x86/traps: Drop exception_table[] and use if/else dispatching

x86/traps: Drop exception_table[] and use if/else dispatching

There is also a lot of redundancy in the table.  8 vectors head to do_trap(),
3 are handled in the IST logic, and that only leaves 7 others not heading to
the do_reserved_trap() catch-all.  This also removes the fragility that any
accidental NULL entry in the table becomes a ticking timebomb.

Function pointers are expensive under retpoline, and different vectors have
wildly different frequences.  Drop the indirect call, and use an if/else chain
instead, which is a code layout technique used by profile-guided optimsiation.

Using Xen's own perfcounter infrastructure, we see the following frequences of
vectors measured from boot until I can SSH into dom0 and collect the stats:

  vec | CFL-R   | Milan   | Notes
  ----+---------+---------+
  NMI |     345 |    3768 | Watchdog.  Milan has many more CPUs.
  ----+---------+---------+
  #PF | 1233234 | 2006441 |
  #GP |   90054 |   96193 |
  #UD |     848 |     851 |
  #NM |       0 |     132 | Per-vendor lazy vs eager FPU policy.
  #DB |      67 |      67 | No clue, but it's something in userspace.

Bloat-o-meter (after some manual insertion of ELF metadata) reports:

  add/remove: 0/1 grow/shrink: 2/0 up/down: 102/-256 (-154)
  Function                                     old     new   delta
  handle_exception_saved                       148     226     +78
  handle_ist_exception                         453     477     +24
  exception_table                              256       -    -256

showing that the if/else chains are less than half the size that
exception_table[] was in the first place.

As part of this change, make two other minor changes.  do_reserved_trap() is
renamed to do_unhandled_trap() because it is the catchall, and already covers
things that aren't reserved any more (#VE/#VC/#HV/#SX).

Furthermore, don't forward #TS to guests.  #TS is specifically for errors
relating to the Task State Segment, which is a Xen-owned structure, not a
guest-owned structure.  Even in the 32bit days, we never let guests register
their own Task State Segments.

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