x86/tlbflush: do not toggle the PGE CR4 bit unless necessary

x86/tlbflush: do not toggle the PGE CR4 bit unless necessary

When PCID is not available Xen does a full tlbflush by toggling the
PGE bit in CR4. This is not necessary if PGE is not enabled, since a
flush can be performed by writing to CR3 in that case.

Change the code in do_tlb_flush to only toggle the PGE bit in CR4 if
it's already enabled, otherwise do the tlb flush by writing to CR3.
This is relevant when running virtualized, since hypervisors don't
usually trap accesses to CR3 when using hardware assisted paging, but
do trap accesses to CR4 specially on AMD hardware, which makes such
accesses much more expensive.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>