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

From: Dave Hansen <dave.hansen@intel.com>
To: Andreas Mohr <andi@lisas.de>
Cc: 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,
	puwen@hygon.cn, mario.limonciello@amd.com, peterz@infradead.org,
	rui.zhang@intel.com, 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
Date: Thu, 22 Sep 2022 14:21:31 -0700	[thread overview]
Message-ID: <4d61b9c0-ee00-c5f6-bef1-622b80c79714@intel.com> (raw)
In-Reply-To: <YyzBLc+OFIN2BMz5@rhlx01.hs-esslingen.de>

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.