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

[Hans de Goede <hdegoede@redhat.com>]

Hi,

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:
>>>>>   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@redhat.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.
>>
>> 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:
>>
>> https://lore.kernel.org/linux-pci/20211005150956.303707-1-hdegoede@redhat.com/
> 
> 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
> (https://git.kernel.org/linus/1af3c2e45e7a), but unfortunately we had
> to revert it.  Even more unfortunately, the revert
> (https://git.kernel.org/linus/5e52f1c5e85f) 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
allocations.

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...

Regards,

Hans







> 
>> 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
>>>
>>
>