[virt-tools-list] [kubevirt-dev] Re: Project for profiles and defaults for libvirt domains

Thu Mar 22 17:18:22 UTC 2018

On Thu, Mar 22, 2018 at 03:54:01PM +0100, Martin Kletzander wrote:
> > 
> > > One more thing could be automatically figuring out best values based on
> > > libosinfo-provided data.
> > > 
> > > 2) Policies
> > > 
> > > 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
> > > predictable way?
> > > 
> > > 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.
> > 
> 
> Sure, this was just meant to be a question as to whether it's worth
> pursuing or not.  You make a good point on why it is not (at least for
> existing apps).
> 
> However, since this was optional, the way this would look without the
> XML abstraction is that both input and output would be valid domain
> definitions, ultimately resulting in something similar to virt-xml with
> the added benefit of applying a policy from a file/string either
> supplied by the application itself.  Whether that policy was taken from
> a common repository of such knowledge is orthogonal to this idea.  Since
> you would work with the same data, the upgrade could be incremental as
> you'd only let virtuned fill in values for new options and could slowly
> move on to using it for some pre-existing ones.  None of the previous
> approaches did this, if I'm not mistaken.  Of course it gets more
> difficult when you need to expose all the bits libvirt does and keep
> them in sync (as you write below).

That has implications for how mgmt app deals with XML. Nova has object
models for representing XML in memory, but it doesn't aim to have
loss-less roundtrip from parse -> object -> format. So if Nova gets
basic XML from virttuned, parses it into its object to let it set
more fields and then formats it again, chances are it will have lost
a bunch of stuff from virttuned. Of course if you know about this
need upfront you can design the application such that it can safely
round-trip, but this is just example of problem with integrating to
existing apps.

The other thing that concerns is that there are dependancies between
different bits of XML for a given device. ie if feature X is set to
a certain value, that prevents use of feature Y. So if virttuned
sets feature X, but  the downstream application uses feature Y, the
final result can be incompatible. The application won't know this
because it doesn't control what stuff virttuned would be setting.
This can in turn cause ordering constraints.

eg the application needs to say that virtio-net is being used, then
virttuned can set some defaults like enabling vhost-net, and then
the application can fill in more bits that it cares about. Or if
we let virttuned go first, setting virtio-net model + vhost-net,
then application wants to change model to e1000e, it has to be
aware that it must now delete the vhost-net bit that virtuned
added. This ends up being more complicated that just ignoring
virttuned and coding up use of vhost-net in application code.

> > 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.
> > 
> 
> The initial idea (which I forgot to mention) was that all the decisions
> libvirt currently does (so that it keeps the guest ABI stable) would be
> moved into data (let's say some DSL) and it could then be switched or
> adjusted if that's not what the mgmt app wants (on a per-definition
> basis, of course).  I didn't feel very optimistic about the upstream
> acceptance for that idea, so I figured that there could be something
> that lives beside libvirt, helps with some policies if requested and
> then the resulting XML could be fed into libvirt for determining the
> rest.

I can't even imagine how we would go about encoding the stable guest
ABI logic libvirt does today in data !

> 
> > There's also the problem with use of native libraries that would
> > impact many apps. We only got OpenStack to grudgingly allow the
> 
> By native you mean actual binary libraries or native to the OpenStack
> code as in python module?  Because what I had in mind for this project
> was a python module with optional wrapper for REST API.

I meant native binary libraries. ie openstack is not happy in general
with adding dependancies on new OS services, because there's a big
time lag for getting them into all distros. By comparison a pure
python library, they can just handle automatically in their deployment
tools, just pip installing on any OS distro straight from pypi. This
is what made use of libosinfo a hard sell in Nova.

The same thing is seen with Go / Rust where some applications have
decided they're better of actually re-implementing the libvirt RPC
protocol in Go / Rust rather than use the libvirt.so client. I think
this is a bad tradeoff in general, but I can see why they like it

Regards,
Daniel
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