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. 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
