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Re: [Xen-devel] Kernel 2.6.39+ hangs when running as HVM guest under Xen

On Thu, Aug 04, 2011 at 02:59:05PM +0200, Stefan Bader wrote:
> Since kernel 2.6.39 we were experiencing strange hangs when booting those as 
> guests in Xen (similar hangs but different places when looking at CentOS 5.4 +
> Xen 3.4.3 as well as Xen 4.1 and a 3.0 based dom0). The problem only happens
> when running with more than one vcpu.

Hey Stefan,

We were all at the XenSummit and I think did not get to think about this at all.
Also the merge window openned so that ate a good chunk of time. Anyhow..

Is this related to this: 
http://marc.info/?i=4E4070B4.1020008@xxxxxxxxxxxxxxxxxxxxxx ?

> I was able to examine some dumps[1] and it always seemed to be a weird
> situations. In one example (booting 3.0 HVM under Xen 3.4.3/2.6.18 dom0) the
> lockup always seemed to occur when the delayed mtrr init took place. Cpu#0
> seemed to have been starting the rendevouz (stop_cpu) but then been 
> interrupted
> and the other (I was using vcpu=2 for simplicity) was idling somewhere else 
> but
> had the mtrr
> rendevouz handler queued up (just seemed to never get started).
> Things seemed to indicate some IPI problem but to be sure I went to bisect 
> when
> the problem started. I ended up with the following patch which, when reverted,
> allows me to bring up a 3.0 HVM guest with more than one CPU without any 
> problems.
> commit 99bbb3a84a99cd04ab16b998b20f01a72cfa9f4f
> Author: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> Date:   Thu Dec 2 17:55:10 2010 +0000
>     xen: PV on HVM: support PV spinlocks and IPIs
>     Initialize PV spinlocks on boot CPU right after native_smp_prepare_cpus
>     (that switch to APIC mode and initialize APIC routing); on secondary
>     Enable the usage of event channels to send and receive IPIs when
>     running as a PV on HVM guest.
> Though I have not yet really understood why exactly this happens, I thought I
> post the results so far. It feels like either signalling an IPI through the
> eventchannel does not come through or goes to the wrong CPU. It did not seem 
> to
> cause the exactly same place to fail. Like said, the 3.0 guest running in the
> CentOS dom0 was locking up early right after all CPUs were brought up. While
> during the bisect (using a kernel between 2.6.38 and .39-rc1) the lockup was 
> later.
> Maybe someone has a clue immediately. I will dig a bit deeper in the dumps in
> the meantime. Looking at the description, which sounds like using event 
> channels

Anything turned up?
> only was intended for PV on HVM guests, it is wrong in the first place to set
> the xen ipi functions on the HVM side...

On true HVM - sure, but on PVonHVM it sounds right.
> -Stefan
> [1] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/791850
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