Xen devel - Oct 2009 - [RFC] Correct/fast timestamping in apps under Xen [3 of 4]: Incarnation number and landlocking

============Premise 3: Xen is able to track the "incarnation" number
for
a guest.  This number will increase whenever a guest is
restored or migrated (and possibly more frequently).
Optionally, an administrator can explicitly mark a guest
as "landlocked", disallowing save/restore/migration for
that guest.
============
Incarnation number is intended to be a monotonically
increasing 64-bit number that never wraps around.
It will certainly increment when a guest is saved
and restored, and when a guest migrates.  Other
events might be useful for an app to track, such
as vcpu-to-pcpu switches, C3 power events, and ??

"Landlocking" (term just made up, suggest a better one
if you have one) enforces the fact that many guests
are never saved and restored and never migrate.
If a system administrator is willing to guarantee
that (and the guarantee may be necessary in many
other circumstances anyway, such as PCI passthrough),
AND if the guest is lauched on a server with reliable
TSC, many TSC problems go away so rdtsc CAN be used
directly.  I''m thinking that a VM config option can
be specified and Xen (or tools) would enforce it.

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