IMO users ought not to be able to use 16-bit segments without
using modify_ldt.  Fortunately, it's impossible to break
espfix64 by loading the PER_CPU segment into SS because it's
PER_CPU is marked read-only and SS cannot contain an RO segment,
but marking PER_CPU as 32-bit is less fragile.
Signed-off-by: NAndy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/179f490d659307873eefd09206bebd417e2ab5ad.1411494540.git.luto@amacapital.netSigned-off-by: NIngo Molnar <mingo@kernel.org>