linux-kernel - Re: [PATCH v2 0/3] x86/PCI: Log E820 clipping

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Date:   Tue, 19 Apr 2022 10:03:44 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Myron Stowe <myron.stowe@...hat.com>,
        Juha-Pekka Heikkila <juhapekka.heikkila@...il.com>,
        Benoit Grégoire <benoitg@...us.ca>,
        Hui Wang <hui.wang@...onical.com>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        linux-acpi@...r.kernel.org, linux-pci@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v2 0/3] x86/PCI: Log E820 clipping

On Tue, Apr 19, 2022 at 11:59:17AM +0200, Hans de Goede wrote:
> On 1/1/70 01:00, Bjorn Helgaas wrote:
> > This is still work-in-progress on the issue of PNP0A03 _CRS methods that
> > are buggy or not interpreted correctly by Linux.
> > 
> > The previous try at:
> >   https://lore.kernel.org/r/20220304035110.988712-1-helgaas@kernel.org
> > caused regressions on some Chromebooks:
> >   https://lore.kernel.org/r/Yjyv03JsetIsTJxN@sirena.org.uk
> > 
> > This v2 drops the commit that caused the Chromebook regression, so it also
> > doesn't fix the issue we were *trying* to fix on Lenovo Yoga and Clevo
> > Barebones.
> > 
> > The point of this v2 update is to split the logging patch into (1) a pure
> > logging addition and (2) the change to only clip PCI windows, which was
> > previously hidden inside the logging patch and not well documented.
> > 
> > Bjorn Helgaas (3):
> >   x86/PCI: Eliminate remove_e820_regions() common subexpressions
> >   x86: Log resource clipping for E820 regions
> >   x86/PCI: Clip only host bridge windows for E820 regions
> 
> Thanks, the entire series looks good to me:
> 
> Reviewed-by: Hans de Goede <hdegoede@...hat.com>

Thank you!

> So what is the plan to actually fix the issue seen on some Lenovo models
> and Clevo Barebones ?   As I mentioned previously I think that since all
> our efforts have failed so far that we should maybe reconsider just
> using DMI quirks to ignore the E820 reservation windows for host bridges
> on affected models ?

I have been resisting DMI quirks but I'm afraid there's no other way.
I think the web we've gotten into, where vendors have used E820 to
interact with _CRS in incompatible and undocumented ways, is not
sustainable.

I'm not aware of any spec that says the OS should use E820 to clip
things out of _CRS, so I think the long term plan should be to
decouple them by default.

Straw man:

  - Disable E820 clipping by default.

  - Add a quirk to enable E820 clipping for machines older than X,
    e.g., 2023, to avoid breaking machines that currently work.

  - Add quirks to disable E820 clipping for individual machines like
    the Lenovo and Clevos that predate X, but E820 clipping breaks
    them.

  - Add quirks to enable E820 clipping for individual machines like
    the Chromebooks (and probably machines we don't know about yet)
    that have devices that consume part of _CRS but are not
    enumerable.

  - Communicate this to OEMs to try to prevent future machines that
    need quirks.

Bjorn