Re: modify NetdevUserOptions through QMP in QEMU 6 - how?

[Thomas Huth <thuth@redhat.com>]

On 15/12/2021 04.31, Jason Wang wrote:
> On Tue, Dec 14, 2021 at 10:53 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>>
>> On Mon, Dec 13, 2021 at 09:02:15AM +0100, Thomas Huth wrote:
>>>   Hi!
>>>
>>> On 10/12/2021 18.02, Alexander Sosedkin wrote:
>>>> With QEMU 5 I could totally issue a QMP netdev_add
>>>> with the same ID to adjust the NetdevUserOptions I want,
>>>> such as restrict or hostfwd. No deleting needed,
>>>> just a netdev_add with what I want changed as a param.
>>>
>>> I'm a little bit surprised that this worked, since AFAIK there is no code in
>>> QEMU to *change* the parameters of a running netdev... likely the code added
>>> a new netdev with the same ID, replacing the old one?
>>>
>>>> With QEMU 6 it started failing, claiming the ID is already used.
>>>> And if I do netdev_del + netdev_add, I just lose connectivity.
>>>> What's even stranger, I still see old netdev attached in info network:
>>>>
>>>>> netdev_del {'id': 'net0'}
>>>> {}
>>>>> human-monitor-command {'command-line': 'info network'}
>>>> virtio-net-pci.0:
>>>> index=0,type=nic,model=virtio-net-pci,macaddr=52:54:00:12:34:56
>>>>    \ net0: index=0,type=user,net=10.0.2.0,restrict=off
>>>
>>> I think that's "normal" - there used to be problems in the past that the
>>> devices (virtio-net-pci in this case) did not like the netdevs to be removed
>>> on the fly. So the netdevs are kept around until you remove the device, too
>>> (i.e. issue a device_del for the virtio-net-pci device).
>>>
>>>>> netdev_add {'type': 'user', 'id': 'net0', 'restrict': False, 'hostfwd': [{'str': 'tcp:127.0.0.1:58239-:22'}]}
>>>> {}
>>>>> human-monitor-command {'command-line': 'info network'}
>>>> unseal: virtio-net-pci.0:
>>>> index=0,type=nic,model=virtio-net-pci,macaddr=52:54:00:12:34:56
>>>>    \ net0: index=0,type=user,net=10.0.2.0,restrict=off
>>>> net0: index=0,type=user,net=10.0.2.0,restrict=off
>>>>
>>>> What's the correct QMP command sequence to modify NetdevUserOptions?
>>>
>>> AFAIK there is no way to modify running netdevs - you'd have to delete the
>>> netdev and the device, and then add both again. But I might have missed
>>> something here, so I CC:-ed some people who might be more familiar with the
>>> details here.
>>>
>>>   Thomas
>>>
>>>
>>>> Please CC me on replies.
>>
>>
>> Wow this really goes to show how wide our feature matrix is.
>>
>> Yes it's probably an unintended side effect but yes it
>> did work it seems, so we really should not just break it
>> without warning.
>>
>>
>> Probably this one:
>>
>> commit 831734cce6494032e9233caff4d8442b3a1e7fef
>> Author: Markus Armbruster <armbru@redhat.com>
>> Date:   Wed Nov 25 11:02:20 2020 +0100
>>
>>      net: Fix handling of id in netdev_add and netdev_del
>>
>>
>>
>> Jason, what is your take here?
> 
> I might be wrong, but I agree with Thomas. Adding a netdev with the
> same ID looks wrong, if it works, it looks like a bug. 

It certainly calls for trouble as soon as you try to delete the netdev again 
- does it delete the first (inactive) instance? Does it delete the second 
active one? Does it delete both? (Otherwise it will leave a dangling 
instance behind) ...
So if changing netdev parameters on the fly is something that we want, we 
should implement this properly instead indeed, and not via such an 
accidental bug.

  Thomas