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

[Gleb Natapov <gleb@redhat.com>]

On Mon, Jul 05, 2010 at 01:36:08PM +0100, Stefan Hajnoczi wrote:
> 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.
> 
That is possible when bios is actually used to boot the guest, but bug
reporter uses -kernel option so no bios boot code should run at all.
Virtio is initialized anyway, but this will happen with boot=on too.

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

--
			Gleb.