Re: [PATCH v10 05/28] x86/cea: Use array indexing to simplify exception stack access

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Unnecessary out-of-line function calls and runtime bounds checks introduced in highly performance-sensitive #VC exception entry paths.
--

commit 6ef3745e3d1d46a5989ce999cb84609a2d082888
Author: Xin Li (Intel) <xin@zytor.com>

x86/cea: Use array indexing to simplify exception stack access

Refactors struct cea_exception_stacks to leverage array indexing for
exception stack access, improving code clarity. Converts
__this_cpu_ist_{bottom,top}_va() from macros to functions.

> diff --git a/arch/x86/mm/cpu_entry_area.c b/arch/x86/mm/cpu_entry_area.c
> index 9fa371af8abc7..a47dfd686bd33 100644
> --- a/arch/x86/mm/cpu_entry_area.c
> +++ b/arch/x86/mm/cpu_entry_area.c
> @@ -18,6 +18,26 @@ static DEFINE_PER_CPU_PAGE_ALIGNED(struct entry_stack_page, entry_stack_storage)
>  static DEFINE_PER_CPU_PAGE_ALIGNED(struct exception_stacks, exception_stacks);
>  DEFINE_PER_CPU(struct cea_exception_stacks*, cea_exception_stacks);
>  
> +/*
> + * Typically invoked by entry code, so must be noinstr.
> + */
> +noinstr unsigned long __this_cpu_ist_bottom_va(enum exception_stack_ordering stack)
> +{
> +	struct cea_exception_stacks *s;
> +
> +	BUILD_BUG_ON(ESTACK_DF != 0);
> +	BUG_ON(stack >= N_EXCEPTION_STACKS);

[Severity: Medium]
Does moving these helpers from macros in the header to out-of-line functions
introduce an unintended performance regression?

Because the functions are defined out-of-line in a different translation unit,
the compiler cannot statically evaluate the BUG_ON(stack >= N_EXCEPTION_STACKS)
bounds checks nor inline the pointer math.

This incurs a penalty of multiple function calls and dynamic checks in
performance-sensitive #VC exception entry paths. For example, hot paths like
is_vc2_stack() in arch/x86/coco/sev/vc-handle.c repeatedly call these functions,
which are evaluated on every #VC exception:

arch/x86/coco/sev/vc-handle.c:is_vc2_stack() {
    ...
    return (sp >= __this_cpu_ist_bottom_va(ESTACK_VC2) &&
            sp < __this_cpu_ist_top_va(ESTACK_VC2));
}

Could this regression in a noinstr entry path be avoided by defining these
helpers as static __always_inline in the header instead?

> +
> +	s = __this_cpu_read(cea_exception_stacks);
> +
> +	return (unsigned long)&s->event_stacks[stack].stack;
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260911213659.2025974-1-sohil.mehta@intel.com?part=5