Re: [PATCH bpf-next v2 07/11] LoongArch: BPF: Support atomics on arena pointers

[Tiezhu Yang]

On 2026/7/2 上午10:23, George Guo wrote:
> From: George Guo <guodongtai@xxxxxxxxxx>
>
> Implement atomic operations on arena pointers (BPF_PROBE_ATOMIC): the
> read-modify-write ops, atomic_fetch_*, xchg, cmpxchg and
> load-acquire/store-release.  For each, the arena base held in REG_ARENA
> is folded into the address and an exception table entry is registered on
> the access so a fault is handled like the other arena probes.
>
> The exception entry must point at the actual memory-accessing
> instruction rather than the last one emitted: the fetch variants append
> a zero-extend after the am* op, and cmpxchg accesses memory with the ll
> of an ll/sc loop.  Generalise add_exception_handler() to take explicit
> fault and resume instruction indices.  A faulting ll resumes past the
> whole ll/sc loop: if the ll faults the sc is never reached, and once the
> ll succeeds the page is mapped so the sc cannot fault, so a single entry
> on the ll suffices.

...

>   
>   bool bpf_jit_supports_insn(struct bpf_insn *insn, bool in_arena)
>   {
> -	if (!in_arena)
> -		return true;
> -
> -	switch (insn->code) {
> -	case BPF_STX | BPF_ATOMIC | BPF_W:
> -	case BPF_STX | BPF_ATOMIC | BPF_DW:
> -		/* Atomics on arena pointers are not implemented yet. */
> -		return false;
> -	}
> -

Adding code in patch 6 and deleting it in patch 7
is unnecessary code noise.

Please implement it cleanly in patch 6 from the start.

> +	/*
> +	 * All arena access instructions are implemented: regular and
> +	 * sign-extending loads/stores (BPF_PROBE_MEM32 / BPF_PROBE_MEM32SX)
> +	 * and atomics (BPF_PROBE_ATOMIC).  The default weak helper rejects
> +	 * everything, so the override is required to enable arena programs.
> +	 */
>   	return true;
>   }

Thanks,
Tiezhu