[libvirt] [Qemu-devel] Problems using netdev_del+netdev_add w/o corresponding device_del+device_add

[Markus Armbruster]

Laine Stump <laine at redhat.com> writes:

> On 10/15/2012 05:25 AM, Daniel P. Berrange wrote:
>> On Mon, Oct 15, 2012 at 10:30:07AM +0200, Stefan Hajnoczi wrote:
>>> On Sat, Oct 13, 2012 at 04:47:14PM -0400, Laine Stump wrote:
>>>> Here is the sequence sent to disconnect only the host side, then
>>>> reconnect it with a new tap device. (although the fd is the same, this
>>>> is because the old tap device had already been closed, so the number is
>>>> just being used - the same thing happens when doing sequential full
>>>> detach/attach cycles, and they all work with no problems):
>>>>
>>>>
>>>> 168.750 > 0x7f8e20000c90
>>>> {"execute":"netdev_del","arguments":{"id":"hostnet0"},"id":"libvirt-30"}
>>>> 168.762 < 0x7f8e20000c90 {"return": {}, "id": "libvirt-30"}

This deletes the backend, and leaves the frontend NIC without a backend.
Such as NIC behaves / should behave like it's not connected to anything
(link down).

>>>> 168.800 > 0x7f8e20000c90
>>>> {"execute":"getfd","arguments":{"fdname":"fd-net0"},"id":"libvirt-31"}
>>>> (fd=27)
>>>> 168.801 < 0x7f8e20000c90 {"return": {}, "id": "libvirt-31"}
>>>> 168.801 > 0x7f8e20000c90
>>>> {"execute":"netdev_add","arguments":{"type":"tap","fd":"fd-net0","id":"hostnet0"},"id":"libvirt-32"}
>>>> 168.802 < 0x7f8e20000c90 {"return": {}, "id": "libvirt-32"}
>>>> 168.802 > 0x7f8e20000c90

This creates a new backend, not connected to any frontend.  The fact
that it has the same ID as some deleted backend is completely
immaterial.

>>>> {"execute":"set_link","arguments":{"name":"net0","up":true},"id":"libvirt-33"}
>>>> 168.803 < 0x7f8e20000c90 {"return": {}, "id": "libvirt-33"}

This orders the NIC to change the link status to "up".  Can't work,
because it's still not connected to anything.  It succeeds anyway, which
could be regarded as a bug.

>>>> After this sequence is done, everything about the network device
>>>> *appears* normal on both the guest and host (at least the things I know
>>>> to look at), but no traffic from the host shows up in a tcpdump of the
>>>> interface on the guest, and no traffic from the guest shows up in a
>>>> tcpdump of the tap device on the host.
>>> What you are trying to do isn't possible today.
>
> Well, at least it's good to know that I should stop trying to make it
> work :-)
>
> Actually, it's a bit disconcerting that 1) the act of creating a guest
> device is split into two commands, implying that they don't necessarily
> have a hardwired a-->b relationship although that is the case, and that

It isn't really the case.

Network frontend and backend are really separate things, but...

> 2) netdev_add even returns success when you use it in this way. Although
> hindsight is 20/20 and all that, if both a and b are required, and must
> always be in the same order, wouldn't it have made more sense for the
> two steps to be a single command? I suppose this is a byproduct of the
> monitor commands being a direct reflection ot the commandline options.
> (At the very least, though, I think netdev_add should report an error if
> the device name alias it uses is already in use by a device.)
>
>>>
>>> The device associates with the netdev during initialization only - there

... the connection between the two can only be made during frontend
initialization.  Not because of design limitations, just because more
dynamic connecting hasn't been implemented.

>>> is no command to associate at a later point in time.  That is why your
>>> example works only when the device is deleted together with the netdev.
>>>
>>> It is certainly possible to implement a command to switch netdevs
>
> At this point yes, it would be better to have a new command rather than
> to make netdev_add work in the way I've attempted - this way there would
> be a new command whose presence libvirt could use to decide whether or
> not to support this functionality.

Besides, I'd oppose ID magic like making netdev_add behave differently
when the ID matches some previously used ID.

>>>  but
>>> I'm curious what the use case is.  Is this necessary just because QEMU
>>> doesn't provide a way to modify the existing netdev or because you
>>> really want to switch to a completely different netdev?
>> We have end users who want to be able to dynamically change the guest'
>> networking attachment, without restarting/hotplugging devices in the
>> guest[1]. If it is just a case of changing from one bridge, to another
>> bridge we can do that just by moving the TAP Device from one to another.
>> This doesn't work if we want to support more general changes in config.
>> eg from a macvtap setup to a TAP setup, or vica-verca.
>
> Beyond that, I haven't determined it conclusively yet, but it so far
> looks to me like a macvtap device can only be linked to a physdev when
> it is created - there is no netlink message to re-link it to a different
> physdev (this is based on my naive examination of the relevant kernel
> source). So if you want to change the attach point for a macvtap-type
> connection, you again need to discard the old macvtap device and create
> a new one, implying that you need to do a new netdev_add.

Wanting to connect a frontend NIC to a different backend seems entirely
fair to me.  Patches welcome :)