Re: HVM Migration of domU on Qemu-upstream DM causes stuck system clock with ACPI

[George Dunlap <george.dunlap@eu.citrix.com>]

On 31/05/13 13:40, Ian Campbell wrote:
> On Fri, 2013-05-31 at 12:57 +0100, Alex Bligh wrote:
>> --On 31 May 2013 12:49:18 +0100 George Dunlap <george.dunlap@eu.citrix.com>
>> wrote:
>>
>>> No -- Linux is asking, "Can you give me an alarm in 5ns?"  And Xen is
>>> saying, "No".  So Linux is saying, "OK, how about 5us?  10us? 20us?"  By
>>> the time it reaches 4ms, Linux has had enough, and says, "If this timer
>>> is so bad that it can't give me an event within 4ms it just won't use
>>> timers at all, thank you very much."
>>>
>>> The problem appears to be that Linux thinks it's asking for something in
>>> the future, but is actually asking for something in the past.  It must
>>> look at its watch just before the final domain pause, and then asks for
>>> the time just after the migration resumes on the other side.  So it
>>> doesn't realize that 10ms (or something) has already passed, and that
>>> it's actually asking for a timer in the past.  The Xen timer driver in
>>> Linux specifically asks Xen for times set in the past to return an error.
>>> Xen is returning an error because the time is in the past, Linux thinks
>>> it's getting an error because the time is too close in the future and
>>> tries asking a little further away.
>>>
>>> Unfortunately I think this is something which needs to be fixed on the
>>> Linux side; I don't really see how we can work around it in Xen.
>> I don't think fixing it only on the Linux side is a great idea, not least
>> as it makes any current Linux image not live migrateable reliably. That's
>> pretty horrible.
> Ultimately though a guest bug is a guest bug, we don't really want to be
> filling the hypervisor with lots of quirky exceptions to interfaces in
> order to work around them, otherwise where does it end?
>
> A kernel side fix can be pushed to the distros fairly aggressively (it's
> mostly just a case of getting an upstream stable backport then filing
> bugs with the main ones, we've done it before) and for users upgrading
> the kernel via the distros is really not so hard and mostly reuses the
> process they must have in place for guest kernel security updates and
> other important kernel bugs anyway.

In any case, it seems I was wrong -- Linux does "look at its watch" 
every time it asks.

The generic timer interface is "set me a timer N nanoseconds in the 
future"; the Xen timer implementation executes 
pvclock_clocksource_read() and adds the delta.  So it may well actually 
be a bug in Xen.

Stand by for further investigation...

  -George