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

[Peter Krempa]

On Fri, Jan 04, 2019 at 17:27:48 +0100, daggs wrote:
> 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)?

If you need programatic logic to decide which device to plug in you
either use the above suggestion to use a daemon using libvirt API or
fork off a separate process or do the logic prior to start by
pre-constructing the XML and using 'virsh create xmlfile' which will be
used as the running config without modifying the persistent one.
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