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

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

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 (default of
the grant table on ARM) of memory because we never give back the page to
Xen due the 1:1 mapping. Although I guess this is not a big deal as it's
quite small and Linux, as said by David, will support memory hotplug soon.

FreeBSD is using memory hole in the address space so there is no issue here.

So I guess we could skip this parameters as 128KB doesn't seem to be a
big deal.

> IOW it should avoid such shattering where it can.

That would work too.

Regards,

-- 
Julien Grall