Re: 2MB page PV guest support clarification

[Mick Jordan <Mick.Jordan@sun.com>]

On 03/02/09 05:45, Dave McCracken wrote:
>
> I am the person at Oracle working on PV guest support for 2MB pages.  I did 
> get an initial patch accepted into the Xen hypervisor that enables basic 2MB 
> page support.  As Keir said, it requires 'allowhugepage' on the Xen 
> hypervisor command line.  It supports the basic ability to specify PSE in the 
> page table, and takes care of the associated type and reference tracking for 
> the mapped page(s).
>
> What this patch does not do is make any guarantee about the alignment of the 
> mapped page, which is a hardware requirement.  The solution I am working on 
> for this is to create domains with 2MB pages.  The hypervisor already 
> supports populating a domain with larger pages.  I am working on supporting 
> 2MB page domains at creation time and restore time.  This approach will also 
> require that balloon drivers understand and work with 2MB pages.
>   
In my world, I make sure I allocate aligned contiguous machine 2MB 
pages. Of course that may not always be possible, depending on what you 
get from Xen. And I've seem some wild outer cases, such as swiss cheese 
memory with every other page missing and no physical run longer than two 
4K pages!

Mick