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From: Peter Maydell <peter.maydell@linaro.org>
Date: Mon, 3 Aug 2015 10:48:15 +0100
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Subject: Re: [Qemu-devel] [PATCH v3] hw/arm/virt: Add high MMIO PCI region
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To: Pavel Fedin <p.fedin@samsung.com>
Cc: QEMU Developers <qemu-devel@nongnu.org>, Igor Mammedov <imammedo@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Alexander Graf <agraf@suse.de>, "Michael S. Tsirkin" <mst@redhat.com>

On 3 August 2015 at 09:09, Pavel Fedin <p.fedin@samsung.com> wrote:
>  Hi!
>
>> What I thought you meant was that a non-LPAE kernel didn't
>> work at all if we told it about the high-MMIO window (which
>> would mean we'd need to *not* put that in the dtb if we
>> wanted to avoid breaking non-LPAE guests that didn't care
>> about the other window.)
>
>  Current generic PCI driver is not so smart. It simply tries to map all resources using devm_request_resource() in a loop. If a single call fails, the driver thinks that it cannot work and fails. It does not try to ignore inaccessible regions.

Then this is a kernel bug and the kernel should be fixed.

>> >  The behavior which i explained above causes boot problems if our
>> > configuration assumes that we boot off emulated PCI device. Because
>> > PCI controller becomes unusable.
>>
>> ...which is what you're saying here.
>>
>> Which is it?
>
>  I don't understand this last question...

I was confused about whether what happened was
(a) the high region is just ignored
(b) the presence of the high region in the device tree blob causes
things which previously worked to stop working

It sounds like the answer is (b).

>  In order to keep this guest working, we need a possibility to
> disable the new MMIO region in qemu. At least to omit it from the
> device tree.

Yes, this is the workaround, which it sounds like we need.

thanks
-- PMM