x86/pv: Flush TLB in response to paging structure changes

x86/pv: Flush TLB in response to paging structure changes

With MMU_UPDATE, a PV guest can make changes to higher level pagetables.  This
is safe from Xen's point of view (as the update only affects guest mappings),
and the guest is required to flush (if necessary) after making updates.

However, Xen's use of linear pagetables (UPDATE_VA_MAPPING, GNTTABOP_map,
writeable pagetables, etc.) is an implementation detail outside of the
API/ABI.

Changes in the paging structure require invalidations in the linear pagetable
range for subsequent accesses into the linear pagetables to access non-stale
mappings.  Xen must provide suitable flushing to prevent intermixed guest
actions from accidentally accessing/modifying the wrong pagetable.

For all L2 and higher modifications, flush the TLB.  PV guests cannot create
L2 or higher entries with the Global bit set, so no mappings established in
the linear range can be global.  (This could in principle be an order 39 flush
starting at LINEAR_PT_VIRT_START, but no such mechanism exists in practice.)

Express the necessary flushes as a set of booleans which accumulate across the
operation.  Comment the flushing logic extensively.

This is XSA-286.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>