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

[Shannon Zhao <zhaoshenglong@xxxxxxxxxx>]

On 2015/8/31 20:34, Julien Grall wrote:
> 
> 
> On 31/08/2015 13:03, Shannon Zhao wrote:
>>>> Currently I use the last RAM bank(kinfo->mem.bank[nr_banks - 1]) to
>>>> calculate the start address of non-RAM place. I'm not sure there
>>>> will be
>>>> no MMIO at that place. Do you have any good idea to find such sure and
>>>> safe non-ram place?
>>>
>>> It's not a safe place because with the direct mapping for the RAM, DOM0
>>> RAM could live in the last bank.
>>
>> I put the tables after the last bank currently.
> 
> I misread your suggestion and though you were planning to use the last
> host memory bank region for the UEFI tables.
> 
> So what you are suggesting is not safe at all, what does ensure you that
> there is no MMIO right after the last DOM0 bank? The DOM0 RAM region may
> lives at the edge of a host RAM bank.
> 

Yes, you're right.

>>
>>> Furthermore, for the grant table
>>> region, we are re-using the RAM region used by Xen (find_gnttab_region),
>>> so it may clash with it too because Xen is relocated as high as
>>> possible.
>>>
>>
>> Oh, I didn't realize this.
>>
>>> Although, given that the grant table region will be found by Linux when
>>> ACPI is used (see your section 3.), we could be able to re-use the Xen
>>> region to store the new tables/structures.
>>>
>>
>> So you mean it's fine to put the tables after last bank of Dom0 RAM?
> 
> No, see my answer above. I'm suggesting to re-use the same trick as we
> do for the grant table region. We know that this region will never be
> allocated in the DOM0 address space either because of the direct mapping
> or because it's very unlikely in the case of the non-direct mapping (Xen
> RAM region is very high).
> 

I tried this. Directly use the "kinfo->gnttab_start = __pa(_stext)" as
the address where these tables are mapped to Dom0. But the value of
gnttab_start is lower than the start of RAM, so Dom0 ingore these
regions and boot failed. see early_init_dt_add_memory_arch()

>>
>> In addition, how does UEFI find the space to place the tables? Could we
>> use the same way?
> 
> I think that those tables are living in the RAM and region used are
> marked as reserved.
> 

So can we use the same way for Dom0? I think the Linux will reserve the
regions for EFI in reserve_regions(). Therefore, Dom0 will not use these
reserved regions for other use.


-- 
Shannon


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