[libvirt-users] script called from qemu hook freezes.

daggs daggs at gmx.com
Fri Jan 4 16:27:48 UTC 2019

Greetings Peter,

> Sent: Friday, January 04, 2019 at 4:47 PM
> From: "Peter Krempa" <pkrempa at redhat.com>
> To: daggs <daggs at gmx.com>
> Cc: libvirt-users at redhat.com
> Subject: Re: [libvirt-users] script called from qemu hook freezes.
> On Thu, Jan 03, 2019 at 18:07:58 +0100, daggs wrote:
> > Greetings,
> > 
> > I'm executing an external script when the qemu hook is called with start or release, the script is rather simple, upon start it iterates over the output of lsusb -t and for each device, it looks if it should be added to the vm we started, if so, it attaches it to the vm as follows:
> > virsh --connect qemu:///system "${cmd}" "${domain}" /dev/stdin << END
> > <hostdev mode='subsystem' type='usb'>
> >   <source>
> >    <address bus='${busnum}' device='${devnum}' />
> >   </source>
> > </hostdev>
> > END
> > 
> > where cmd is attach-device, domain is the vm's name, busnum and devnum come from the output of the lsusb -t.
> > my issue is that upon the first attach attempt, the cmd hangs, I need to kill it and after than I cannot preform any virsh cmd, I must restart the host.
> > if I try to execute the same cmd after the vm is up, it works great.
> > 
> > why the attach process gets stuck? do I need to execute it under different stage?
> Hook scripts shall never call any libvirt API (even through virsh). At
> the point when the hook script is called the VM startup process is
> paused until the script returns. If the script attempts to modify the VM
> it gets stuck as the VM is locked at that point.
> You either need to add the device prior to startup, but AFAIK that is
> not possible with a hook script or after but the script needs to return.
> So you either fork off a process which will wait for the startup to
> finish from the hook script or write an APP using the libvirt API which
> will wait for the VM start event and then execute what's necessary.

I see, is there another way to do what I need (on startup usb hotplug) or maybe optional hotplug (e.g. don't fail the device isn't present)?

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