Re: [PATCH bpf v2] bpf, riscv: Make arena support depend on ZACAS

["Björn Töpel" <bjorn@kernel.org>]

Chen Pei <cp0613@linux.alibaba.com> writes:

> The arena range tree allocates its nodes with kmalloc_nolock() since
> commit f8c67d8550ee ("bpf: Use kmalloc_nolock() in range tree").
> kmalloc_nolock() requires slab caches with cmpxchg128 support
> (__CMPXCHG_DOUBLE); on riscv cmpxchg128 is provided by the ZACAS
> extension. On systems without ZACAS every arena map creation fails
> with a misleading -ENOMEM.
>
> Report the missing support instead: make bpf_jit_supports_arena()
> return system_has_cmpxchg128() where it is defined, so arena map
> creation fails with -EOPNOTSUPP on systems without ZACAS. The macro
> is only defined when both CONFIG_RISCV_ISA_ZACAS and
> CONFIG_TOOLCHAIN_HAS_ZACAS are enabled, so guard it with #ifdef the
> same way mm/slab.h consumes it, and reject arena otherwise. This
> matches how arena BPF_CMPXCHG instructions are already gated on ZACAS
> in bpf_jit_supports_insn().
>
> Fixes: f8c67d8550ee ("bpf: Use kmalloc_nolock() in range tree")
> Cc: stable@vger.kernel.org
> Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
> ---
> Changes since v1:
> - Guard system_has_cmpxchg128() with #ifdef instead of calling it
>   unconditionally: the macro is only defined when both
>   CONFIG_RISCV_ISA_ZACAS and CONFIG_TOOLCHAIN_HAS_ZACAS are enabled
>   (as reported by sashiko-bot), so v1 broke the build when either
>   was disabled. This mirrors how mm/slab.h consumes the macro.
>
> Why #ifdef rather than rv_ext_enabled(ZACAS)? The predicates differ
> exactly in the configurations that matter:
>
>   scenario (ISA_ZACAS/TOOLCHAIN/hw)  v1           rv_ext_enabled  #ifdef
>   ISA=n or TOOLCHAIN=n               build fails  rejects         rejects
>   ISA=y TOOLCHAIN=n hw has ZACAS     build fails  accepts, then   rejects
>                                                   -ENOMEM again
>   ISA=y TOOLCHAIN=y hw has ZACAS     exact        exact           exact

Thanks for clarifying this; Yeah, ZACAS requires runtime *and* toolchain
support.

Acked-by: Björn Töpel <bjorn@kernel.org>