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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 3 Jun 2013, Roger Pau Monnà wrote:
> On 31/05/13 17:10, Roger Pau Monnà wrote:
> > On 31/05/13 15:07, George Dunlap wrote:
> >> 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@xxxxxxxxxxxxx>
> >>>> 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...
> 
> I've been investigating further during the weekend, and although I'm not
> familiar with the timer code in Xen, I think the problem comes from the
> fact that in __update_vcpu_system_time when Xen detects that the guest
> is using a vtsc it adds offsets to the time passed to the guest, while
> in VCPUOP_set_singleshot_timer Xen compares the time passed from the
> guest using NOW(), which is just the Xen uptime, without taking into
> account any offsets.
> 
> This only happens after migration because Xen automatically switches to
> vtsc when it detects that the guest has been migrated. I'm currently
> setting up a Linux PVHVM on shared storage to perform some testing, but
> one possible solution might be to add tsc_mode="native_paravirt" to the
> PVHVM config file, and another one would be fixing
> VCPUOP_set_singleshot_timer to take into account the vtsc offsets and
> correctly translate the time passed from the guest.

Good analisys!
I think that the right solution would be to fix
VCPUOP_set_singleshot_timer.

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