Re: [Qemu-devel] Rethinking missed tick catchup

[Anthony Liguori]

Jan Kiszka <address@hidden> writes:

> On 2012-09-12 15:54, Anthony Liguori wrote:
>> 
>> 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.
>
> As with our current discussion around fixing the PIC and its impact on
> the PIT, we should try on the userspace model first and then check if
> the design can be adapted to support in-kernel as well.
>
> For which guests is the PIT important again? Old Linux kernels? Windows
> should be mostly happy with the RTC - or the HPET.

I thought that only 64-bit Win2k8+ used the RTC.

I thought win2k3 and even 32-bit win2k8 still used the PIT.

>> Are there other issues with reinjection that people are aware of?  Does
>> anything seem obviously wrong with the above?
>
> We should take the chance and design everything in a way that the HPET
> can finally be (left) enabled.

I thought the issue with the HPET was access frequency and the cost of
heavy weight exits.

I don't have concrete data here.  I've only heard it second hand.  Can
anyone comment more?

Regards,

Anthony Liguori

>
> Jan
>
> -- 
> Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
> Corporate Competence Center Embedded Linux