Re: [PATCH bpf-next v4 0/6] Mixing bpf2bpf and tailcalls for RV64

[Björn Töpel]

Hey!

On Mon, 29 Jun 2026 at 15:56, Pu Lehui <pulehui@xxxxxxxxxxxxxxx> wrote:
>
> From: Pu Lehui <pulehui@xxxxxxxxxx>
>
> 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 updated 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. As for the tailcall hierarchy issue,
> inspired by the s390's low-overhead approach, we store TCC from
> RV_REG_TCC back to stack after calling bpf2bpf call or calling orig bpf
> func in bpf trampoline.
>
> In addition, some code cleans are also attached to this patchset.
>
> Tests test_bpf.ko and test_verifier have passed, as well as the relative
> testcases of test_progs*.
>
> v4:
> - Fix tailcall hierarchy issue.
> - use is_struct_ops_tramp helper in bpf trampoline

Alexei outlined some (major) concerns on the whole mixing side of
things [0]. I haven't been following that space -- did Leon have a
workaround/fix? Can you explain more how you fixed it, and why it
would make sense to add it to RISC-V now?

Thanks,
Björn


[0] https://lore.kernel.org/bpf/CAADnVQ+rLneO4t=YYmLYtc945Fz0=ucNTWZBxgvs8toFY-onRg@xxxxxxxxxxxxxx/