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

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Message-ID: <4d61b9c0-ee00-c5f6-bef1-622b80c79714@intel.com>
Date:   Thu, 22 Sep 2022 14:21:31 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andreas Mohr <andi@...as.de>
Cc:     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, peterz@...radead.org,
        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 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.