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Re: [Xen-devel] [PATCH] libxl: allow an <emulator> to be selected in the domain config XML


[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?


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