[libvirt] [Qemu-devel] Re: Libvirt debug API

Daniel Veillard veillard at redhat.com
Mon Apr 12 08:56:21 UTC 2010

On Sun, Apr 11, 2010 at 11:17:38PM +0100, Jamie Lokier wrote:
> Richard W.M. Jones wrote:
> > On Fri, Apr 09, 2010 at 10:06:51PM +0100, Jamie Lokier wrote:
> > > Daniel P. Berrange wrote:
> > > > I think this alteration of existing args is faaaar too complex & fragile,
> > > > and way overkill.
> > > 
> > > Would it not be simpler, for the target audience, for the config to
> > > contain a one-line shell script to transform particular matched
> > > arguments in any way that's wanted?
> > 
> > I might be missing something but I thought you could already do this
> > in current libvirt.  ie. Change the <emulator> element [1] to point to
> > your shell script:
> > 
> >   <emulator>/usr/local/bin/my-qemu-wrapper.sh</emulator>
> > 
> > The wrapper should get called with the command line arguments and you
> > can alter them however you like.
> > 
> > Isn't that right, Dan?
> That forces the shell script to be relatively complicated and
> recognise all qemu arguments (including version specific ones) to skip
> the ones with parameters - and to know where to look for the real
> target executable is.
> It's not a very friendly way to add or change parts of specific
> subsystems, for example modifying a blockdev parameter, but it can be
> done that way if necessary.
> It's not that hard to write this for trivial extra options:
>    <emulator>/bin/sh -c 'qemu "$0" "$@" -extra-flag'</emulator>
> (if that works).

  That won't work because we expect the emulator to be a path to
the emulator program, so yes that has to be done in the wrapper script.

> Parsing libvirt output and having to guess which option corresponds to
> what from the libvirt config sounds very fragile and also a rather
> large amount of effort for something which should be easy.

  it's not that easy because we change the way we build the command line
for qemu as qemu evolves. for example before and after the support for
-device there have been a lot of changes.

> And then there's modifying how network tap devices are set up and
> other networking, outside qemu, but requested by libvirt.  Doing that
> in <emulator> sounds like a bad idea, but script hooks are the only
> sane way to make networking fit every environment and requirement.
> Ideally without forcing the hook writer to reimplement the normal
> network setup from scratch just to tweak it a bit.  I apologise if
> libvirt already provides such hooks - I haven't looked at that part of
> it.

  Well it's very new, and I still didn't wrote the documentation for it
so that's notmal, not your fault :-)


Daniel Veillard      | libxml Gnome XML XSLT toolkit  http://xmlsoft.org/
daniel at veillard.com  | Rpmfind RPM search engine http://rpmfind.net/
http://veillard.com/ | virtualization library  http://libvirt.org/

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