[PATCH] xen pmtimer: correct pmtimer accuracy

[Andrew Jones <drjones@redhat.com>]

From: Ulrich Obergfell <uobergfe@redhat.com>

Several seconds of backward time drift per minute can be seen on a
RHEL6 HVM guest by switching the clocksource to 'acpi_pm' and then
running gettimeofday() in a loop. This is due to the accumulation
of small inaccuracies that are caused by shifting out the lower 32
bits when pmt_update_time() computes 'tmr_val'.

The patch makes sure that the lower 32 bits of the computed value
are not lost. They are saved in a new field 'not_accounted' in the
PMTState structure and are accounted the next time pmt_update_time()
is called.

diff --git a/xen/arch/x86/hvm/pmtimer.c b/xen/arch/x86/hvm/pmtimer.c
--- a/xen/arch/x86/hvm/pmtimer.c
+++ b/xen/arch/x86/hvm/pmtimer.c
@@ -84,13 +84,16 @@
 static void pmt_update_time(PMTState *s)
 {
     uint64_t curr_gtime;
+    uint64_t tmp;
     uint32_t msb = s->pm.tmr_val & TMR_VAL_MSB;
     
     ASSERT(spin_is_locked(&s->lock));
 
     /* Update the timer */
     curr_gtime = hvm_get_guest_time(s->vcpu);
-    s->pm.tmr_val += ((curr_gtime - s->last_gtime) * s->scale) >> 32;
+    tmp = ((curr_gtime - s->last_gtime) * s->scale) + s->not_accounted;
+    s->not_accounted = tmp & 0xffffffff;
+    s->pm.tmr_val += tmp >> 32;
     s->pm.tmr_val &= TMR_VAL_MASK;
     s->last_gtime = curr_gtime;
     
@@ -276,6 +279,7 @@
     spin_lock_init(&s->lock);
 
     s->scale = ((uint64_t)FREQUENCE_PMTIMER << 32) / SYSTEM_TIME_HZ;
+    s->not_accounted = 0;
     s->vcpu = v;
 
     /* Intercept port I/O (need two handlers because PM1a_CNT is between
diff --git a/xen/include/asm-x86/hvm/vpt.h b/xen/include/asm-x86/hvm/vpt.h
--- a/xen/include/asm-x86/hvm/vpt.h
+++ b/xen/include/asm-x86/hvm/vpt.h
@@ -117,6 +117,7 @@
     struct hvm_hw_pmtimer pm;   /* 32bit timer value */
     struct vcpu *vcpu;          /* Keeps sync with this vcpu's guest-time */
     uint64_t last_gtime;        /* Last (guest) time we updated the timer */
+    uint64_t not_accounted;     /* time not accounted at last update */
     uint64_t scale;             /* Multiplier to get from tsc to timer ticks */
     struct timer timer;         /* To make sure we send SCIs */
     spinlock_t lock;