macvtap with disconnected physical interface

[Laine Stump]

On 5/3/22 4:31 AM, Daniel P. Berrangé wrote:
> On Mon, May 02, 2022 at 03:42:05AM +0200, Gionatan Danti wrote:
>> Dear list,
>> I just discovered the hard way that if the lower lever physical interface of
>> a macvlan bridge is disconnected (ie: by unplugging the eth cable, resulting
>> in no carrier), inter-guest network traffic from all virtual machines bound
>> to the disconnected interface is dropped.
>>
>> This behavior surprises me, as with the classic bridges I can disconnect the
>> underlying physical interface without causing any harm to inter-guest
>> traffic.
>>
>> Am I doing something wrong, or this really is the expected behavior? If so,
>> can I force the macvtap interfaces to bridge traffic even when the
>> underlying physical interface is disconnected?
> 
> Can you share the <interface> configuration for your guest NIC so we
> can see how it is setup.

I can't say that I've ever tried this, since my only reason for using 
macvtap is to provide the guests with direct connectivity to the 
physical network, and unplugging the physdev negates that. The behavior 
you describe doesn't surprise me all that much though, since the 
physical device in the case of a host bridge isn't an integral part of 
the bridge (it's just one more device attached to a port), while the 
physical device and macvlan bridge a much more closely associated.

I'm Cc'ing Michael Tsirkin to see if he has more authoritative 
information on whether or not the macvtaps connected to a macvlan bridge 
can communicate amongst themselves when the physdev is disconnected.

In the meantime, is there a reason you don't want to just use a standard 
host bridge that's not connected to any physdev? The one thing I can 
think of is that you might not want to allow communication between the 
host and guests, but as long as the bridge itself isn't given an IP 
address, that won't be possible (at least at the level of IP).