Re: [Xen-devel] "Time went backwards" messages

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel