On Mon, Jul 13, 2026 at 06:08:26PM +0300, Vladimir Sementsov-Ogievskiy wrote: > On 13.07.26 16:29, Markus Armbruster wrote: > > Vladimir Sementsov-Ogievskiy <[email protected]> writes: > > > > > On 13.07.26 10:57, Markus Armbruster wrote: > > > > Vladimir Sementsov-Ogievskiy <[email protected]> writes: > > > > > > > > > Support transferring of TAP state (including open fd). > > > > > > > > > > Add new property "local-migration-supported", which defines > > > > > whether local-migration is actually supported for this TAP device. > > > > > Starting from 11.2 QEMU Machine Types it's enabled by default. > > > > > > > > > > Note, that local-migration (including migrating opened FDs > > > > > > > > Scratch the comma after "Note", ... > > > > > > > > > through migration channel, which must be UNIX socket), is > > > > > > > > ... and the one before "is". > > > > > > > > > enabled by global "local" migration parameters. But individual > > > > > devices may have additional options to enable/disable it > > > > > per device. > > > > > > > > > > The tricky thing, is that we need to know, should we do > > > > > > > > Do you mean "The tricky thing is that we ween to know whether to call"? > > > > > > yes > > > > > > > > > > > > call open/connect in TAP initialization code. That means, > > > > > that on incoming migration we want to know status of > > > > > "local" migration parameter at time of creating TAP device. > > > > > So, currently, for incoming local migration only TAP device > > > > > created through QMP is supported, as you need to set migration > > > > > parameter first (through QMP as well). > > > > Possibly clearer: > > > > The tricky thing is that we need to know whether to call open/connect > > in TAP initialization code, i.e. we need to know the value of > > migration parameter "local" when creating the TAP device. For > > incoming migration, we can know only for TAP devices created with QMP > > after setting the migration parameter with QMP. > > > > > > > > > > > > So the full picture is: > > > > > > > > > > On source, to start outgoing "local" migration you need: > > > > > > > > > > - migration parameter "local" set to true > > > > > - "local-migration-supported" TAP option set to true (the > > > > > default, starting from 11.2 QEMU Machine Types) > > > > > > > > > > If at least one of these options is not set, TAP backend > > > > > doesn't participate in migration. > > > > > > > > > > On target, things are more difficult: > > > > > > > > > > Same, you need both "local" and "local-migration-supported" > > > > > be set. And same, if one of them is not set, TAP backend > > > > > is initialized as usual, and doesn't accept any incoming > > > > > state. > > > > > > > > > > Additionally, if you are going to set "local", it must be > > > > > set before creating the TAP device. If TAP device created > > > > > with "local" unset, it initializes as usual. If you enable > > > > > "local" after it and start incoming migration, it will fail > > > > > in .pre_load handler of TAP backend. > > > > > > > > Out of curiosity, how would such a failure look like? > > > > > > TAP is initialized as usual, so the answer depends on the options. > > > > > > For incoming migration we should open TAP without "ifname" argument. > > > If so (and "local" is not set), new TAP device will be created. It > > > shouldn't trigger any additional problems I think. > > > > > > > > > Checking with test: > > > > > > diff --git a/tests/functional/x86_64/test_tap_migration.py > > > b/tests/functional/x86_64/test_tap_migration.py > > > index bcc0a60dfe9..94982bc7b55 100755 > > > --- a/tests/functional/x86_64/test_tap_migration.py > > > +++ b/tests/functional/x86_64/test_tap_migration.py > > > @@ -316,8 +316,8 @@ def prepare_and_launch_vm( > > > else: > > > tap_name = TAP_ID > > > > > > - self.set_migration_capabilities(vm, local) > > > self.add_virtio_net(vm, vhost, tap_name, local, incoming) > > > + self.set_migration_capabilities(vm, local) > > > > > > def add_virtio_net( > > > self, vm, vhost: bool, tap_name: str, local: bool, incoming: > > > bool > > > > > > > > > Leads to > > > > > > qemu-system-x86_64: TAP is already initialized and cannot receive > > > incoming fd > > > qemu-system-x86_64: warning: qemu_fclose: received fd 27 was never claimed > > > qemu-system-x86_64: load of migration failed: Invalid argument: error > > > while loading state for instance 0x0 of device 'netdev.1/0/net-tap': pre > > > load hook failed for: 'net-tap', version_id: 0, minimum version_id: 0, > > > ret: -22 > > > > > > on target, and failed migration, as expected. > > > > How is the user supposed to reason their way from here to "oh, I need to > > create my TAP device with QMP and after setting @local"? > > Not trivial, yes. > > I can update error message to > > TAP is already initialized and cannot receive incoming fd. For local > migration, "local" migration parameter must be set _before_ creating TAP > device.
Another reason to just make it a machine property globally and be done with it, maybe? > > > > > > > > > > > > > > Moreover, there are interface restrictions: if you create TAP > > > > > device when QEMU is in INCOMING state, and both "local" > > > > > and "local-migration-supported" set, most of TAP options are > > > > > not allowed, and script/downscript are required to be explicitly > > > > > unset (set to "" or "no"). > > > > > > > > > > Signed-off-by: Vladimir Sementsov-Ogievskiy > > > > > <[email protected]> > > > > [...] > > > > > -- > Best regards, > Vladimir
