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 01:07 PM, Daniel P. Berrange wrote:
> 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.
>    

I don't know how this is being implemented in the new qemu code that's 
implementing Xen support but from what I saw the upstream Xen-4.1 is not 
using it yet so implementing this into the new xen management layer 
could be a good idea for somebody working on the xen layer for qemu 
because I'm not familiar with this yet.

>    
>> 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
>    
Well, this way I guess we should have 2, maybe 3 different approaches - 
different for qemu itself and for upstream version Xen using the older 
version of qemu-dm and RHEL-5 version.

Therefore I think we should drop the patch for qemu (the one sent to 
this list) and decide about the others using their own lists/discussions.

Michal

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