Re: [libvirt] [PATCH 0/3] allow fixing tap-bridge connections when network is restarted

It's been a long-standing problem that when you stop and restart a libvirt network, the guest tap devices are no longer connected to the network. Until now the only way to recover from this was to either shutdown and restart all the affected guests, or to detach and then re-attach all affected guest network devices. These patches permit the situation to be remedied by just restarting libvirtd - when the hypervisor drivers are notifying the network drivers of each connection, the network can retrieve the current "master" for the tap device and compare it to the network's bridge device. If they don't match, then it disconnects the tap from any incorrect bridge and connects to the correct bridge. *** For now, that's as far as we go - *semi*-automated (since you need to restart libvirtd for it to happen). My intent is for this to be completely automated, but the logic to do that is a bit "convoluted" so I've deferred trying to implement it until I've given it more thought - a few different trains of thought have led to dead-ends, and so far only one seems reasonably doable: * add a networkStartCallback list to the network driver state object. * as each hypervisor driver that uses the network driver is initialized, it will call an internal-only function in the network driver to register a callback. * whenever a network is started, it will call all the registered callback functions (sending the name of the network being started, and some HV-specific context pointer). * Each hypervisor's callback function will look through all of its active domains for interfaces that are using the given network, and for each such interface, will call networkNotifyActualDevice() (the function that has been updated in Patch 3/3 to reconnect taps to bridges). The "convoluted" part here is that we need to make sure there is enough (but not too much!) locking/refcounting both when adding items to the callback list (which should only be done single threaded, since it happens during the driver initializations) and when the networkStart() function (in some state of lockedness/refcountedness) calls over to some e.g. qemu function that will then need to do some amount of locking (at least on each domain as it is processed) and then calls back into the network driver (which will need "some amount" of locking/refcounting). Since we're calling from network driver into qemu into network driver, while the normal nesting is just calling from qemu into network, I want to be sure there is no possibility for a deadlock. (Also, I want to avoid making the list of callbacks any more complicated than absolutely necessary - the "in" thing to do these days seems to be to allocate a hash table, but there's a lot of extra code required for that (see util/virclosecallbacks.[hc]) which seems like overkill for a list of maximum 4-5 items that will *never* change after the driver initialization - if anyone has ideas/opinions about that, please speak up. Laine Stump (2): util: new function virNetDevGetMaster() util: new function virNetDevTapAttachBridge() root (1): network: reconnect tap devices during networkNotifyActualDevice

src/libvirt_private.syms | 2 + src/network/bridge_driver.c | 30 +++++++++++- src/util/virnetdev.c | 49 +++++++++++++++++++ src/util/virnetdev.h | 3 ++ src/util/virnetdevtap.c | 111 +++++++++++++++++++++++++++++--------------- src/util/virnetdevtap.h | 12 +++++ 6 files changed, 169 insertions(+), 38 deletions(-)