64's case JIT can not do a (much) better job compared to the
> current approach in the verifier.

This direct memory read (using gs segment) ought to be a bit faster
than calculating offset and then doing memory dereference, but yes,
the difference won't be as big as you got with RISC-V and ARM64. Ok,
never mind, we can always benchmark and add that later, no big deal.

>
> On RISC-V and ARM64, JIT was able to do it better because both of these
> architectures save a pointer to the task struct in a special CPU
> register. As x86-64 doesn't have enough extra registers, it uses a
> percpu variable to store task struct, thread_info, and the cpu
> number.
>
> P.S. - While doing this for BPF, I realized that ARM64 kernel code is
> also not optimal as it is using the percpu variable and is not reading
> the CPU register directly. So, I sent a patch[1] to fix it in the kernel
> and get rid of the per-cpu variable in ARM64.
>
>
> [1] https://lore.kernel.org/all/20240502123449.2690-2-puranjay@xxxxxxxxxx/
>
> > Anyways, great work, a small nit below.
> >
> > Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
>
> Thanks,
> Puranjay