[Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift

[Jan Kiszka]

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.

> 
> This is an RFC through which I would like to get feedback on how the
> idea of a patch to compensate those lost interrupts would be received:
> 
> The patch determines the number of lost timer interrupts based on the
> number of elapsed comparator periods. Lost interrupts are compensated

That neglects coalescing of the HPET IRQs: If the timer is run regularly
but the guest is not able to retrieve the injected IRQs, you should
still see drifts with your patches.

> by gradually injecting additional interrupts during the subsequent
> timer intervals, starting at a rate of one additional interrupt per
> interval. If further interrupts are lost while compensation is still
> in progress, the rate is increased. The algorithm imposes a limit on
> the rate and on the 'backlog' of lost interrupts to be injected. The
> patch can be enabled via a qemu command line option.
> 
>   -hpet [device=none|present][,driftfix=none|slew]
> 
> The 'device=none' option is equivalent to the '-no-hpet' option, and
> the 'driftfix=slew' option enables the patch (similar to RTC).
> 
> 
> The second and third part of this series of email contain the patch:
> 
> - Code part 1 introduces the qemu command line option.
> - Code part 2 implements compensation of lost interrupts.
> 
> Please review and please comment.
> 

Generally, this issue needs to be attacked at qemu level (added to CC),
not qemu-kvm.

We had a lengthy discussion on the list last year. We (including qemu
people) basically agreed that we needs a generic feedback infrastructure
to track coalesced IRQs for periodic, clock providing devices to allow
reinjection (which would include reinjection of completely missed timer
events like in your series).

However, there was one unsolved design issue remain IIRC:

http://thread.gmane.org/gmane.comp.emulators.qemu/73181

Once we have a proper answer for this, we can resume creating the
de-coalescing framework.

Jan

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