KVM git hangs with if=virtio (works under kvm 0.12.3)

[ewheeler <kvm@ew.ewheeler.org>]

Hello all,

I'm booting a CentOS kernel under today's KVM git and it hangs after
initializing the serial port when the drive if=virtio, but not when
drive if=ide.  Look close---this is not a "forgot to add virtio_blk"
problem.  If I use 0.12.3 from Ubuntu 10.04 it works properly.

Reproduction:

Using kvm 0.12.3 on ubuntu 10.04 (1:84+dfsg-0ubuntu16+0.12.3+noroms
+0ubuntu9) it will work properly:

  qemu-system-x86_64 -drive file=dummy-disk-image,if=virtio \
 			-kernel vmlinuz-2.6.18-194.3.1.el5.centos.plus 

As expected, the kernel panics unable to mount root (good-boot.png).
This makes sense, as "dummy-disk-image" is 1MB of 0x00 bytes.
  
---However---if I use today's git (2010-07-01) of kvm:

   /usr/local/kvm-git/bin/qemu-system-x86_64 -drive file=dummy-disk-image,if=virtio \
   	-kernel vmlinuz-2.6.18-194.3.1.el5.centos.plus    

This hangs just after initializing the Serial device (obtained by adding
-serial stdio -append console=ttyS0):

Note that this only happens with the disk interface set to virtio
(if=virtio).  It works fine for ide (if=ide).


Am I doing something wrong here?  
Is anyone else having this problem?

The pictures, disk and kernel image is available here: 
  http://www.portlandlinuxsupport.com/src/kvm-20100701-regression/

-Eric


More detail:

	==== snip ====   (see also bad-boot.png)
	PCI: PIIX3: Enabling Passive Release on 0000:00:01.0
	Activating ISA DMA hang workarounds.
	pci_hotplug: PCI Hot Plug PCI Core version: 0.5
	Real Time Clock Driver v1.12ac
	Non-volatile memory driver v1.2
	Linux agpgart interface v0.101 (c) Dave Jones
	Serial: 8250/16550 driver $Revision: 1.90 $ 4 ports, IRQ sharing enabled
	  [[ hangs here and spins at >100% cpu ]]
	==== snip ====   

And the qemu-system-x86_64 process spins at >100% cpu:

== snip from top ==
  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND                                                              
 8895 root      20   0  287m  28m 3124 S  102  0.5   0:56.82 qemu-system-x86                                                      
== snip from top ==