Re: [Xen-devel] [PATCH 1/2] PV hugepages - Xen patch

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

On Wed, Oct 08, 2008 at 03:50:56PM -0700, Jeremy Fitzhardinge wrote:
> Keir Fraser wrote:
> >Actually this is an interesting one. For a PV guest it may be in general
> >unsolvable, since the target machine may not have allocatable 2MB extents.
> >It may also screw live migration since 2MB is a very coarse granularity to
> >do dirty-page tracking. One option: perhaps the PV kernel could shatter and
> >then reconstruct (as best it can) superpage mappings across save/restore?
> 
> That means you need to notify the guest when you're starting a live 
> migration, rather than just springing it on them at the last moment as 
> we do now.
> 
> But shattering large pages all over the place is going to be pretty 
> expensive, and possibly awkward if it suddenly needs to come up with a 
> pile of pages for the new L1 entries.

Or you could just take the view this is a pre-migration capability check,
and that admin (or mgmt app) must ensure sufficient free hugepages on the 
destination before attempting migration. If this isn't satisfied then
XenD can just fail / abort the migration op and leave it running on original
host.

Dainel
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