Re: [Xen-devel] Design doc of adding ACPI support for arm64 on Xen - version 5

[Christoffer Dall <christoffer.dall@xxxxxxxxxx>]

On Wed, Sep 2, 2015 at 3:54 PM, Ian Campbell <ian.campbell@xxxxxxxxxx> wrote:
> On Wed, 2015-09-02 at 14:48 +0100, Julien Grall wrote:
>> 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).
>
> It's hard to believe this isn't a bug... It's probably worth asking the
> Linux maintainers about this.
>

wasn't this that whole workaround to make sure Linux maps the data as
regular RAM, because otherwise architecture generic code would map it
as IO memory, and generic routines such as memcpy would fault on
unaligned accesses, or am I confusing ACPI with EFI here?

Leif (added to the to-field) had some insight on this earlier on.

-Christoffer

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