Re: [PATCH 03/16] x86/amd_nb: Clean up early_is_amd_nb()

Sohil Mehta
2024-10-31T00:53:37Z

> On Tue, Oct 29, 2024 at 04:15:33PM +0000, Luck, Tony wrote:
>>>>>> - if (boot_cpu_data.x86_vendor == X86_VENDOR_HYGON)
>>>>>> - misc_ids = hygon_nb_misc_ids;
>>>>>> + if (boot_cpu_has(X86_FEATURE_ZEN))
>>>>>
>>>>> check_for_deprecated_apis: WARNING: arch/x86/kernel/amd_nb.c:395: Do not use boot_cpu_has() - use cpu_feature_enabled() instead.
>>>>

Do the comments in cpufeature.h need updating? It seems to recommend
boot_cpu_has() in most cases and suggests using static_cpu_has() (which
is used by cpu_feature_enabled()) only in fast paths.

/*
* Static testing of CPU features. Used the same as boot_cpu_has(). It
* statically patches the target code for additional performance. Use
* static_cpu_has() only in fast paths, where every cycle counts. Which
* means that the boot_cpu_has() variant is already fast enough for the
* majority of cases and you should stick to using it as it is generally
* only two instructions: a RIP-relative MOV and a TEST.
*
...

*/
static __always_inline bool _static_cpu_has(u16 bit)

>
> And if not to checkpatch, then maybe it can be included in the TIP
> maintainers' handbook? That is, if others are using it or something
> similar.
>