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Date:   Thu, 22 Sep 2022 09:28:55 +0530
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Subject: Re: [PATCH] ACPI: processor_idle: Skip dummy wait for processors
 based on the Zen microarchitecture
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To:     Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@intel.com>
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, tglx@linutronix.de, andi@lisas.de,
        puwen@hygon.cn, mario.limonciello@amd.com, peterz@infradead.org,
        rui.zhang@intel.com, gpiccoli@igalia.com,
        daniel.lezcano@linaro.org, gautham.shenoy@amd.com,
        Calvin Ong <calvin.ong@amd.com>, stable@vger.kernel.org,
        regressions@lists.linux.dev
References: <20220921063638.2489-1-kprateek.nayak@amd.com>
 <2b6143b6-9db4-05bc-1e8d-c5d129126f99@intel.com> <YytrV2UMah8555+s@zn.tnic>
From:   Ananth Narayan <ananth.narayan@amd.com>
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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@intel.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