On Tue, Jan 16, 2018 at 05:43:44PM +0000, Daniel P. Berrange wrote:
> On Tue, Jan 16, 2018 at 03:08:15PM -0200, Eduardo Habkost wrote:
> > [CCing Daniel]
[...]
> > I still don't understand why OpenStack doesn't let users add or
> > modify elements on the domain XML. This isn't the first time I
> > see this preventing users from fixing problems or optimizing
> > their systems.
> >
> > Is there a summary of the reasons behind this limitation
> > somewhere?
>
> Exposing ability to control every aspect of Libvirt XML is a non-goal of
> Nova. A great many of the features require different modelling and/or
> explicit handling by Nova to work well in the context of OpenStack's
> architecture. The domain XML is automatically generated on the fly by
> Nova based on the info it gets from various inputs, so there's nothing
> that can be editted directly to add custom elements. The only way that
> would allow modification is for Nova to send the XML it generates to
> an external plugin script and read back modified XML. Historically Nova
> did have alot of plugin points that allowed arbitrary admin hacks like
> this, but they end up being a support burden in themselves, as they
> end up being black boxes which change Nova behaviour in unpredictable
> ways. Thus Nova has actually worked to remove as many of the plugins
> as possible.
>
> In this case there is a clear benefit to being able to add extra CPU
> features, over the base named model. It is easy for Nova to wire this
> up and it should do so as a priority.
Agreed, it has long been pending in Nova; I also recall you've
identified other use cases for it (e.g. ability to mention 1G huge pages
with the CPU flag "pdpe1gb"). So I began a quick WIP Nova change to
allow it to specify CPU feature flags. I haven't worked out all the
details yet, and still addressing the quick comment you (DanPB) made on
IRC.
https://review.openstack.org/#/c/534384 -- [WIP] libvirt: Allow to
specify granular CPU feature flags
PS: Thursday and Friday I'll be a bit sporadic in my availability, but
this change is on top of my TODO list.
--
/kashyap
