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

[Gleb Natapov <gleb@redhat.com>]

On Wed, Jul 07, 2010 at 10:36:17AM +0100, Stefan Hajnoczi wrote:
> 2010/7/7 Gleb Natapov <gleb@redhat.com>:
> > On Wed, Jul 07, 2010 at 10:03:31AM +0100, Stefan Hajnoczi wrote:
> >> 2010/7/5 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.
> >>
> >> Okay, I got to the bottom of this.  Here's the story, see bottom of the mail for
> >> the solution and workarounds:
> >>
> > Great. Thanks Stefan. I was sure that fix you found is in qemu.git
> > already. It is good to know that Linux actually uses int13 once. Didn't
> > know that. To be absolutely sure no interrupt will stuck we should probably
> > clear interrupt after each virtio read in bios by reading status register.
> 
> Yes, this is the approach I took when rewriting gPXE's virtio-net
> driver recently:
> 
> http://git.etherboot.org/?p=people/stefanha/gpxe.git;a=blob;f=src/drivers/net/virtio-net.c;h=50c580049c3663f0d1332d51e30c8a59b5663b68;hb=d58bf6fc8db4a55075e370c7e1d436a570400f55#l299
> 
> Are you spinning a patch or should I submit one?
> 
If you have time do it please. I am a little bit busy right now :(

--
			Gleb.