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Date:   Fri, 23 Sep 2022 09:42:06 +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,
        linux-acpi@vger.kernel.org, linux-pm@vger.kernel.org,
        dave.hansen@linux.intel.com, bp@alien8.de, tglx@linutronix.de,
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        gpiccoli@igalia.com, daniel.lezcano@linaro.org,
        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:
> 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.

Except you need to add VENDOR_HYGON at the very least. All of this turns
into a trainwreck real quick.