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xen-devel
Re: [Xen-devel] "Time went backwards" messages
On 5 Apr 2005, at 00:40, Don Fry wrote:
I have a high end IBM system with 4 HT CPUs, am running xen-unstable
with only Dom0 active, and I get lots of "Timer ISR/n: Time went
backwards" messages. This is a short segment from dmesg:
The errors are coming from the per-vcpu timer interrupt that is only
used in SMP guest builds. I can well imagine there are some 'latent
correctness issues' around that code. :-) If you get messages starting
'Timer ISR: Time went backwards' then that would be morew worrying.
I looked at the archives and noticed this was reported when running
under VMware, but I see this happening with only xen and dom0.
That was a different issue, most likely.
Are there any plans to restore Cyclone or HPET support to linux, and
add
it to xen, to eliminate this kind of problem?
I have some prototype cyclone code added to xen, but have not done
anything to Linux yet.
Do HPET/Cyclone actually give greater long-term accuracy than the
legacy PIT? The only thing we use the non-local-APIC timer for is to
track wallclock time, so if the timers are driven off the same crystal
then it doesn't seem there will be a win.
As I recall the HPET allows you to do things like set events to a
precision of picoseconds, but it wasn't clear to me it was actually any
more accurate than the PIT. :-) And we can always accumulate the lost
precision in setting the PIT registers and periodically add it into the
wallclock time.
I have no problem with HPET/Cyclone code in Xen *if* it buys us
anything.
-- Keir
--
Don Fry
brazilnut@xxxxxxxxxx
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