On 02/03/2011 03:24 PM, Jan Kiszka wrote:
On 2011-02-03 21:07, Anthony Liguori wrote:
On 02/03/2011 09:28 AM, Jan Kiszka wrote:
On 2011-02-03 14:43, Ulrich Obergfell wrote:
Hi,
I am observing severe backward time drift in a MS Windows Vista(tm)
guest running on a Fedora 14 KVM host. I can reproduce the problem
with the following steps:
1. Use 'vncviewer' to connect to the guest's desktop.
2. Click on the menu title bar of a window on the guest's desktop.
3. Move that window around on the guest's desktop.
While I keep on moving the window around for one minute, the guest
time falls up to 15 seconds behind host time.
The problem is caused by delayed callbacks of hpet_timer(). A timer
interrupt is injected into the guest during each callback. However,
interrupts are lost if delays are greater than a comparator period.
Yes, that's a well known limitation of qemu, in fact. We are lacking a
generic irq coalescing infrastructure. That, once designed and
available, would also allow to fix the HPET.
I don't think it requires anything that sophisticated.
It's just the period calculation of the HPET that's wrong and doesn't
account for loss.
Blind (/wrt the guest state) reinjection from the iothread will
compensate for lost time of *that* thread but not of the target vcpu(s).
Is the case your concern about where you try to set an interrupt from
the I/O thread and the VCPU is not scheduled such that it doesn't
actually get injected into KVM until after the period is over?
This should be a rare event. If you are missing 50% of your
notifications, not amount of gradual catchup is going to help you out.
So, depending on your workload, you may reduce the drift more or less,
but you won't fix it this way.
There is no such thing as "fix" it. Time drift can happen on bare metal
too. Interrupts can be coalesced due to crappy SMM code. It's
something we see quite a lot in practice.
My point is that there's really low hanging fruit and while for some
curious reason I don't actually see this patch, I believe that a patch
like this probably can help us quite a lot in the short term.
The think the two biggest problems we have right now are bad period
calculations due to sloppy unit conversion (PIT/RTC) and lack of
accounting for missed periods.
It's worth noting again that if you don't use a gradual catchup policy,
interrupt notifiers are extremely important because you're not going to
inject before the end of the interrupt window. However, with a gradual
policy, it shouldn't be a huge issue.
Regards,
Anthony Liguori
Jan
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