Re: [Xen-devel] [PATCH v3 38/62] arm/acpi: Add placeholder for efi and acpi load address

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Stefano,

On 26/11/15 16:04, Stefano Stabellini wrote:
> On Wed, 18 Nov 2015, Julien Grall wrote:
>> On 18/11/15 03:01, Shannon Zhao wrote:
>>> "All above tables will be mapped to Dom0 non-RAM space. Since when
>>> booting through ACPI it doesn't need the grant table region(see below
>>> section 3), it could use this region to store the tables or use the same
>>> way to find one memory region to store them."
>>>
>>> Firstly, as Jan suggested, these tables should not be in RAM space, so
>>> we drop the previous way that copying these tables to Dom0 RAM.
>>> Then I suggested map these tables to the space after the Dom0 RAM space,
>>> but this not right because Dom0 RAM region might be at the edge of
>>> physical RAM space and there might be device MMIO regions.
>>> Then you suggest it could map these tables to the region which is used
>>> for grant table(or the region found by the same way) while it's not used
>>> when it boots with ACPI. These regions are not used by Xen and will not
>>> be used by Dom0 either currently. But as you say, it will be wrong if
>>> Dom0 memory is not 1:1 mapped.
>>
>> Will you remember in 6 months why you wrote the code like that?
>>
>> My point on the previous mail is you don't describe what you did,
>> neither in the code nor in the commit message.
>>
>> Most of the place in the code are trying to avoid the assumption that
>> DOM0 is using direct mapped. If not, we always have a comment/commit
>> message explaining why we are doing like that and the implication (see
>> the grant table example [1]).
>>
>>> So how about below idea:
>>> We still copy these tables to Dom0 RAM space but when we create
>>> EFI_MMAP_TABLE, we remove the space occupied by these tables from the
>>> EfiConventionalMemory descriptor.
>>
>> As you don't need the grant table region, why don't you re-use it fopr
>> your tables? It may also be possible that we have some space just after
>> for the EFI table.
> 
> I think that the current approach is good, please just extend the
> in-code comment in patch #40.

In fact my suggestion was wrong :/. We need real memory for EFI/ACPI
placeholder.

Regards,

-- 
Julien Grall

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