linux-kernel - Re: [PATCH v5 1/2] x86/PCI: Ignore E820 reservations for bridge windows on newer systems

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Date:   Sat, 6 Nov 2021 11:15:07 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Myron Stowe <myron.stowe@...hat.com>,
        Juha-Pekka Heikkila <juhapekka.heikkila@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, linux-acpi@...r.kernel.org,
        linux-pci@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Benoit Grégoire <benoitg@...us.ca>,
        Hui Wang <hui.wang@...onical.com>, stable@...r.kernel.org,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH v5 1/2] x86/PCI: Ignore E820 reservations for bridge
 windows on newer systems

Hi Bjorn,

On 10/20/21 23:14, Bjorn Helgaas wrote:
> On Wed, Oct 20, 2021 at 12:23:26PM +0200, Hans de Goede wrote:
>> On 10/19/21 23:52, Bjorn Helgaas wrote:
>>> On Thu, Oct 14, 2021 at 08:39:42PM +0200, Hans de Goede wrote:
>>>> Some BIOS-es contain a bug where they add addresses which map to system
>>>> RAM in the PCI host bridge window returned by the ACPI _CRS method, see
>>>> commit 4dc2287c1805 ("x86: avoid E820 regions when allocating address
>>>> space").
>>>>
>>>> To work around this bug Linux excludes E820 reserved addresses when
>>>> allocating addresses from the PCI host bridge window since 2010.
>>>> ...
> 
>>> I haven't seen anybody else eager to merge this, so I guess I'll stick
>>> my neck out here.
>>>
>>> I applied this to my for-linus branch for v5.15.
>>
>> Thank you, and sorry about the build-errors which the lkp
>> kernel-test-robot found.
>>
>> I've just send out a patch which fixes these build-errors
>> (verified with both .config-s from the lkp reports).
>> Feel free to squash this into the original patch (or keep
>> them separate, whatever works for you).
> 
> Thanks, I squashed the fix in.
> 
> HOWEVER, I think it would be fairly risky to push this into v5.15.
> We would be relying on the assumption that current machines have all
> fixed the BIOS defect that 4dc2287c1805 addressed, and we have little
> evidence for that.
>
> I'm not sure there's significant benefit to having this in v5.15.
> Yes, the mainline v5.15 kernel would work on the affected machines,
> but I suspect most people with those machines are running distro
> kernels, not mainline kernels.

I understand that you were reluctant to add this to 5.15 so close
near the end of the 5.15 cycle, but can we please get this into
5.16 now ?

I know you ultimately want to see if there is a better fix,
but this is hitting a *lot* of users right now and if we come up
with a better fix we can always use that to replace this one
later.

So cam we please just go with this fix now, so that we can
fix the issues a lot of users are seeing caused by the current
*wrong* behavior of taking the e820 reservations into account ?

Regards,

Hans