Re: [PATCH bpf-next 4/4] riscv, bpf: Mixing bpf2bpf and tailcalls

Pu Lehui
2023-09-28T10:39:53Z

On 2023/9/28 17:59, Björn Töpel wrote:
> Pu Lehui <pulehui@huaweicloud.com> writes:
>
>> From: Pu Lehui <pulehui@huawei.com>
>>
>> In the current RV64 JIT, if we just don't initialize the TCC in subprog,
>> the TCC can be propagated from the parent process to the subprocess, but
>> the TCC of the parent process cannot be restored when the subprocess
>> exits. Since the RV64 TCC is initialized before saving the callee saved
>> registers into the stack, we cannot use the callee saved register to
>> pass the TCC, otherwise the original value of the callee saved register
>> will be destroyed. So we implemented mixing bpf2bpf and tailcalls
>> similar to x86_64, i.e. using a non-callee saved register to transfer
>> the TCC between functions, and saving that register to the stack to
>> protect the TCC value. At the same time, we also consider the scenario
>> of mixing trampoline.
>
> Hi!
>
> The RISC-V JIT tries to minimize the stack usage, e.g. it doesn't have a
> fixed pro/epilogue like some of the other JITs. I think we can do better
> here, so that the pass-TCC-via-register can be used, and the additional
> stack access can be avoided.
>
> Today, the TCC is passed via a register (a6) and can be viewed as a
> "state" variable/transparent argument/return value. As you point out, we
> loose this when we do a call. On (any) calls we move the TCC to a
> callee-saved register.
>
> WDYT about the following scheme:
>
> 1 Pickup the arm64 bpf2bpf/tailmix mechanism of just clearing the TCC
> for the main program.
> 2 For BPF helper calls, move TCC to s6, perform the call, and restore
> a6. Dito for kfunc calls (BPF_PSEUDO_KFUNC_CALL).
> 3 For all other calls, a6 is passed transparently.
>
> For 2 bpf_jit_get_func_addr() can be used to determine if the callee is
> a BPF helper or not.
>
> In summary; Determine in the JIT if we're leaving BPF-land, and need to
> move the TCC to a callee-saved reg, or not, and save us a bunch of stack
> store/loads.
>

Sorry, I am on holiday, will deal with it after the holiday.

>
> Björn