Re: [libvirt] [PATCH 2/3] qemu: remove unnecessary setting of tap device online state

On 08/25/2016 05:41 AM, Vasiliy Tolstov wrote: (tl;dr - it doesn't seem to me like this should be a problem, since we don't add the IP addresses or routes to the tap device until just before starting the guest CPUs) Hmm, so a L3 analog to the problem that we have at L2 with macvtap interfaces (the presence of an IFF_UP interface with the same MAC address as the guest causes traffic for that MAC to be sent to the destination too soon. When connecting a tap device to a bridge, it's important for it to be IFF_UP as soon as possible, because the STP forward delay timer doesn't start until it's IFF_UP (and since the MAC address of the tap device itself isn't used for forwarding any traffic, there's no harm to an L2 forwarding tables caused by this). Of course in the case of taps connected to bridges, we don't have any IP address set, and also no routes set (although I'm wondering if in the future we might have routes (but still no IPs)), so we never encounter the issue with L3 forwarding that we do for type='ethernet'. Setting the tap device offline does have the effect of eliminating all IP addresses and routes in a single operation, which works properly for you. But if that tap happened to be connected to a bridge (outside the scope of libvirt), waiting to set it IFF_UP could result in a much longer-than-desired wait before the interface was usable. But of course if we're not adding the routes and IPs until qemuInterfaceStartDevice(), the issue wouldn't exist at domain start time - that function isn't called until right before the CPUs are started, which is exactly when you want it, so there shouldn't be any case where either the IP address of the tap device or the routes associated with it are visible prior to the exact time when you want it to happen. There is one issue that may still need to be addressed - there are a few cases where we stop the guest CPUs temporarily, and then restart them; qemuInterfaceStopDevice *is* called before the CPUs are stopped, but because we don't have anything in there to remove the routes or IPs on the tap device, it would still be seen as a destination for the given IPs during this time. I'm not sure this is really a problem though, because we do fully intend to start the same CPU up again and in the meantime there isn't any other valid destination for the traffic - removing and re-adding the routes during, e.g a qemuDomainRevertToSnapshot() would only have the effect of causing a mini (and single iteration) route flap. So I don't think anything needs to be done about this either. Does this all make sense, or am I missing something?