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

[Michal Novotny <minovotn@redhat.com>]

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.

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.

Michal

-- 
Michal Novotny<minovotn@redhat.com>, RHCE
Virtualization Team (xen userspace), Red Hat