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

Pu Lehui
2023-09-19T11:23:31Z

On 2023/9/19 18:04, Conor Dooley wrote:
> On Tue, Sep 19, 2023 at 11:57:11AM +0800, Pu Lehui wrote:
>> 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.
>>
>> Tests test_bpf.ko and test_verifier have passed, as well as the relative
>> testcases of test_progs*.
>>
>> Signed-off-by: Pu Lehui <pulehui@huawei.com>
>
> Breaks the build:
> ../arch/riscv/net/bpf_jit_comp64.c:846:14: error: use of undeclared identifier 'BPF_TRAMP_F_TAIL_CALL_CTX'
>

Hi Conor,

BPF_TRAMP_F_TAIL_CALL_CTX rely on commit [0], and it has been merged
into bpf-next tree.

Link:
https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=2b5dcb31a19a2e0acd869b12c9db9b2d696ef544
[0]

> Thanks,
> Conor.