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

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

>>  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.

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