[RFC PATCH 0/3] xen/privcmd: support for paged-out frames

[David Vrabel <david.vrabel@citrix.com>]

This series is a straight forward-port of some functionality from
classic kernels to support Xen hosts that do paging of guests.

This isn't functionality the XenServer makes use of so I've not tested
these with paging in use (GridCentric requested that our older kernels
supported this and I'm just doing the forward port).

I'm not entirely happy about the approach used here because:

1. It relies on the meaning of the return code of the update_mmu
hypercall and it assumes the value Xen used for -ENOENT is the same
the kernel uses. This does not appear to be a formal part of the
hypercall ABI.

Keir, can you comment on this?

2. It seems more sensible to have the kernel do the retries instead of
libxc doing them.  The kernel has to have a mechanism for this any way
(for mapping back/front rings).

3. The current way of handling paged-out frames by repeatedly retrying
is a bit lame.  Shouldn't there be some event that the guest waiting
for the frame can wait on instead?  By moving the retry mechanism into
the kernel we can change this without impacting the ABI to userspace.

David