Re: [Qemu-devel] [Xen-devel] Xen hypervisor inside KVM guest with x2apic CPU feature fails to boot

[Gerd Hoffmann]

  Hi,

> > (XEN) ****************************************
> > (XEN) Panic on CPU 0:
> > (XEN) IO-APIC + timer doesn't work!  Boot with apic_verbosity=debug and 
> > send a report.  Then try booting with the 'noapic' option
> > (XEN) ****************************************
> >

Just tried the same a few days ago.  Adding a second vcpu made things
work.

> I am going to go out on a limb and say it is unlikely for the Xen side
> of things to be broken.  We have machines in XenServers test lab which
> support x2apic but have it disabled in this manor because of a lack of
> EIM (or due to errata).

I'm not so sure.  non-SMP setup with x2apic is probably pretty rare on
physical hardware and I wouldn't be surprised if that happens to hit a
untested code path in Xen ...

> I would look into what KVM/qemu does differently as a result of the
> "+x2apic" flag, and whether it is simply the reported cpuid feature, or
> whether it affects the legacy ioapic emulation.

... but enabling x2apic for a guest depending on legacy ioapic stuff is
pretty unlikely too, so that setup being emulated incorrectly without
being noticed so far wouldn't surprise me too.

cheers,
  Gerd