[PATCH 0/6] xen/x86: PV boot speedup

[Jan Beulich]

The observed (by the human eye) performance difference of early boot
between native and PV-on-Xen was just too large to not look into. As
it turns out, gaining performance back wasn't all that difficult.

While the series (re)introduces a small number of PTWR emulations on
the boot path (from phys_pte_init()), there has been a much larger
number of them post-boot. Hence I think if this was of concern, the
post-boot instances would want eliminating first.

Some of the later changes aren'r directly related to the main goal of
the series; these address aspects noticed while doing the investigation.

1: streamline set_pte_mfn()
2: restore (fix) xen_set_pte_init() behavior
3: adjust xen_set_fixmap()
4: adjust handling of the L3 user vsyscall special page table
5: there's no highmem anymore in PV mode
6: restrict PV Dom0 identity mapping

Jan