[libvirt] RFC: virt-xml: command line tool for altering guest configuration

[Cole Robinson]

Hi all,

Attached is a patch to virtinst.git that provides a new tool named
virt-xml. virt-xml provides a scriptable command line interface for
editing libvirt guest XML.

A couple examples:

Change boot order and boot menu for 'myguest':
$ ./virt-xml --guest myguest --boot network,hd,cdrom,menu=on
Requested changes:
--- Original XML
+++ New XML
@@ -10,6 +10,8 @@
   <os>
     <type arch="i686">hvm</type>
     <loader>/usr/lib/xen/boot/hvmloader</loader>
+    <boot dev="network"/>
+    <bootmenu enable="yes"/>
     <boot dev="hd"/>
+    <boot dev="cdrom"/>
   </os>
   <features>
     <acpi/>

Change disk #7 to use cache=none for 'myguest':
$ ./virt-xml --guest myguest --device 7 --disk cache=none
Requested changes:
--- Original XML
+++ New XML
@@ -74,6 +74,7 @@
     <disk type="file" device="disk">
       <source file="/tmp/foobar3"/>
       <target dev="sdd" bus="usb"/>
+      <driver cache="none"/>
     </disk>
     <disk type="file" device="disk">
       <driver name="tap" type="qcow" cache="writethrough"/>


Change watchdog action for 'myguest':
$ ./virt-xml --guest myguest --device 1 --watchdog action=pause
Requested changes:
--- Original XML
+++ New XML
@@ -220,7 +220,7 @@
         <address domain="0x0003" bus="0x00" slot="0x19" function="0x0"/>
       </source>
     </hostdev>
-    <watchdog model="ib700" action="poweroff"/>
+    <watchdog model="ib700" action="pause"/>
     <memballoon model="xen"/>
   </devices>
 </domain>


--boot, --disk, and --watchdog come straight from virt-install, see man
virt-install for detailed docs of the option format. Those options shown
are the only things wired up right now, but it's a straightforward task
to wire up most of the relevant remaining virt-install opts. Also the
tool doesn't actually apply the changes yet, but that is also a trivial
addition. I think the interesting bit here is nailing down the scope of
the tool and deciding on good CLI syntax.

Not sure if we should do hotplug/hotunplug as a subcommand or just
another option like:

virt-xml ... --hotplug --disk /tmp/foo.img

Also not really sure how to hotunplug using the --device syntax of the above
examples:

virt-xml ... --hotunplug --device 1 --disk <what goes here?>

We could make --device just an option to whatever device string the user
gives

virt-xml ... --hotunplug --disk id=1

We can easily allow different XML input methods like:

virt-xml --guestxml file.xml ...

or even batch processing

virt-xml --guest * ...

Maybe even have allow more intelligence about what guests we edit, say
we want to set cache=none for all disk devices, not CDROM or floppy
(this would take a bit of work to sort out).

virt-xml --guest * --disk match,device=disk --disk cache=none

Then there is also the question of extending the tool to also edit other XML
like storage, network, interface, etc.

Any recommendations appreciated: option syntax, future capabilities, or even a
better name for the tool :)

A quick word about why I think we should do this with a separate tool rather
than virsh. For starters, I think the need for this functionality on the
command line is clear by now, so it's just a question of where to implement it.

Why not virsh? Text and command line processing in C is a serious pain
compared to a higher level language like python. I don't think this is a
controversial statement, in fact I think it's partly responsible for why the
virsh XML building commands have typically been in a pretty sorry state. virsh
could conceivably use the internal domain_conf API to make this task much
easier, but it would still require a lot of command line options for every XML
parameter.

In contrast, virt-install already needs to do most of this command line
processing, and virt-manager needs to do all the XML building, parsing, and
editing. Since this code already lives in one place (virtinst), it's almost
trivial for us to reuse it. Supporting a new XML property on the command line
is a very simple task and will immediately benefit both virt-install and
virt-xml. We also already have over 75% of the domain XML schema available via
existing CLI options that are already documented :)

Questions or comments appreciated!

Thanks,
Cole
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