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

[Stefan Hajnoczi]

On Mon, Oct 15, 2012 at 11:15:30AM -0400, Laine Stump wrote:
> 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 commands are historic (at least to me) and we have to make the best
of them.

> >>  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.

Yep, I just checked too.  macvlan_dev->lowerdev is only set in
macvlan_common_newlink().  There is no way to change it once the link
has been created.

Stefan