[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] RE: s_time going backwards on same processor?
OK, I am definitely recording stime going backwards observed with the attached patch. I have recorded dozens over a few hours. It appears to have no obvious pattern between reports, but one thing is consistent: In all cases, t->tsc_scale.shift is -1. I'll try to run some more tests over the weekend (e.g. with different clocksources... this is with clocksource=hpet), but thought I'd report what I have seen. I'm running on a dual-core single socket ("Conroe"). Dan P.S. If you try the patch, ensure you set the boot parameter "measurestime". > -----Original Message----- > From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx] > Sent: Wednesday, July 23, 2008 1:06 AM > To: dan.magenheimer@xxxxxxxxxx; Xen-Devel (E-mail) > Cc: Dave Winchell > Subject: Re: s_time going backwards on same processor? > > > On 22/7/08 22:58, "Dan Magenheimer" > <dan.magenheimer@xxxxxxxxxx> wrote: > > > I *do* know that get_s_time() on different processors > > can have this behavior and I know it is possible for > > hvm_get_guest_time() to go backwards (timer_mode=0), > > but I thought s_time was monotonically non-decreasing > > on any given processor and that read_platform_stime() > > is also monotonically non-decreasing. > > > > Does dom0 maybe have direct hardware access to the hardware > > platform timer that xen system time is dependent on? > > No matter what happens to the underlying platform timer, it should be > impossible for Xen system time to go backwards on any given > processor. The > calibration function never sets the TSC and system timestamps > for the next > time record any earlier than current TSC value and current > computed system > time value. Hence it should be impossible for system time to > be computed as > earlier than that time record. > > -- Keir > > > Attachment:
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