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Re: [Xen-devel] Re: [libvirt] [RFC] libxenlight driver

To: Jim Fehlig <jfehlig@xxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [libvirt] [RFC] libxenlight driver
From: Gianni Tedesco <gianni.tedesco@xxxxxxxxxx>
Date: Fri, 21 Jan 2011 16:33:55 +0000
Cc: LibVir <libvir-list@xxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, "Daniel P. Berrange" <berrange@xxxxxxxxxx>
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On Fri, 2011-01-21 at 15:48 +0000, Jim Fehlig wrote:
> Daniel P. Berrange wrote:
> > On Thu, Jan 20, 2011 at 04:49:25PM -0700, Jim Fehlig wrote:
> >   
> >> I'm looking into creating a driver for the new Xen xl/libxl toolstack
> >> (aka libxenlight [1]), set to become the default in upcoming Xen 4.1.0
> >> release.
> >>
> >> My first hurdle is deciding whether this should be a new driver or
> >> integrated with existing xen-unified driver.  Initially I thought a new
> >> driver would be a better approach - a clean break from the old code,
> >> similar to the xenapi driver.  libxenlight is also stateless (no managed
> >> domains), which seems like another good argument for a new driver.  But
> >> libxenlight is really just another interface into the same hypervisor,
> >> so in that regard it should be a xen-unified subdriver.
> >>     
> >
> > Something on the system must be stateful, continually monitoring
> > guests & taking neccessary actions ? eg If XenD isn't used, then
> > what is responsible for restarting guests which crash, or performing
> > core dumps on crashed guests, etc, etc ?
> >   
> Good questions.  I have just started looking at the new toolstack, and
> frankly don't yet know how this is handled.  Adding xen-devel for
> comment ...

Yes and no, in xen, the hypervisor internal data structures combined
with xenstore is the state (pretty much anyway). The purpose of
libxenlight is to abstract away these details.

However, when a domain is created with libxenlight one must wait for a
few events to ensure the proper clean-up of the domain. The events are
basically 'domain death' or 'device ejected'. These exist so the
toolstack can handle rebooting, core dumping the domain, inserting the
next CD image, or whatever. I am not sure how libvirt handles such
events normally but you may either integrate with a poll/select based
mainloop or have threads or processes with self-pipes handling these.

So in summary, state is maintained by the xen system outside of any
libvirt driver, but handling the kind of events you mention will require
some special work to be done for domains created by libvirt.

> > This would have a bearing on how best to design a libvirt driver
> >
> >   
> >> There are certainly benefits to the xen-unified subdriver approach, e.g.
> >> the existing hypervisor and xenstore subdrivers can be leveraged, the
> >> former providing all the capabilities code.  But AFAIK, libxenlight and
> >> xend should not be used together, so I don't think we would want the
> >> xend subdriver activated if libxenlight is detected.  Supposedly xl can
> >> be used as a direct replacement for xm, allowing unconditional use of
> >> that subdriver.
> >>
> >> BTW, Ian Jackson responded [2] to some of my questions regarding
> >> compatibility between old and new toolstack if you are interested.
> >>
> >> I'd like to hear other's opinions on a new driver vs. a xen-unified
> >> subdriver.
> >>     
> >
> > Due to the number of revisions of Xen userspace stack, and the
> > need to talk to so many pieces to get an efficient driver, the
> > current Xen unified driver is rather hairy. Particularly if
> > XenD itself is deprecated as a control mechanism, then I'd
> > go for a new standalone driver, that runs from libvirtd context
> > and leverages the standard libvirt storage/network/inteface
> > drivers for non-HV stuff (which I assume libxenlight doesn't
> > cover).
> >   
> Correct.  AFAICT, libxenlight does not cover any host storage or network
> management.  A libxenlight driver will be a hypervsisor driver only.

Yes definitely. We handle attaching and detaching NIC's and block
devices. Any other management is orthogonal to libxenlight (as it is for

BTW. I would also add my vote for a separate code-base for
xend/traditional vs. libxenlight drivers as there is no similarity of
interface. The xend HTTP RPC calls are unrelated to the libxl API.

In any case, please keep xen-devel in the distribution list :)


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