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

[Peter Zijlstra]

On Thu, Sep 22, 2022 at 02:21:31PM -0700, Dave Hansen wrote:
> 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.

Some might be broken; apparently their SoC/Entertainment divisions has a
few out of order SKUs that were not listed in their regular documents.
(yay interweb)

I ran into this when I tried doing a Zen2 range check for retbleed. In
the end we ended up using the availablility of STIBP as a heuristic to
indentify Zen2+ or something.