On Tue, Mar 20, 2018 at 03:20:31PM +0100, Martin Kletzander wrote:
1) Default devices/values
Libvirt itself must default to whatever values there were before any
particular element was introduced due to the fact that it strives to
keep the guest ABI stable. That means, for example, that it can't just
add -vmcoreinfo option (for KASLR support) or magically add the pvpanic
device to all QEMU machines, even though it would be useful, as that
would change the guest ABI.
For default values this is even more obvious. Let's say someone figures
out some "pretty good" default values for various HyperV enlightenment
feature tunables. Libvirt can't magically change them, but each one of
the projects building on top of it doesn't want to keep that list
updated and take care of setting them in every new XML. Some projects
don't even expose those to the end user as a knob, while others might.
This gets very tricky, very fast.
Lets say that you have an initial good set of hyperv config
tunables. Now sometime passes and it is decided that there is a
different, better set of config tunables. If the module that is
providing this policy to apps like OpenStack just updates itself
to provide this new policy, this can cause problems with the
existing deployed applications in a number of ways.
First the new config probably depends on specific versions of
libvirt and QEMU, and you can't mandate to consuming apps which
versions they must be using. So you need a matrix of libvirt +
QEMU + config option settings.
Even if you have the matching libvirt & QEMU versions, it is not
safe to assume the application will want to use the new policy.
An application may need live migration compatibility with older
versions. Or it may need to retain guaranteed ABI compatibility
with the way the VM was previously launched and be using transient
guests, generating the XML fresh each time.
The application will have knowledge about when it wants to use new
vs old hyperv tunable policy, but exposing that to your policy module
is very tricky because it is inherantly application specific logic
largely determined by the way the application code is written.
One more thing could be automatically figuring out best values based
Lot of the time there are parts of the domain definition that need to be
added, but nobody really cares about them. Sometimes it's enough to
have few templates, another time you might want to have a policy
per-scenario and want to combine them in various ways. For example with
the data provided by point 1).
For example if you want PCI-Express, you need the q35 machine type, but
you don't really want to care about the machine type. Or you want to
use SPICE, but you don't want to care about adding QXL.
What if some of these policies could be specified once (using some DSL
for example), and used by virtuned to merge them in a unified and
3) Abstracting the XML
This is probably just usable for stateless apps, but it might happen
that some apps don't really want to care about the XML at all. They
just want an abstract view of the domain, possibly add/remove a device
and that's it. We could do that as well. I can't really tell how much
of a demand there is for it, though.
It is safe to say that applications do not want to touch XML at all.
Any non-trivial application has created an abstraction around XML,
so that they have an API to express what they want, rather than
manipulating of strings to format/parse XML.
The libvirt-gconfig project aims to provide a C API for manipulating
XML documents, with language bindings available via GObject
etc. Go is notable missing, but for that we have libvirt-go-xml
which provides a set of native Go structs to represent the XML.
The problem we've faced with libvirt-gconfig is that it is a really
hard sell to get applications to convert existing code to use it.
We've only had success where an applicaiton has been written to use
libvirt-gconfig from day one - eg GNOME Boxes and libvirt-sandbox.
Virt-manager is the poster-child for using libvirt-gconfig, but I
don't see it adopting it any time soon, as it is a massive effort
to change all existing code - even if libvirt-gconfig had full
XML schema coverge, which it doesn't :-(
I also wanted to use libvirt-gconfig in OpenStack, but there was
resistance for adding a dependancy on another native library, so
there we've basically copied what virt-manager does and defined a
set of pure python objects to represent config which is serialized
to / from XML :-( If libvirt-gconfig were to be used by OpenStack
it would need to deal with fact that not every distro has that
available, so the existing pure python config objects would need
to be maintained in parallel for an indefinite amount of time.
So in fact using libvirt-gconfig would increase maint burden for
OpenStack, rather than reduce it.
libvirt-gconfig would be a hard sell for Go apps when you compare
it to libvirt-go-xml, because the latter is following the common
Go paradigm for XML manipulation.
If there was something higher level that gets more interesting,
but the hard bit is that you still need a way to get at all the
low level bits becuase a higher level abstracted API will never
cover every niche use case.
4) Identifying devices properly
In contrast to the previous point, stateful apps might have a problem
identifying devices after hotplug. For example, let's say you don't
care about the addresses and leave that up to libvirt. You hotplug a
device into the domain and dump the new XML of it. Depending on what
type of device it was, you might need to identify it based on different
values. It could be <target dev=''/> for disks, <mac
interfaces etc. For some devices it might not even be possible and you
need to remember the addresses of all the previous devices and then
parse them just to identify that one device and then throw them away.
