From mboxrd@z Thu Jan  1 00:00:00 1970
From: Julien Grall <julien.grall@citrix.com>
Subject: Re: Design doc of adding ACPI support for arm64 on Xen
 - version 5
Date: Wed, 2 Sep 2015 14:48:01 +0100
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To: Ian Campbell <ian.campbell@citrix.com>, Shannon Zhao <shannon.zhao@linaro.org>, Shannon Zhao <zhaoshenglong@huawei.com>, xen-devel <xen-devel@lists.xen.org>
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On 02/09/15 14:26, Ian Campbell wrote:
>>>> I think the problem is how you reserved this region in the EFI memory
>>>> table. From what I saw, you marked this new memory with EFI_MEMORY_WB
>>>> (which means that the region can be usable by Linux).
>>>>
>>> Yes, I mark it with EFI_MEMORY_WB. Is this right?
>>
>> I would say no, but it's only because I looked at the kernel code 
>> quickly.
>>
>> You have to looks how ACPI region/UEFI tables are described in the host
>> EFI memory map and mimicking for the DOM0 EFI memory map.
> 
> Surely it is the type (EfiACPIReclaimMemory, EfiACPIMemoryNVS etc) and not
> the mapping attributes which should control whether an OS considers a
> region usable? At least until the OS is done parsing tables neither of
> those are usable (which implies we want NVS as our type, unless the memory
> is intended to be reclaimed by dom0, implying it should own it).

It looks like that Linux on ARM64 is considering any region with
EFI_MEMORY_WB set as normal RAM and will try to add as System RAM (see
reserve_regions in arch/arm64/kernel/efi.c).

At the same time, having WB set for a region that should be read-only
looks like wrong to me.

Regards,

-- 
Julien Grall