Re: [PATCH v6 1/8] x86/runtime-const: Introduce runtime_const_mask_32()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The commit introduces `runtime_const_mask_32` for x86 but omits a generic fallback in `include/asm-generic/runtime-const.h`.
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--- Patch [1]: [PATCH v6 1/8] x86/runtime-const: Introduce runtime_const_mask_32() ---
commit 664c491c817106934caf3b46117ff57570227576
Author: Peter Zijlstra <peterz@infradead.org>

x86/runtime-const: Introduce runtime_const_mask_32()

This commit introduces the runtime_const_mask_32() macro for the x86
architecture to optimize the mask operation in the futex hash computation
hot path. It also adds the __runtime_fixup_mask() helper to support
this functionality.

Link: https://patch.msgid.link/20260227161841.GH606826@noisy.programming.kicks-ass.net

> diff --git a/arch/x86/include/asm/runtime-const.h b/arch/x86/include/asm/runtime-const.h
> index 4cd94fdcb45e2..b13f7036c1c9b 100644
> --- a/arch/x86/include/asm/runtime-const.h
> +++ b/arch/x86/include/asm/runtime-const.h
[ ... ]
> @@ -41,6 +41,15 @@
>  		:"+r" (__ret));					\
>  	__ret; })
>  
> +#define runtime_const_mask_32(val, sym) ({			\
> +	typeof(0u+(val)) __ret = (val);				\
> +	asm_inline("and $0x12345678, %k0\n1:\n"				\
> +		   ".pushsection runtime_mask_" #sym ",\"a\"\n\t"\
> +		   ".long 1b - 4 - .\n"				\
> +		   ".popsection"				\
> +		   : "+r" (__ret));				\
> +	__ret; })
> +

[Severity: Medium]
Will this cause a build regression on non-x86 architectures when the generic
code starts using runtime_const_mask_32()?

The commit message mentions this will be used by the futex hash computation in
a subsequent commit. However, there doesn't appear to be a generic fallback
for this macro in include/asm-generic/runtime-const.h.

Without a generic fallback, it seems architectures that don't support or
haven't yet implemented runtime constants would fail to compile once the
generic futex code uses this macro.

Should a fallback like the following be introduced in
include/asm-generic/runtime-const.h alongside this x86 implementation to
maintain cross-architecture compatibility and strict bisectability?

#define runtime_const_mask_32(val, sym) ((val) & (sym))

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260728052540.4728-1-kprateek.nayak@amd.com?part=1