On Fri, Oct 25, 2019 at 02:19:19PM +0200, Jens Freimann wrote: > This is implementing the host side of the net_failover concept > (https://www.kernel.org/doc/html/latest/networking/net_failover.html) > > Changes since v5: > * rename net_failover_pair_id parameter/property to failover_pair_id > * in PCI code use pci_bus_is_express(). This won't fail on functions > 0 > * make sure primary and standby can't be added to same PCI slot > * add documentation file in docs/ to virtio-net patch, add file to > MAINTAINERS (added to networking devices section) > * add comment to QAPI event for failover negotiation, try to improve > commit message > > The general idea is that we have a pair of devices, a vfio-pci and a > virtio-net device. Before migration the vfio device is unplugged and data > flows to the virtio-net device, on the target side another vfio-pci device > is plugged in to take over the data-path. In the guest the net_failover > module will pair net devices with the same MAC address. > > * Patch 1 adds the infrastructure to hide the device for the qbus and qdev > APIs > > * Patch 2 adds checks to PCIDevice for only allowing ethernet devices as > failover primary and only PCIExpress capable devices > > * Patch 3 sets a new flag for PCIDevice 'partially_hotplugged' which we > use to skip the unrealize code path when doing a unplug of the primary > device > > * Patch 4 sets the pending_deleted_event before triggering the guest > unplug request > > * Patch 5 and 6 add new qmp events, one sends the device id of a device > that was just requested to be unplugged from the guest and another one > to let libvirt know if VIRTIO_NET_F_STANDBY was negotiated > > * Patch 7 make sure that we can unplug the vfio-device before > migration starts > > * Patch 8 adds a new migration state that is entered while we wait for > devices to be unplugged by guest OS > > * Patch 9 just adds the new migration state to a check in libqos code > > * Patch 10 In the second patch the virtio-net uses the API to defer adding > the vfio > device until the VIRTIO_NET_F_STANDBY feature is acked. It also > implements the migration handler to unplug the device from the guest and > re-plug in case of migration failure > > * Patch 11 allows migration for failover vfio-pci devices > > Previous discussion: > RFC v1 https://patchwork.ozlabs.org/cover/989098/ > RFC v2 https://www.mail-archive.com/qemu-devel@nongnu.org/msg606906.html > v1: https://lists.gnu.org/archive/html/qemu-devel/2019-05/msg03968.html > v2: https://www.mail-archive.com/qemu-devel@nongnu.org/msg635214.html > v3: https://patchew.org/QEMU/20191011112015.11785-1-jfreim...@redhat.com/ > v4: https://patchew.org/QEMU/20191018202040.30349-1-jfreim...@redhat.com/ > v5: https://patchew.org/QEMU/20191023082711.16694-1-jfreim...@redhat.com/ > > To summarize concerns/feedback from previous discussion: > 1.- guest OS can reject or worse _delay_ unplug by any amount of time. > Migration might get stuck for unpredictable time with unclear reason. > This approach combines two tricky things, hot/unplug and migration. > -> We need to let libvirt know what's happening. Add new qmp events > and a new migration state. When a primary device is (partially) > unplugged (only from guest) we send a qmp event with the device id. When > it is unplugged from the guest the DEVICE_DELETED event is sent. > Migration will enter the wait-unplug state while waiting for the guest > os to unplug all primary devices and then move on with migration. > 2. PCI devices are a precious ressource. The primary device should never > be added to QEMU if it won't be used by guest instead of hiding it in > QEMU. > -> We only hotplug the device when the standby feature bit was > negotiated. We save the device cmdline options until we need it for > qdev_device_add()
The status of the feature support in the guest can change. E.g. a guest reboot will clear it for certain, and the guest may boot into another OS that doesn't support pv-pt failover, and will become confused by two network devices with the same MAC. AFAICS from a brief skimming, the patchset doesn't appear to address this scenario (which is probably not so uncommon). > Hiding a device can be a useful concept to model. For example a > pci device in a powered-off slot could be marked as hidden until the > slot is > powered on (mst). > 3. Management layer software should handle this. Open Stack already has > components/code to handle unplug/replug VFIO devices and metadata to > provide to the guest for detecting which devices should be paired. > -> An approach that includes all software from firmware to > higher-level management software wasn't tried in the last years. This is > an attempt to keep it simple and contained in QEMU as much as possible. > One of the problems that stopped management software and libvirt from > implementing this idea is that it can't be sure that it's possible to > re-plug the primary device. By not freeing the devices resources in QEMU > and only asking the guest OS to unplug it is possible to re-plug the > device in case of a migration failure. Frankly I'm failing to see the point in requiring 100%-reliable re-plug on migration rollback. The whole idea of this failover is to allow temporary QOS degradation; if this isn't allowed you don't even consider migrating. So if the migration fails, you can leave the guest in the degraded state on the source host until a better migraion target is found or the conditions on the source host allow the re-plug to succeed. Thanks, Roman.
