[Qemu-devel] Rethinking missed tick catchup

[Anthony Liguori]

Hi,

We've been running into a lot of problems lately with Windows guests and
I think they all ultimately could be addressed by revisiting the missed
tick catchup algorithms that we use.  Mike and I spent a while talking
about it yesterday and I wanted to take the discussion to the list to
get some additional input.

Here are the problems we're seeing:

1) Rapid reinjection can lead to time moving faster for short bursts of
   time.  We've seen a number of RTC watchdog BSoDs and it's possible
   that at least one cause is reinjection speed.

2) When hibernating a host system, the guest gets is essentially paused
   for a long period of time.  This results in a very large tick catchup
   while also resulting in a large skew in guest time.

   I've gotten reports of the tick catchup consuming a lot of CPU time
   from rapid delivery of interrupts (although I haven't reproduced this
   yet).

3) Windows appears to have a service that periodically syncs the guest
   time with the hardware clock.  I've been told the resync period is an
   hour.  For large clock skews, this can compete with reinjection
   resulting in a positive skew in time (the guest can be ahead of the
   host).

I've been thinking about an algorithm like this to address these
problems:

A) Limit the number of interrupts that we reinject to the equivalent of
   a small period of wallclock time.  Something like 60 seconds.

B) In the event of (A), trigger a notification in QEMU.  This is easy
   for the RTC but harder for the in-kernel PIT.  Maybe it's a good time to
   revisit usage of the in-kernel PIT?

C) On acculumated tick overflow, rely on using a qemu-ga command to
   force a resync of the guest's time to the hardware wallclock time.

D) Whenever the guest reads the wallclock time from the RTC, reset all
   accumulated ticks.

In order to do (C), we'll need to plumb qemu-ga through QMP.  Mike and I
discussed a low-impact way of doing this (having a separate dispatch
path for guest agent commands) and I'm confident we could do this for
1.3.

This would mean that management tools would need to consume qemu-ga
through QMP.  Not sure if this is a problem for anyone.

I'm not sure whether it's worth trying to support this with the
in-kernel PIT or not either.

Are there other issues with reinjection that people are aware of?  Does
anything seem obviously wrong with the above?

Regards,

Anthony Liguori