Re: [libvirt] [PATCH] libxl: allow an <emulator> to be selected in the domain config XML
Hi,
[added xen-devel: FYI this is about how to properly set the libxl
device_model_version when the user has provided a manual device_model
override (aka a path to a qemu) in the libvirt domain XML.]
On 30/04/13 16:10, Jim Fehlig wrote:
David Scott wrote:
> The emulator path supplied can be any valid path on the system.
>
> Note that when setting a device_model, libxl needs us to set the
> device_model_version too. The device_model_version can be either
>
> ...QEMU_XEN: meaning "upstream qemu", the default in xen-4.3 onwards
> ...QEMU_XEN_TRADITIONAL: the old xen-specific fork
>
> We detect the device_model_version by examining the qemu filename:
> if it is "qemu-dm" then it's the old xen-specific fork. If anything
> else then we assume "upstream qemu" (whose filename may change
> in future). Note that if you are using a wrapper script to (eg)
> adjust the arguments of the old qemu during development, you will
> have to ensure the wrapper script also has the name "qemu-dm", by
> placing it in a separate directory.
>
That is unfortunate. Users could have existing config with
<emulator>/usr/bin/my-qemu-dm</emulator> which works with the legacy
stack but not with libxl right? Is it possible to safely query the
binary to determine if it is qemu-dm?
From my reading of libxl, it doesn't seem to have any way to detect the
type of a given qemu binary (or at least I couldn't spot it). I think
that if we were to write some detection code we should probably add it
to libxl rather than libvirt -- what do you think?
The other options I can think of are:
1. weaken the test so we interpret any filename containing the substring
"qemu-dm" as traditional-- this would catch your case at least
2. flip the default around so that if an <emulator> is provided we
assume traditional unless the filename is "qemu-system-i386" (or maybe
just "contains qemu-system-i386" or "contains qemu-system")
3. add libxl driver-specific XML (is that possible?) to allow the user
to override a libvirt default. It would be a shame to expose the
complexity to the libvirt client though.
What do you think?
Cheers,
Dave