libvirt users - Mar 2015 - How to burn a CD using libvirt?

Hi,

My first e-mail... my apologies if this is not the appropriate e-mail list.

My goal is to *write* from the guest OS to a CD or DVD reader/writer on
my host system.  Currently, the write operation is failing.  I believe
it is because the virtual CD-ROM device is set to be "readonly".  Is
it
possible to write to a QEMU CDROM device and, if so, how?  (The host 
machine's CD-ROM drive does support writing/burning.)

I'm using the "virt-manager" GUI to start the virtual machine. 
Once
started, the "qemu" command line from my virtual machine process
includes the following, where the host's built-in CD reader/writer is
"/dev/sr0".

    /usr/bin/qemu-system-x86_64 ... -drive
          file=/dev/sr0,if=none,id=drive-ide0-0-1,readonly=on,format=raw

This obviously says that the device is "readonly".

A snippet of XML from my virtual machine from the CDROM is:

<domain type='kvm'>
...
  <devices>
    <disk type='block' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <source dev='/dev/sr0'/>
      <target dev='hdb' bus='ide'/>
      <readonly/>
      <address type='drive' controller='0' bus='0'
target='0' unit='1'/>
    </disk>


I've tried removing the <readonly/> tag from the above VM definition,
and then running the VM, but the tag magically reappears in the VM
definition when it runs.

Also, the "virt-manager" GUI shows the CDROM device as having the
option
of "Readonly" being checked (and the GUI does not allow me to uncheck
it.)

Other details:

  * qemu version 2.1.3 on Fedora 21.
  * libvirt version 1.2.0.2


Thanks for any help or pointers you can provide,
-- Steve

On Wed, Mar 25, 2015 at 05:02:33PM -0500, Steve Ross
wrote:
>Hi,
>
>My first e-mail... my apologies if this is not the appropriate e-mail list.
>
You did, sorry for the super-late response :)
>My goal is to *write* from the guest OS to a CD or DVD reader/writer on
>my host system.  Currently, the write operation is failing.  I believe
>it is because the virtual CD-ROM device is set to be "readonly". 
Is it
>possible to write to a QEMU CDROM device and, if so, how?  (The host
>machine's CD-ROM drive does support writing/burning.)
>
>I'm using the "virt-manager" GUI to start the virtual machine.
Once
>started, the "qemu" command line from my virtual machine process
>includes the following, where the host's built-in CD reader/writer is
>"/dev/sr0".
>
>    /usr/bin/qemu-system-x86_64 ... -drive
>          file=/dev/sr0,if=none,id=drive-ide0-0-1,readonly=on,format=raw
>
>This obviously says that the device is "readonly".
>
>A snippet of XML from my virtual machine from the CDROM is:
>
><domain type='kvm'>
>...
>  <devices>
>    <disk type='block' device='cdrom'>
>      <driver name='qemu' type='raw'/>
>      <source dev='/dev/sr0'/>
>      <target dev='hdb' bus='ide'/>
>      <readonly/>
>      <address type='drive' controller='0'
bus='0' target='0' unit='1'/>
>    </disk>
>
Libvirt adds that automatically to cdrom devices.
>
>I've tried removing the <readonly/> tag from the above VM
definition,
>and then running the VM, but the tag magically reappears in the VM
>definition when it runs.
>
>Also, the "virt-manager" GUI shows the CDROM device as having the
option
>of "Readonly" being checked (and the GUI does not allow me to
uncheck it.)
>
And this is for the same reason -- libvirt adds it by default.

On the root cause as to why even libvirt does that -- I was thinking
about few scenarios and I came to only one unclear conclusion.  There
might have been historical causes etc., but I think there's a safety
feature smell behind this.  If you pass it like this, it won't be
opened exclusively and you might get to a problem where your burned CD
is not working, although this should be handled by the device itself.
Anyway, after thinking about how to fix it I realized there is a
workaround for this.  You just need to pass-through the CD writer as a
device.  QEMU will have exclusive access and all the ioctl()s and
fcntl()s will for sure work in the guest.

Hope that helps at least a bit.

Have a nice day,
Martin
>Other details:
>
>  * qemu version 2.1.3 on Fedora 21.
>  * libvirt version 1.2.0.2
>
>
>Thanks for any help or pointers you can provide,
>-- Steve
>
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