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

[Daniel P. Berrange]

On Mon, Sep 20, 2010 at 01:05:33PM +0200, Michal Novotny wrote:
> On 09/20/2010 12:53 PM, Daniel P. Berrange wrote:
> >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
> >   
> When the qemu-dm has crashed we can't do anything with the guest, that's 
> correct. Nevertheless do you think that we should bail with error and 
> just fix the layer of xen management to check whether there's a device 
> model still running or not? It's being spawned by XenD itself so we 
> would need to check whether this process is not a zombie using the 
> /proc/$PID/stat or use some better way to get the state. Unfortunately 
> using /proc/$PID/stat would kill the portability of the code.
> 
> The other way is to implement the thread that will be (periodically or 
> "on change") checking the device model state and that will be 
> terminating the domain when device model dies/quits. I'm not saying this 
> is the bad approach but we've been talking with Mirek about at least 
> RHEL-5 version and he told me that he recommends to implement a fallback 
> to the default NIC.

If XenD holds open a connection to the QEMU monitor socket, then it
can easily receive a POLLHUP when QEMU dies. This is the approach 
most mgmt tools use for detecting QEMU death.

> Daniel, if you consider RHEL-5 version, what do you prefer to do with 
> this one? Fix it somehow in the XenD or is altering the device model OK 
> for this version? Also, the patch has been already sent upstream Xen for 
> consideration about an hour ago.

RHEL5 Xen maintenance isn't a concern of qemu-devel really, so this
is not the place to decide that.

Daniel
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