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Date: Mon, 13 Jul 2015 18:20:41 +0200
From: Alexander Graf <agraf@suse.de>
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Subject: Re: [Qemu-devel] [PATCH 2.4] hw/arm/boot: Increase fdt alignment
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: QEMU Developers <qemu-devel@nongnu.org>

On 07/13/15 18:13, Peter Maydell wrote:
> On 13 July 2015 at 16:56, Alexander Graf <agraf@suse.de> wrote:
>> The Linux kernel on aarch64 creates a page table entry at early bootup
>> that spans the 2MB range on memory spanning the fdt start address:
>>
>>    [ ALIGN_DOWN(fdt, 2MB) ... ALIGN_DOWN(fdt, 2MB) + 2MB ]
>>
>> This means that when our current 4k alignment happens to fall at the end
>> of the aligned region, Linux tries to access memory that is not mapped.
>>
>> The easy fix is to instead increase the alignment to 2MB, making Linux's
>> logic always succeed.
> This is a kernel bug and should be fixed there. The booting
> protocol document is pretty clear:
> https://www.kernel.org/doc/Documentation/arm64/booting.txt
>
> "The device tree blob (dtb) must be placed on an 8-byte boundary"

While I tend to agree, we can't fix old / released kernels. So maybe we 
should just fix it in both?


Alex