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[80.230.24.117]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47f464c1f84sm1827633f8f.29.2026.07.13.12.36.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 Jul 2026 12:36:06 -0700 (PDT) Date: Mon, 13 Jul 2026 15:36:03 -0400 From: "Michael S. Tsirkin" To: Vladimir Sementsov-Ogievskiy Cc: jasowang@redhat.com, armbru@redhat.com, peterx@redhat.com, farosas@suse.de, raphael.s.norwitz@gmail.com, bchaney@akamai.com, qemu-devel@nongnu.org, berrange@redhat.com, pbonzini@redhat.com, yc-core@yandex-team.ru, mark.caveayland@nutanix.com, Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , Zhao Liu , Jason Wang , Eric Blake Subject: Re: [PATCH v18 12/14] net/tap: support local migration with virtio-net Message-ID: <20260713153340-mutt-send-email-mst@kernel.org> References: <20260710191809.721154-1-vsementsov@yandex-team.ru> <20260710191809.721154-13-vsementsov@yandex-team.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260710191809.721154-13-vsementsov@yandex-team.ru> Received-SPF: permerror client-ip=170.10.129.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.01, SPF_HELO_PASS=-0.001, T_SPF_PERMERROR=0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Fri, Jul 10, 2026 at 10:18:01PM +0300, Vladimir Sementsov-Ogievskiy wrote: > 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 > through migration channel, which must be UNIX socket), 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 > 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). > > 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. > > 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 I am still confused why is this per device. I just do not see where is it useful to set it for some devices but not others. So it looks like a machine property "incoming local migration" to me? Sorry if I am being dense. > --- > > In v17 thread we considered making new qapi parameters "unstable". > Still, it was because of doubtful "incoming-fds" parameter. In this > v18 it's completely removed, and we may live only with "local" > migration parameter + "local-migration-supported" TAP option. > This way the series doesn't conflict with suggested by Peter > [PATCH RFC 0/2] migration/vl: new -incoming config:* for early migration parameters > So, I think, worth making one more iteration as "stable candidate". > If still some doubts, not problem for me to resend with "unstable" features. > > hw/core/machine.c | 7 ++ > net/tap.c | 162 ++++++++++++++++++++++++++++++++++++++++++++-- > qapi/net.json | 23 ++++++- > 3 files changed, 185 insertions(+), 7 deletions(-) > > diff --git a/hw/core/machine.c b/hw/core/machine.c > index 15886a56b19..2a1abb29041 100644 > --- a/hw/core/machine.c > +++ b/hw/core/machine.c > @@ -36,10 +36,16 @@ > #include "hw/virtio/virtio-pci.h" > #include "hw/virtio/virtio-net.h" > #include "hw/virtio/virtio-iommu.h" > +#include "net/tap.h" > #include "hw/acpi/generic_event_device.h" > #include "qemu/audio.h" > #include "hw/arm/smmuv3.h" > > +/* > + * TODO: When hw_compat_11_1 appears, local-migration-supported=false > + * for TYPE_TAP_NETDEV should be move to it from hw_compat_11_0. > + */ > + > GlobalProperty hw_compat_11_0[] = { > { "chardev-vc", "encoding", "cp437" }, > { "tpm-crb", "cap-chunk", "off" }, > @@ -49,6 +55,7 @@ GlobalProperty