With new enough libvirt you could use the user aliases for that, but
turns out it's not that easy to use them properly anyway. Also the
aliases won't help users identify that device inside the guest.
NB, relating between host device config and guest visible device
config is a massive problem space in its own right, and not very
easy to address. In OpenStack we ended up defining a concept of
"device tagging" via cloud-init metadata, where openstack allows
users to set opaque string tags against devices their VM has.
OpenStack that generates a metadata file that records various
pieces of identifying hardware attributes (PCI address, MAC
addr, disk serial, etc) alongside the user tag. This metadata
file is exposed to the guest with the hope that there's enough
info to allow the user to decide which device is to be used for
We really should've gone with new attribute for the user alias instead
of using an existing one, given how many problems that is causing.
5) Generating the right XML snippet for device hot-(un)plug
This is kind of related to some previous points.
When hot-plugging a device and creating an XML snippet for it, you want
to keep the defaults from point 1) and policies from 2) in mind. Or
something related to the already existing domain which you can describe
systematically. And adding something for identification (see previous
Doing the hot-unplug is easy depending on how much information about
that device is saved by your application. The less you save about the
device (or show to the user in a GUI, if applicable) the harder it might
be to generate an XML that libvirt will accept. Again, some problems
with this should be fixed in libvirt, some of them are easy to
workaround. But having a common ground that takes care of this should
help some projects.
Hot-unplug could be implemented just based on the alias. This is
something that would fit into libvirt as well.
To mention some pre-existing solutions:
- I understand OpenStack has some really sensible and wisely chosen
and/or tested default values.
In terms of default devices and OS specific choices, OpenStack's
decisions have been largely inspired by previous work in oVirt
and / or virt-manager. So there's obviously overlap in the
conceptual area, but there's also plenty that is very specific
to OpenStack - untangling the two extract the common bits from
the app specific bits is hard.
- I know KubeVirt has VirtualMachinePresets. That is something
related to points 1) and 2). Also their abstraction of the XML might
be usable for point 3).
- There was an effort on creating policy based configuration of libvirt
objects called libvirt-designer. This is closely related to points 2)
and 3). Unfortunately there was no much going on lately and part of
virt-manager repository has currently more features implemented with
the same ideas in mind, just not exported for public use.
This is the same kind of problem we faced wrt libvirt-gconfig and
libvirt-gobject usage from virt-manager - it has an extensive code
base that already works, and rewriting it to use something new
is alot of work for no short-term benefit. libvirt-gconfig/gobject
were supposed to be the "easy" bits for virt-manager to adopt, as
they don't really include much logic that would step on virt-manager's
toes. libvirt-designer was going to be a very opinionated library
and in retrospective that makes it even harder to consider adopting
it for usage in virt-manager, as it'll have signficant liklihood
of making functionally significant changes in behaviour.
There's also the problem with use of native libraries that would
impact many apps. We only got OpenStack to grudgingly allow the
use of libosinfo native library via GObject Introspection, by
promising to do work to turn the osinfo database into an approved
stable format which OpenStack could then consume directly, dropping
the native API usage :-( Incidentally, the former was done (formal
spec for the DB format), but the latter was not yet (direct DB usage
BTW, I don't like that I'm being so negative to your proposal :-(
I used to hope that we would be able to build higher level APIs on
top of libvirt to reduce the overlap between different applications
reinventing the wheel. Even the simplest bits we tried like the
gconfig/gobject API are barely used. libvirt-designer is basically
a failure. Though admittedly it didn't have enough development resource
applied to make it compelling, in retrospect adoption was always going
to be a hard sell except in greenfield developments.
Libosinfo is probably the bit we've had most success with, and has
most promise for the future, particularly now that we formally allow
apps to read the osinfo database directly and bypass the API. It is
quite easy to fit into existing application codebases which helps alot.
Even there I'm still disappointed that we only have GNOME Boxes using
the kickstart generator part of osinfo - oVirt and Oz both still have
their own kickstart generator code for automating OS installs.
In general though, I fear anything API based is going to be a really
hard sell to get wide adoption for based on what we've seen before.
I think the biggest bang-for-buck is identifying more areas where we
can turn code into data. There's definitely scope for recording more
types of information in the osinfo database. There might also be
scope for defining entirely new databases to complement the osinfo
data, if something looks out of scope for libosinfo.