2.6.35-rc1 regression with pvclock and smp guests

Andre Przywara
2010-07-22T12:59:36Z

Hi,

I found a regression with pvclock and SMP KVM _guests_.
PVCLOCK enabled guest kernels boot with qemu-kvm.git and with smp=1, but
with smp=2 halt at:

Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
(last line shown)

I bisected this down to:
commit 489fb490dbf8dab0249ad82b56688ae3842a79e8
Author: Glauber Costa <glommer@redhat.com>
Date: Tue May 11 12:17:40 2010 -0400

x86, paravirt: Add a global synchronization point for pvclock

One commit before works, smp=1 always works, disabling PVCLOCK works.
Using qemu-kvm-0.12.4 works, too.
Having PVCLOCK enabled and with smp=2 the kernel halts without any
further message.
This is still the case with the lastest tip.
Even pinning both VCPU threads to the same host core show the bug.
The bug triggers on all hosts I tested, an single socket quadcore
Athlon, a dual socket dualcore K8-Opteron and a quad socket 12core Opteron.

Please note that this is the guest kernel, the host kernel does not matter.

I have no idea (and don't feel like ;-) debugging this, so I hope
someone will find and fix the bug.

Regards,
Andre.

--
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448-3567-12