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

[Julien Grall <julien.grall@citrix.com>]

Hi Shannon,

On 02/09/15 10:18, Christoffer Dall wrote:
> On Wed, Sep 02, 2015 at 02:41:36AM -0600, Jan Beulich wrote:
>>>>> Shannon Zhao <zhaoshenglong@huawei.com> 09/02/15 8:03 AM >>>
>>> There are some descriptions in Documentation/arm64/booting.txt of Linux:
>>>
>>> "The Image must be placed text_offset bytes from a 2MB aligned base
>>> address near the start of usable system RAM and called there. Memory
>>> below that base address is currently unusable by Linux, and therefore it
>>> is strongly recommended that this location is the start of system RAM.
>>> At least image_size bytes from the start of the image must be free for
>>> use by the kernel."
>>>
>> >From this, it says "Memory below that base address is currently unusable
>>> by Linux". So if we put these tables below Dom0 RAM address and even
>>> describe these regions as RAM, the Linux could not use them.
>>
>> May I remind you that a design should not take specific guest OS
>> implementation details (which even for that one OS may change over time)
>> as the basis for decisions?
>>
> While I agree that the guest behavior should not dictate an unfortunate
> design, surely factoring in the behavior of the expected guests in the
> design is a reasonable thing to do?
> 
> Changing the boot requirements of Linux for an architecture is a really
> invasive change, IMHO, and should be avoided if possible.
> 
> Are there other acceptable solutions for placing the EFI tables
> somewhere else that would work?

Before speaking about other acceptable solutions, I'd like to have more
information about where the ACPI tables are living in memory on bare
metal.

Is it mandatory to have the ACPI tables living above the kernel Image
in the RAM?

As mentioned to Shannon, I would be surprised this is the case given
that ACPI should not live in memory usable by the kernel.

Regards,

-- 
Julien Grall