Re: [PATCH 00/11] Alternate p2m: support multiple copies of host p2m

[Ed White <edmund.h.white@intel.com>]

> Or: declare in the interface that the altp2ms are soft state that can
> be dropped on migration, with some suitable callback (#VE injection?)
> to the guest when an altp2m 'view' is not available.  That depends on
> whether the in-guest agent can reconstruct the state it needs from
> scratch.
> 

I've been wondering about this too, although it's not something the
existing software that we want to enable on Xen has to cope with.

I still don't understand under what circumstances a machine page
can be removed from a guest, especially given that we are only
going to use remapping when we are actively using both copies of
the page. However, if such a page can be removed, the agent
(in-domain or out-of-domain) has to be able to know about that
and handle it. There's also the issue that access permissions
are soft state and can be reverted to default in certain cases.

Maybe the solution to all of this is a mechanism by which the
agent can be notified that some of its modifications have been
invalidated, so it can rebuild state as appropriate.

We could then add a piece of state to an alternate p2m to indicate
that it contains remapped gfn->mfn translations, and invalidate
the entire p2m if any mfn is removed from the guest.

Migration could invalidate everything.

Ed