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Date:   Fri, 23 Sep 2022 09:57:51 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Andreas Mohr <andi@lisas.de>,
        K Prateek Nayak <kprateek.nayak@amd.com>,
        linux-kernel@vger.kernel.org, rafael@kernel.org, lenb@kernel.org,
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        ananth.narayan@amd.com, gautham.shenoy@amd.com,
        Calvin Ong <calvin.ong@amd.com>, stable@vger.kernel.org,
        regressions@lists.linux.dev
Subject: Re: [PATCH] ACPI: processor_idle: Skip dummy wait for processors
 based on the Zen microarchitecture
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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.