Mailing List Archive

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;

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