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

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

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...

Regards,

-- 
Julien Grall