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

[Shannon Zhao <zhaoshenglong@huawei.com>]

On 2015/8/18 15:01, 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.

Oh, sorry. Will add more details explanation in next version.

-- 
Shannon