From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: Xen balloon driver improvement (version 1)
Date: Thu, 23 Oct 2014 12:42:03 +0100
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To: Ian Campbell <Ian.Campbell@citrix.com>, Wei Liu <wei.liu2@citrix.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>, David Vrabel <david.vrabel@citrix.com>, xen-devel@lists.xen.org
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On 23/10/14 12:00, Ian Campbell wrote:
> On Wed, 2014-10-22 at 19:29 +0100, Wei Liu wrote:
>
>>>> For instance, balloon driver can maintain three queues:
>>>>
>>>> 1. queue for 2 MB pages
>>>> 1. queue for 4 KB pages (delegated to core balloon driver)
>>>> 1. queue for pages used to mapped pages from other domain
>>> What about 1GB pages?
>>>
>> I wouldn't bother with 1GB pages here.
> Guests which don't have special privileges are limited to 2M contiguous
> allocations anyway, to stop them from consuming "precious" higher order
> mappings.
>
> I'm not sure that's still worthwhile (e.g. is it valid on ARM or
> shadow/HAP x86? I'm not sure).
>
>> It would require too much work to coalesce 4KB pages to 1GB pages. 
> FWIW in practice You'd probably coalesce 4K pages into 2M  and then 2M
> into 1G.
>
>

Wouldn't it be wonderful to be able to run a 1GB PVH guest on a 1GB HAP
mapping, with the PVH guest making use of 2MB mapping where possible.

PVH (ought) to be able to do away with the MTRR caching issues, the
legacy IO regions, and so long as the guest doesn't balloon pages out or
map a foreign grant, it won't shatter the host superpage.

But in principle, I agree that making better use of 2MB pages is more
important than considering 1GB pages at the moment.

~Andrew