Re: Design doc of adding ACPI support for arm64 on Xen - version 2

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

On 12/08/2015 09:46, Ian Campbell wrote:
> On Tue, 2015-08-11 at 17:01 +0100, Julien Grall wrote:
>> On 11/08/15 16:19, Ian Campbell wrote:
>>>> IIRC we talked about it few months ago and you said that using
>>>> balloon
>>>> page will split in 4K the 1G/2M mapping done in the stage-2 p2m.
>>>
>>> Did I? Odd because I'm also of the opinion that alloc_ballooned_pages
>>> should operate in chunks of 2M at the hypercall layer and keep any
>>> resulting spare 4K pages on a free list to use for future such
>>> allocations.
>>
>> That from what I recall from an IRL talk.
>>
>> Anyway, I've looked in my archive to see why we decided to keep the
>> grant table parameters (in Xen ACPI table at this point). We were not
>> sure that the domain as all the key in hand in order to find memory hole.
>>
>> I think it's quite important to not think only about Linux but all other
>> Operating Systems. If we ever require a parameters later, it would mean
>> that the OS won't be able to run as DOM0 on older Xen.
>>
>> Linux is using ballooned page, which means loosing ~128KB
>
> It's not "loosing" anything. It is _using_ 128KB for a legitimate data
> structure needed to support the platform (the fact that it happens to not
> really be using the underlying RAM is irrelevant here).
>
> It's not really inherently any different to "loosing" memory in order to
> allocate e.g. page tables.

If you read my sentence until the end, you would have noticed why I say 
"loosing". It's because we never give back this memory to Xen due to the 
1:1 mapping...

The grant table are already allocated in Xen separately which means 
doubling the size of memory used for DOM0 grant table.

Anyway, that's small amount of data and can optimized later using the 
hotplug.

Cheers,

-- 
Julien Grall