PPC: BookE: Make FIT/WDT timers at best millisecond grained

The default granularity for the FIT timer on 440 is on every 0x1000th
transition of TB from 0 to 1. Translated that means 48828 times a second.

Since interrupts are quite expensive for 440 and we don't really care
about the accuracy of the FIT to that significance, let's force FIT and
WDT to at best millisecond granularity.

This basically restores behavior as it was in QEMU 1.6, where timers
could only deal with millisecond granularities at all.

This patch greatly improves performance with the 440 target and restores
roughly the same performance level that QEMU 1.6 had for me.

Signed-off-by: Alexander Graf <agraf@suse.de>
Message-id: 1385416015-22775-3-git-send-email-agraf@suse.de
Signed-off-by: Anthony Liguori <aliguori@amazon.com>

hw/ppc/ppc_booke.c

@@ -174,6 +174,12 @@ static void booke_update_fixed_timer(CPUPPCState         *env,
 
     if (*next == now) {
         (*next)++;
+    } else {
+        /*
+         * There's no point to fake any granularity that's more fine grained
+         * than milliseconds. Anything beyond that just overloads the system.
+         */
+        *next = MAX(*next, now + SCALE_MS);
     }
 
     /* Fire the next timer */