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

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Date:   Fri, 23 Sep 2022 09:42:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andreas Mohr <andi@...as.de>,
        K Prateek Nayak <kprateek.nayak@....com>,
        linux-kernel@...r.kernel.org, rafael@...nel.org, lenb@...nel.org,
        linux-acpi@...r.kernel.org, linux-pm@...r.kernel.org,
        dave.hansen@...ux.intel.com, bp@...en8.de, tglx@...utronix.de,
        puwen@...on.cn, mario.limonciello@....com, rui.zhang@...el.com,
        gpiccoli@...lia.com, daniel.lezcano@...aro.org,
        ananth.narayan@....com, gautham.shenoy@....com,
        Calvin Ong <calvin.ong@....com>, stable@...r.kernel.org,
        regressions@...ts.linux.dev
Subject: Re: [PATCH] ACPI: processor_idle: Skip dummy wait for processors
 based on the Zen microarchitecture

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.