Re: [Qemu-devel] dataplane bug: fail to start Windows VM with dataplane enable

[Stefan Hajnoczi <stefanha@gmail.com>]

On Wed, Mar 27, 2013 at 03:02:01PM +0100, Stefan Hajnoczi wrote:
> On Wed, Mar 27, 2013 at 02:32:19PM +0100, Stefan Hajnoczi wrote:
> > On Tue, Mar 26, 2013 at 11:10:35PM +0800, 张磊强 wrote:
> > > Hi,  Paolo && Stefan:
> > > 
> > > When I test the dataplane feature with qemu master, I find that
> > > Windows (Windows 7 and Windows 2003) VM will hang if dataplane is
> > > enabled. But if I try to start a Fedora VM, it can start normally.
> > > 
> > > The command I boot QEMU is:
> > > x86_64-softmmu/qemu-system-x86_64 -enable-kvm -m 1024 -smp 2 -drive
> > > file=win7.img,if=none,id=drive-virtio-disk,format=raw,cache=none,aio=native
> > > -device virtio-blk-pci,config-wce=off,scsi=off,x-data-plane=on,drive=drive-virtio-disk,id=virtio-disk
> > > 
> > > I found the similar bug has reported some days ago:
> > > http://lists.nongnu.org/archive/html/qemu-devel/2013-03/msg02200.html
> > > . And a patch for this bug has already committed by Paolo at Mar 13:
> > > http://lists.nongnu.org/archive/html/qemu-devel/2013-03/msg02200.html
> > > .
> > > 
> > > But it cannot work under my environment. Could you give me some advise
> > > to debug this problem ? I can provide more information if need.
> > 
> > Hi,
> > I haven't gotten to the bottom of it yet but wanted to let you know that
> > I'm seeing a hang at the boot screen after the installer reboots.  The
> > Windows logo animation runs but the guest seems unable to make progress.
> > 
> > Will let you know when there is a fix.  The guest I'm testing is Windows
> > 7 Professional 64-bit.
> > 
> > As a workaround you can set x-data-plane=off to boot the guest for the
> > first time.  Subsequent boots will succeed with x-data-plane=on.
> 
> Next data point, it's not caused by the Paolo's AioContext conversion.
> This means it's unrelated to the recent bug that you mentioned.
> 
> The boot gets stuck immediately after the switch from the BIOS
> virtio-blk driver to the Windows viostor driver.  We get two requests
> (VIRTIO_BLK_T_GET_ID and VIRTIO_BLK_T_READ) and then the guest stops.
> The descriptor index of the second request has been incremented, perhaps
> the guest is not receiving host->guest notifies.

Okay, getting closer to the root cause now.  The guest driver has not
enabled MSI-X and the guest notifier that dataplane uses is not hooked
up to anything.  When MSI-X is enabled the guest notifier is hooked up
to irqfd and when MSI-X is disabled it is supposed to bounce back into
QEMU which issues an ioctl for the interrupt.  We've gotten into a state
where the guest notifier exists but nothing is listening to it :).

Therefore the guest is stuck waiting for virtio-blk I/O requests to
complete.

Stefan