XSA-273 changes required, among other things, making any PTE updates
restartable. The changes making PTE updates restartable assumed that L2
pagetables would always be promoted preemptibly; but this turns out not
to be the case when using the 'linear pagetable' feature; the result was
that interrupted operations are not handled properly in certain cases.

Furthermore, previous security work making pagetable update preemptible
failed to account for 'linear pagetables' at L3 and L4 levels, making it
possible for operations to run for longer than acceptable times.