hw_compat_11_0[] = { > { TYPE_ARM_SMMUV3, "ssidsize", "0" }, > { TYPE_ARM_SMMUV3, "oas", "44" }, > { "migration", "switchover-ack-legacy", "on" }, > + { TYPE_TAP_NETDEV, "local-migration-supported", "false" }, > }; > const size_t hw_compat_11_0_len = G_N_ELEMENTS(hw_compat_11_0); > > diff --git a/net/tap.c b/net/tap.c > index 605e5867b01..adcac70b7ea 100644 > --- a/net/tap.c > +++ b/net/tap.c > @@ -38,12 +38,17 @@ > #include "monitor/monitor.h" > #include "system/runstate.h" > #include "system/system.h" > +#include "migration/misc.h" > #include "qapi/error.h" > #include "qemu/cutils.h" > #include "qemu/error-report.h" > #include "qemu/main-loop.h" > #include "qemu/sockets.h" > #include "hw/virtio/vhost.h" > +#include "hw/core/vmstate-if.h" > +#include "migration/vmstate.h" > +#include "qom/object.h" > +#include "qom/compat-properties.h" > > #include "net/tap.h" > #include "net/util.h" > @@ -71,6 +76,8 @@ static const int kernel_feature_bits[] = { > > OBJECT_DECLARE_SIMPLE_TYPE(TAPState, TAP_NETDEV) > > +static const VMStateDescription vmstate_tap; > + > struct TAPState { > Object parent_obj; > > @@ -95,6 +102,7 @@ struct TAPState { > int queue_index; > bool read_poll_detached; > VMChangeStateEntry *vmstate; > + bool local_migration_supported; > }; > > static void launch_script(const char *setup_script, const char *ifname, > @@ -405,6 +413,8 @@ static void tap_cleanup(NetClientState *nc) > tap_write_poll(s, false); > close(s->fd); > s->fd = -1; > + > + vmstate_unregister(VMSTATE_IF(s), &vmstate_tap, s); > } > > static void tap_poll(NetClientState *nc, bool enable) > @@ -441,6 +451,73 @@ static VHostNetState *tap_get_vhost_net(NetClientState *nc) > return s->vhost_net; > } > > +static bool tap_is_wait_incoming(NetClientState *nc) > +{ > + TAPState *s = container_of(nc, TAPState, nc); > + assert(nc->info->type == NET_CLIENT_DRIVER_TAP); > + return s->fd == -1; > +} > + > +static int tap_pre_load(void *opaque) > +{ > + TAPState *s = opaque; > + > + if (s->fd != -1) { > + error_report( > + "TAP is already initialized and cannot receive incoming fd"); > + return -EINVAL; > + } > + > + return 0; > +} > + > +static bool tap_setup_vhost(TAPState *s, Error **errp); > + > +static int tap_post_load(void *opaque, int version_id) > +{ > + TAPState *s = opaque; > + Error *local_err = NULL; > + > + tap_read_poll(s, true); > + > + if (s->fd < 0) { > + return -1; > + } > + > + if (!tap_setup_vhost(s, &local_err)) { > + error_prepend(&local_err, > + "Failed to setup vhost during TAP post-load: "); > + error_report_err(local_err); > + return -1; > + } > + > + return 0; > +} > + > +static bool tap_needed(void *opaque) > +{ > + TAPState *s = opaque; > + > + return s->local_migration_supported && migrate_local(); > +} > + > +static const VMStateDescription vmstate_tap = { > + .name = "net-tap", > + .priority = MIG_PRI_BACKEND, > + .pre_load = tap_pre_load, > + .post_load = tap_post_load, > + .needed = tap_needed, > + .fields = (const VMStateField[]) { > + VMSTATE_FD(fd, TAPState), > + VMSTATE_BOOL(using_vnet_hdr, TAPState), > + VMSTATE_BOOL(has_ufo, TAPState), > + VMSTATE_BOOL(has_uso, TAPState), > + VMSTATE_BOOL(has_tunnel, TAPState), > + VMSTATE_BOOL(enabled, TAPState), > + VMSTATE_UINT32(host_vnet_hdr_len, TAPState), > + VMSTATE_END_OF_LIST() > + } > +}; > > static char *tap_vmstate_if_get_id(VMStateIf *obj) > { > @@ -449,17 +526,42 @@ static char *tap_vmstate_if_get_id(VMStateIf *obj) > return res; > } > > +static bool tap_get_local_migration_supported_prop(Object *obj, Error **errp) > +{ > + TAPState *s = TAP_NETDEV(obj); > + return s->local_migration_supported; > +} > + > +static void tap_set_local_migration_supported_prop(Object *obj, bool value, > + Error **errp) > +{ > + TAPState *s = TAP_NETDEV(obj); > + s->local_migration_supported = value; > +} > + > +static void tap_instance_init(Object *obj) > +{ > + TAPState *s = TAP_NETDEV(obj); > + s->local_migration_supported = true; > +} > + > static void tap_class_init(ObjectClass *klass, const void *data) > { > VMStateIfClass *vc = VMSTATE_IF_CLASS(klass); > > vc->get_id = tap_vmstate_if_get_id; > + > + object_class_property_add_bool(klass, "local-migration-supported", > + tap_get_local_migration_supported_prop, > + tap_set_local_migration_supported_prop); > } > > static const TypeInfo tap_netdev_info = { > .name = TYPE_TAP_NETDEV, > .parent = TYPE_OBJECT, > .instance_size = sizeof(TAPState), > + .instance_init = tap_instance_init, > + .instance_post_init = object_apply_compat_props, > .class_init = tap_class_init, > .interfaces = (const InterfaceInfo[]) { > { TYPE_VMSTATE_IF }, > @@ -492,13 +594,16 @@ static NetClientInfo net_tap_info = { > .set_vnet_le = tap_set_vnet_le, > .set_vnet_be = tap_set_vnet_be, > .set_steering_ebpf = tap_set_steering_ebpf, > + .is_wait_incoming = tap_is_wait_incoming, > .get_vhost_net = tap_get_vhost_net, > }; > > static TAPState *new_tap(NetClientState *peer, > const char *model, > const char *name, > - int queue_index) > + int queue_index, > + bool has_local_migration_supported, > + bool local_migration_supported) > { > TAPState *s = TAP_NETDEV(object_new(TYPE_TAP_NETDEV)); > > @@ -507,6 +612,12 @@ static TAPState *new_tap(NetClientState *peer, > > s->queue_index = queue_index; > > + if (has_local_migration_supported) { > + s->local_migration_supported = local_migration_supported; > + } > + > + vmstate_register(VMSTATE_IF(s), VMSTATE_INSTANCE_ID_ANY, &vmstate_tap, s); > + > return s; > } > > @@ -515,10 +626,14 @@ static TAPState *net_tap_fd_init(NetClientState *peer, > const char *name, > int fd, > int vnet_hdr, > - int queue_index) > + int queue_index, > + bool has_local_migration_supported, > + bool local_migration_supported) > { > NetOffloads ol = {}; > - TAPState *s = new_tap(peer, model, name, queue_index); > + TAPState *s = new_tap(peer, model, name, queue_index, > + has_local_migration_supported, > + local_migration_supported); > > s->fd = fd; > s->host_vnet_hdr_len = vnet_hdr ? sizeof(struct virtio_net_hdr) : 0; > @@ -755,7 +870,7 @@ int net_init_bridge(const Netdev *netdev, const char *name, > close(fd); > return -1; > } > - s = net_tap_fd_init(peer, "bridge", name, fd, vnet_hdr, 0); > + s = net_tap_fd_init(peer, "bridge", name, fd, vnet_hdr, 0, true, false); > > qemu_set_info_str(&s->nc, "helper=%s,br=%s", helper, br); > > @@ -835,7 +950,9 @@ static bool net_init_tap_one(const NetdevTapOptions *tap, NetClientState *peer, > Error **errp) > { > TAPState *s = net_tap_fd_init(peer, tap->helper ? "bridge" : "tap", > - name, fd, vnet_hdr, queue_index); > + name, fd, vnet_hdr, queue_index, > + tap->has_local_migration_supported, > + tap->local_migration_supported); > bool sndbuf_required = tap->has_sndbuf; > int sndbuf = > (tap->has_sndbuf && tap->sndbuf) ? MIN(tap->sndbuf, INT_MAX) : INT_MAX; > @@ -983,6 +1100,7 @@ int net_init_tap(const Netdev *netdev, const char *name, > /* for the no-fd, no-helper case */ > char