*Here's the commit that causes the problem:*

$ git checkout d185c094b404b4ff392b77d1244c0233da7d53bd
qemu-char: eliminate busy waiting on can_read returning zero

*Symptoms:*

When the VM boots everything crawls and the host CPU is at 100%,
messages are printing extremely slowly on the VM console
and the VM is not making any boot progress......... printing kernel
messages once a second or so.......

Something in QEMU appears to be spinning.

Here's my command line:

qemu-system-x86_64 -enable-kvm -m 2048 -smp 
2,sockets=1,cores=1,threads=1 -nodefconfig -rtc base=utc -no-shutdown 
disk.img -serial pty -monitor unix:/tmp/devguest,server,nowait -netdev 
type=tap,vhost=on,id=hostnet0,script=kvmifup.sh,downscript=kvmifdown.sh 
-device virtio-net-pci,netdev=hostnet0,mac=12:34:36:00:00:02 -usb 
-device usb-tablet,id=input0  -device 
virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5

- Michael Hines