Re: [Qemu-devel] Re: [PATCH] Make NIC model fallback to default when specified model is not supported

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Sep 20, 2010 at 12:48:50PM +0200, Michal Novotny wrote:
> On 09/20/2010 12:34 PM, Paolo Bonzini wrote:
> >On 09/20/2010 11:47 AM, Michal Novotny wrote:
> >>Hi,
> >>
> >>this is the patch to introduce a NIC model fallback to default when 
> >>model
> >>specified is not supported. It's been tested on i386-softmmu target on
> >>i386 host using the Windows XP x86 virtual machine and by trying to 
> >>setup
> >>the invalid (unsupported) model of NIC device. Also, the new constant in
> >>the net.h called the DEFAULT_NIC_MODEL has been introduced to be able to
> >>change the default NIC model easily. This variable is being used to set
> >>the default NIC model when necessary.
> >
> >Why?  If it's not supported, it shouldn't run.
> >
> >Paolo
> 
> I don't think so. It makes sense it shouldn't run for case of pure qemu 
> but since there's newly added support for xen (and also there's support 
> for other virtualization platforms to be used with the qemu device 
> model) it should fallback with just a warning since otherwise those 
> platforms, like e.g. mentioned Xen, will leave defunct device models 
> there and the guests won't run be running at all ending up with no 
> state. If there's a warning with information it's falling back to 
> default the user can notice if he wants to but it won't leave the 
> defunct device models anymore which can be pretty hard to determine 
> what's going on there for standard user that doesn't have much 
> experience with e.g. Xen yet.

IMHO this is just a bug in the xen mgmt layer. If the QEMU device model
dies/quits, then XenD should teardown the guest, since you can't do any
useful work once the device model has crashed. Silently switching to a
different NIC model than the one requested is definitely a wrong approach.

Daniel
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