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

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Date:   Thu, 22 Sep 2022 09:28:55 +0530
From:   Ananth Narayan <ananth.narayan@....com>
To:     Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...el.com>
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, tglx@...utronix.de, andi@...as.de,
        puwen@...on.cn, mario.limonciello@....com, peterz@...radead.org,
        rui.zhang@...el.com, gpiccoli@...lia.com,
        daniel.lezcano@...aro.org, 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 22-09-2022 01:21 am, Borislav Petkov wrote:
> On Wed, Sep 21, 2022 at 07:15:07AM -0700, Dave Hansen wrote:
>> In the end, the delay is because of buggy, circa 2006 chipsets?  So, we
>> use a CPU vendor specific check to approximate that the chipset is
>> recent and not affected by the bug?  If so, is there no better way to
>> check for a newer chipset than this?
> 
> So I did some git archeology but that particular addition is in some
> conglomerate, glued-together patch from 2007 which added the cpuidle
> tree:
> 
> commit 4f86d3a8e297205780cca027e974fd5f81064780
> Author: Len Brown <len.brown@...el.com>
> Date:   Wed Oct 3 18:58:00 2007 -0400
> 
>     cpuidle: consolidate 2.6.22 cpuidle branch into one patch

In fact, the code has moved around a fair bit and the check in its initial form
goes as far back as ACPI's posting for inclusion in the kernel in March 2002
[1]. We could not find any way of digging further back, yet.

Prior to that, I think the ACPI enablement code was being released independent
of the kernel per https://kernel.org/doc/ols/2004/ols2004v1-pages-121-132.pdf
and was included in Andrew's mm tree for a while.

>From https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git
the first tag that contains code with the dummy read is v2.5.7 AFAICS.

Ananth

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/mpe/linux-fullhistory.git/commit/?id=972c16130d9dc182cedcdd408408d9eacc7d6a2d