[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH] xen pmtimer: correct pmtimer accuracy
From: Ulrich Obergfell <uobergfe@xxxxxxxxxx> 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; _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
|
Lists.xenproject.org is hosted with RackSpace, monitoring our |