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

[Gleb Natapov <gleb@redhat.com>]

On Mon, Jul 05, 2010 at 01:11:25PM +0100, Stefan Hajnoczi wrote:
> On Fri, Jul 2, 2010 at 9:07 AM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > On Fri, Jul 2, 2010 at 4:31 AM, ewheeler <kvm@ew.ewheeler.org> wrote:
> >> 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?
> >
> > I have seen this issue with a RHEL 5.5 guest running under
> > qemu-kvm.git.  It boots a new guest fine but hangs as you described
> > with the RHEL 5.5 kernel.  I have not investigated.
> 
> This issue is affected by extboot, a feature that enables booting from
> virtio-blk devices.  I have just sent a patch to the KVM mailing list
> to restore extboot functionality which has been broken in
> qemu-kvm.git.  That patch can be used to work around this issue by
> using "-drive ...,boot=on" but it doesn't explain why the RHEL 5.5
> kernel hangs during serial initialization when extboot is not present.
> 
Hang that happens during guest boot (after bootloader started the
kernel) cannot be worked around by extboot. extboot is also not needed
with latest qemu git to boot from virtio disks since the support for
that is in the bios now.
 
--
			Gleb.