Re: 2MB page PV guest support clarification

[Dave McCracken <dcm@mccr.org>]

On Saturday 28 February 2009, Keir Fraser wrote:
> On 28/02/2009 00:03, "Ian Pratt" <Ian.Pratt@eu.citrix.com> wrote:
> >> Its a work in progress, but there's nothing usable yet, as far as I
> >> know.
> >
> > Oracle have been working on PV 2MB page support, and I expect they'll
> > pitch in with an update.
> >
> > Over the last 18 months or so there have been a number of changes to
> > xen's PV PT handling that make support of 2MB pages significantly easier
> > than it was previously. However, the guest has to be careful how it uses
> > them as it can't alias any memory that may be used for storing pagetables
> > pages (that must be RO).
>
> Oracle already got their code checked in. You have to specify
> 'allowhugepage' on Xen's command line to enable it. It has limitations,
> such as save/restore doesn't work.

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.

Dave McCracken
Oracle Corp.