x86/xpti: avoid copying L4 page table contents when possible

x86/xpti: avoid copying L4 page table contents when possible

For mitigation of Meltdown the current L4 page table is copied to the
cpu local root page table each time a 64 bit pv guest is entered.

Copying can be avoided in cases where the guest L4 page table hasn't
been modified while running the hypervisor, e.g. when handling
interrupts or any hypercall not modifying the L4 page table or %cr3.

So add a per-cpu flag indicating whether the copying should be
performed and set that flag only when loading a new %cr3 or modifying
the L4 page table.  This includes synchronization of the cpu local
root page table with other cpus, so add a special synchronization flag
for that case.

A simple performance check (compiling the hypervisor via "make -j 4")
in dom0 with 4 vcpus shows a significant improvement:

- real time drops from 112 seconds to 103 seconds
- system time drops from 142 seconds to 131 seconds

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>