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

[Stefan Hajnoczi <stefanha@gmail.com>]

2010/7/5 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.

I agree that something else is going on here and needs to be
investigated, but I do think that extboot can indirectly affect the
guest boot.

With extboot the virtio-blk PCI adapter is not touched by the
firmware/bootloader.  Is it possible that a virtio-blk interrupt is
raised and not acknowledged before entering Linux.  When Linux brings
up the serial port it gets swamped with interrupts?  That's just a
guess.

Newer (non-RHEL 5.5) kernels boot fine without extboot, so another
path to solving this bug is to bisect guest kernels.

Stefan