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    SubjectRe: [PATCH v2 0/3] x86/PCI: Log E820 clipping

    Sorry for the late reply.

    On 4/19/22 18:45, Bjorn Helgaas wrote:
    > On Tue, Apr 19, 2022 at 05:16:44PM +0200, Hans de Goede wrote:
    >> Hi,
    >> On 4/19/22 17:03, Bjorn Helgaas wrote:
    >>> 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:
    >>>>> caused regressions on some Chromebooks:
    >>>>> 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 <>
    >>> 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.
    >> Well there is the first match adjacent windows returned by _CRS and
    >> only then do the "covers whole region" exception check. I still
    >> think that would work at least for the chromebook regression...
    > Without a crystal clear strategy, I think we're going to be tweaking
    > the algorithm forever as the _CRS/E820 mix changes. That's why I
    > think that in the long term, a "use _CRS only, with quirks for
    > exceptions" strategy will be simplest.

    Looking at the amount of exception we already now about I'm
    not sure if that will work well.

    >> So do you want me to give that a try; or shall I write a patch
    >> using DMI quirks. And if we go the DMI quirks, what about
    >> matching cmdline arguments? If we add matching cmdline arguments,
    >> which seems to be the sensible thing to do then to allow users
    >> to test if they need the quirk, then we basically end up with my
    >> first attempt at fixing this from 6 months ago:
    > So I think we should go ahead with DMI quirks instead of trying to
    > make the algorithm smarter, and yes, I think we will need commandline
    > arguments, probably one to force E820 clipping for future machines,
    > and one to disable it for old machines.

    So what you are suggesting is to go back to a bios-date based approach
    (to determine old vs new machines) combined with DMI quirks to force
    E820 clipping on new machines which turn out to need it despite them
    being new ?

    >>> 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.
    >> Right and AFAICT the reason Windows is getting away with this is
    >> the same as with the original Dell _CRS has overlap with
    >> physical RAM issue (1), Linux assigns address to unassigneds BAR-s
    >> starting with the lowest available address in the bridge window,
    >> where as Windows assigns addresses from the highest available
    >> address in the window.
    > Right, I agree. I'm guessing Chromebooks don't get tested with
    > Windows at all, so we don't even have that level of testing to help.
    >> So the real fix here might very well be
    >> to rework the BAR assignment code to switch to fill the window
    >> from the top rather then from the bottom. AFAICT all issues where
    >> excluding _E820 reservations have helped are with _E820 - bridge
    >> window overlaps at the bottom of the window.
    >> IOW these are really all bugs in the _CRS method for the bridge,
    >> which Windows does not hit because it never actually uses
    >> the lowest address(es) of the _CRS returned window.
    > Yes. We actually did try this
    > (, but unfortunately we had
    > to revert it. Even more unfortunately, the revert
    > ( doesn't have any details
    > about what went wrong.

    When I first started working on this I did read the entire old
    email thread and IIRC this approach was reverted because the
    e820 based approach was deemed to be a cleaner fix. Also the
    single resource_alloc_from_bottom flag influenced all types
    of resource allocations, not just PCI host bridge window

    Note that the current kernel no longer has the resource_alloc_from_bottom
    flag. Still I think it might be worthwhile to give switching to
    top-down allocating for host bridge window allocs a try. Maybe we
    can make the desired allocation strategy a flag in the resource ?

    I have the feeling that if we switch to top-down allocating
    that we can then switch to just using _CRS and that everything
    will then just work, because we then match what Windows is doing...



    >> 1) At least I read in either a bugzilla, or email thread about
    >> this that Windows allocating bridge window space from the top
    >> was assumed to be why Windows was not impacted.
    >>> 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

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