ifname[128]; > int *fds = NULL, *vhost_fds = NULL; > + bool incoming_fds; > > assert(netdev->type == NET_CLIENT_DRIVER_TAP); > tap = &netdev->u.tap; > @@ -1005,6 +1123,23 @@ int net_init_tap(const Netdev *netdev, const char *name, > return -1; > } > > + incoming_fds = tap->local_migration_supported && migrate_local() && > + runstate_check(RUN_STATE_INMIGRATE); > + > + if (incoming_fds && > + (tap->fd || tap->fds || tap->helper || tap->br || tap->ifname || > + tap->has_sndbuf || tap->has_vnet_hdr || > + !tap_is_explicit_no_script(tap->script) || > + !tap_is_explicit_no_script(tap->downscript))) { > + error_setg(errp, "Local incoming migration of TAP device (-incoming, " > + "migration parameter @local is set, " > + "TAP parameter @local-migration-supported is set) " > + "is incompatible with " > + "fd=, fds=, helper=, br=, ifname=, sndbuf= and vnet_hdr=, " > + "and requires explicit empty script= and downscript="); > + return -1; > + } > + > queues = tap_parse_fds_and_queues(tap, &fds, errp); > if (queues < 0) { > return -1; > @@ -1023,7 +1158,22 @@ int net_init_tap(const Netdev *netdev, const char *name, > goto fail; > } > > - if (fds) { > + if (incoming_fds) { > + for (i = 0; i < queues; i++) { > + TAPState *s = new_tap(peer, "tap", name, i, > + tap->has_local_migration_supported, > + tap->local_migration_supported); > + qemu_set_info_str(&s->nc, "incoming"); > + > + s->fd = -1; > + if (vhost_fds) { > + s->vhostfd = vhost_fds[i]; > + s->vhost_busyloop_timeout = tap->has_poll_us ? tap->poll_us : 0; > + } else { > + s->vhostfd = -1; > + } > + } > + } else if (fds) { > for (i = 0; i < queues; i++) { > if (i == 0) { > vnet_hdr = tap_probe_vnet_hdr(fds[i], errp); > diff --git a/qapi/net.json b/qapi/net.json > index ada0329ef9d..2070979f20e 100644 > --- a/qapi/net.json > +++ b/qapi/net.json > @@ -435,6 +435,26 @@ > # @poll-us: maximum number of microseconds that could be spent on busy > # polling for tap (since 2.7) > # > +# @local-migration-supported: enable local migration for this TAP > +# backend. When set, local migration is enabled/disabled by > +# migration parameter @local for this TAP backend. When unset, > +# migration parameter @local is ignored for this TAP backend. > +# To be able to do incoming local migration of TAP backend, > +# migration parameter @local must be set _before_ creating the > +# TAP backend. Otherwise, TAP backend is initialized as usual, > +# opening/creating TAP devices in kernel. In this case further > +# local incoming migration (with migration parameter @local set > +# after creating TAP backend with @local-migration-supporeted > +# parameter set) will simply fail. > +# Moreover, when QEMU is in incoming migration state, migration > +# parameter @local is set and @local-migration-supported is set, > +# following options are not supported and must not be set: > +# @ds, @fds, @helper, @br, @ifname, @sndbuf, @vnet_hdr. > +# Additionally in this case @script and @downscipt must be > +# explicitly disabled (empty strings or "no"). > +# (Since 11.2. Defaults to true for QEMU Machine Types >= 11.2, > +# and to false for QEMU Machine Types < 11.2) > +# > # Since: 1.2 > ## > { 'struct': 'NetdevTapOptions', > @@ -453,7 +473,8 @@ > '*vhostfds': 'str', > '*vhostforce': 'bool', > '*queues': 'uint32', > - '*poll-us': 'uint32'} } > + '*poll-us': 'uint32', > + '*local-migration-supported': 'bool' } } > > ## > # @NetdevSocketOptions: > -- > 2.43.0