Re: Design doc of adding ACPI support for arm64 on Xen - version 3

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

On 18/08/2015 00:01, Jan Beulich wrote:
>>>> On 18.08.15 at 08:43, <julien.grall@citrix.com> wrote:
>> Hi Jan,
>>
>> On 17/08/2015 22:10, Jan Beulich wrote:
>>>>>> Julien Grall <julien.grall@citrix.com> 08/17/15 6:27 PM >>>
>>>> On 17/08/2015 08:33, Jan Beulich wrote:
>>>>>>>> On 14.08.15 at 16:59, <shannon.zhao@linaro.org> wrote:
>>>>>> b) Create EFI_MEMORY_DESCRIPTOR table. This will add memory start and
>>>>>> size information of Dom0. And Dom0 will get the memory information
>>>>>> through this EFI table.
>>>>>
>>>>> To some degree the same applies here: While I see that you have no
>>>>> legacy vehicle like x86's E820, I also don't see how Dom0 - not being
>>>>> able to make EFI boot or runtime services calls - would get hold of this
>>>>> table. And if a non-EFI mechanism is to be used here, using the EFI
>>>>> data structure would turn out to be just an arbitrary (or convenience)
>>>>> decision, not something inherently required. Which I think should be
>>>>> said explicitly if so, rather than leaving the reader guess.
>>>>
>>>> It's not an arbitrary decision, when UEFI stub in Linux is using device
>>>> tree properties to pass the UEFI table to the kernel ([1]).
>>>>
>>>> When booting on Xen with ACPI, dom0 will use the non-EFI entry point.
>>>> The easiest way to pass the memory information to Linux is using the
>>>> UEFI DT properties.
>>>
>>> In which case it is even more arbitrary to use the EFI data structure to
>>> convey memory information (instead of expressing it in plain DT, which is
>>> how I blindly assume non-EFI does it). Of course there's the small chance
>>> that "UEFI DT properties" implies a certain binary format, but it's still
>> odd
>>> for a non-EFI entry point to assume EFI properties to be there...
>>
>> Linux is able to boot either on ACPI or DT. When ACPI is used, the EFI
>> stub (mandatory) will create a small DT in order to pass the command
>> line and other informations (such as the EFI memory table) to the
>> kernel. This is because the stub is self-contained and and never use
>> variable living in the kernel.
>>
>> In order to know whether you are booting using DT or ACPI, they check if
>> the DT contains only the /chosen node.
>>
>> Actually the EFI-stub will always jumped on the non-EFI path.
>>
>> By passing the minimal DT as suggested on the design doc, we are
>> avoiding to get a different code path for Xen in the kernel and we are
>> finally acting as the EFI-stub was a nop for DOM0.
>>
>> So I don't see how this would be arbitrary...
>
> Indeed with your explanation this doesn't look arbitrary anymore.
> But none of this was spelled out in the original document.

Sorry I'm forgetting every time that you may not be aware of all the 
justifications.

Regards,

-- 
Julien Grall