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.