Versions of the two final patches were submitted standalone earlier
on. The series here tries to carry out a suggestion from Andrew,
which the two of us have been discussing. Then said previously posted
patches are re-based on top, utilizing the new functionality.

Xen:

1: spec-ctrl: add logic to issue IBPB on exit to guest
2: spec-ctrl: defer context-switch IBPB until guest entry
3: limit issuing of IBPB during context switch
4: PV: issue branch prediction barrier when switching 64-bit guest to kernel mode

Linux (new):

1: x86/Xen: make use of IBPB controlling VM assist

Jan