Instead of polling the fences regularly, have a thread
that blocks for a single fence using a separate shared
context, then uses eventfd to wake up the main thread
when something happens.

Inside the guest, glmark2 typicially runs twice as fast with the thread
sync. Although in general, the performances seems to be about +30%. The
benefits is mostly for CPU-bounds tasks (when main the thread hits 100%)

A naive perf stat of the vtest renderer with glmark2 "build" test with a
fixed number of frames (500) results in the following stats data:
(do not value timing related informations, since the renderer is ran and
stopped manually)

without thread:

       3032.282265      task-clock (msec)         #    0.420 CPUs utilized
             4,277      context-switches          #    0.001 M/sec
               102      cpu-migrations            #    0.034 K/sec
             9,020      page-faults               #    0.003 M/sec
     7,884,098,254      cycles                    #    2.600 GHz
     4,440,126,451      stalled-cycles-frontend   #   56.32% frontend cycles idle
   <not supported>      stalled-cycles-backend
    11,024,091,578      instructions              #    1.40  insns per cycle
                                                  #    0.40  stalled
                                                  #    cycles per insn
     1,091,831,588      branches                  #  360.069 M/sec
         5,426,846      branch-misses             #    0.50% of all branches

with thread:

       3403.592921      task-clock (msec)         #    0.452 CPUs utilized
             7,145      context-switches          #    0.002 M/sec
               410      cpu-migrations            #    0.120 K/sec
             6,191      page-faults               #    0.002 M/sec
     7,475,038,064      cycles                    #    2.196 GHz
     4,487,043,071      stalled-cycles-frontend   #   60.03% frontend cycles idle
   <not supported>      stalled-cycles-backend
     9,925,205,494      instructions              #    1.33  insns per cycle
                                                  #    0.45  stalled
                                                  #    cycles per insn
       834,375,503      branches                  #  245.146 M/sec
         4,919,995      branch-misses             #    0.59% of all branches

Signed-off-by: Marc-André Lureau <marcandre.lureau@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>