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

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Message-ID: <YytrV2UMah8555+s@zn.tnic>
Date:   Wed, 21 Sep 2022 21:51:51 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     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, 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 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


so the most precise check here should be to limit that dummy read to
that Intel chipset which needed it. Damned if I knew how to figure out
which...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette