Re: [PATCH] ACPI: processor_idle: Skip dummy wait for processors based on the Zen microarchitecture

[Dave Hansen]

On 9/22/22 13:10, Andreas Mohr wrote:
>   (- but then what about other more modern chipsets?)
> 
> --> we need to achieve (hopefully sufficiently precisely) a solution which
> takes into account Zen3 STPCLK# improvements while
> preserving "accepted" behaviour/requirements on *all* STPCLK#-hampered chipsets
> ("STPCLK# I/O wait is default/traditional handling"?).

Ideally, sure.  But, we're talking about theoretically regressing the
idle behavior of some indeterminate set of old systems, the majority of
which are sitting in a puddle of capacitor goo at the bottom of a
landfill right now.  This is far from an ideal situation.

FWIW, I'd much rather do something like

	if ((boot_cpu_data.x86_vendor == X86_VENDOR_AMD) &&
	    (boot_cpu_data.x86_model >= 0xF))
		return;

	inl(slow_whatever);

than a Zen check.  AMD has, as far as I know, been a lot more sequential
and sane about model numbers than Intel, and there are some AMD model
number range checks in the codebase today.

A check like this would also be _relatively_ future-proof in the case
that X86_FEATURE_ZEN stops getting set on future AMD CPUs.  That's a lot
more likely than AMD going and reusing a <0xF model.