Re: [PATCH bpf] bpf, riscv: Make arena support depend on ZACAS

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] The unconditional use of `system_has_cmpxchg128()` in `bpf_jit_supports_arena()` causes a build failure when `CONFIG_RISCV_ISA_ZACAS` or `CONFIG_TOOLCHAIN_HAS_ZACAS` is disabled.
--

commit c73d679b84188e98d5cff0171b751fa3f4ce5627
Author: Chen Pei <cp0613@linux.alibaba.com>

bpf, riscv: Make arena support depend on ZACAS

This patch updates bpf_jit_supports_arena() to return the result of
system_has_cmpxchg128(). This ensures that BPF arena map creation fails
with -EOPNOTSUPP on RISC-V systems that do not have the ZACAS extension.

> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
> index e7378be171a9f..a5870d386f983 100644
> --- a/arch/riscv/net/bpf_jit_comp64.c
> +++ b/arch/riscv/net/bpf_jit_comp64.c
> @@ -2141,7 +2141,11 @@ bool bpf_jit_supports_ptr_xchg(void)
>  
>  bool bpf_jit_supports_arena(void)
>  {
> -	return true;
> +	/*
> +	 * The arena range tree uses kmalloc_nolock(), which needs
> +	 * cmpxchg128, provided by ZACAS on riscv.
> +	 */
> +	return system_has_cmpxchg128();

[Severity: Low]
Does this code cause a build regression when compiling without ZACAS support?

Looking at arch/riscv/include/asm/cmpxchg.h, system_has_cmpxchg128() is
conditionally defined without a fallback:

#if defined(CONFIG_64BIT) && defined(CONFIG_RISCV_ISA_ZACAS) && defined(CONFIG_TOOLCHAIN_HAS_ZACAS)
#define system_has_cmpxchg128()        riscv_has_extension_unlikely(...)
...
#endif

If a kernel is built with CONFIG_BPF_JIT=y but CONFIG_RISCV_ISA_ZACAS=n
(or CONFIG_TOOLCHAIN_HAS_ZACAS=n), will the unconditional use of
system_has_cmpxchg128() in bpf_jit_supports_arena() result in an implicit
declaration error?

>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260901131132.16241-1-cp0613@linux.alibaba.com?part=1