On Mon, Jul 13, 2026 at 08:55:37PM +0300, Vladimir Sementsov-Ogievskiy wrote: > On 13.07.26 14:21, Daniel P. Berrangé wrote: > > On Mon, Jul 13, 2026 at 01:55:48PM +0300, Vladimir Sementsov-Ogievskiy > > wrote: > > > On 13.07.26 12:51, Daniel P. Berrangé wrote: > > > > On Fri, Jul 10, 2026 at 10:17:58PM +0300, Vladimir Sementsov-Ogievskiy > > > > wrote: > > > > > We are going to implement local-migration feature: some devices will > > > > > be > > > > > able to transfer open file descriptors through migration stream (which > > > > > must UNIX domain socket for that purpose). This allows to transfer > > > > > the > > > > > whole backend state without reconnecting and restarting the backend > > > > > service. For example, virtio-net will migrate its attached TAP netdev, > > > > > together with its connected file descriptors. > > > > > > > > > > In this commit we introduce a migration parameter, which enables > > > > > the feature for devices that support it (none at the moment). > > > > > > > > > > Signed-off-by: Vladimir Sementsov-Ogievskiy > > > > > <[email protected]> > > > > > Acked-by: Markus Armbruster <[email protected]> > > > > > Acked-by: Peter Xu <[email protected]> > > > > > Reviewed-by: Ben Chaney <[email protected]> > > > > > --- > > > > > include/migration/misc.h | 2 ++ > > > > > migration/options.c | 18 +++++++++++++++++- > > > > > qapi/migration.json | 12 ++++++++++-- > > > > > 3 files changed, 29 insertions(+), 3 deletions(-) > > > > > > > > > > > > > diff --git a/qapi/migration.json b/qapi/migration.json > > > > > index 66bd900465a..03bdf19f3a6 100644 > > > > > --- a/qapi/migration.json > > > > > +++ b/qapi/migration.json > > > > > @@ -830,7 +830,8 @@ > > > > > 'zero-page-detection', > > > > > 'direct-io', > > > > > { 'name': 'x-rdma-chunk-size', 'features': [ 'unstable' > > > > > ] }, > > > > > - 'cpr-exec-command'] } > > > > > + 'cpr-exec-command', > > > > > + 'local'] } > > > > > ## > > > > > # @migrate-set-parameters: > > > > > @@ -1012,6 +1013,12 @@ > > > > > # Must be set to the same value on both source and destination > > > > > # before migration starts. (Since 11.1) > > > > > # > > > > > +# @local: Enable local migration for devices that support it. > > > > > Backend > > > > > +# state and its file descriptors can then be passed to the > > > > > +# destination in the migration channel. The migration channel > > > > > +# must be a Unix domain socket. Usually needs to be enabled per > > > > > +# device. (Since 11.2) > > > > > > > > What happens when this is enabled and there are devices which > > > > don't support it ? > > > > > > These devices are migrated as usual, ignoring "local" option. > > > > > > > > > > > What happens when this is enabled and a device on the src > > > > does not support it but a device on the (newer) QEMU dest > > > > does support it ? How does the mgmt app know which devices > > > > support it ? > > > > > > The only supporting device (TAP-backend) declares support by > > > "local-migration-supported", so mgmt app should care to set > > > same value for it on host and target (like for other options > > > actually). Or simply relay on default, which is true starting > > > from 11.2 Machine Type. > > > > > > > > > > > Can we document the semantics in these cases. > > > > > > That's: "Usually needs to be enabled per device." > > > > ....except that we have it enabled by default for TAP backends > > in new machine types.... except that it will then break if the > > app/user has enabled any of the "forbidden" options. > > > > The way this doc is written makes it sound like "local" is an > > optimization you can turn on in any scenario, and it will do > > the right thing if the device supports it. That is very much > > not the case - enabling this flag will definitely break many > > configs out of the box, with new machine types due to the TAP > > device defaults & incompatible options. > > Ok, now we decided to NOT make default support for TAP in newer > machine types. > > So, enabling "local" is safe in existing scenarios, only explicit > enabling TAP.local-migration-supported will change the behavior. > > Keep "local" documentation as is? > > I see the only way to make it more informative: directly describe > connection with TAP.local-migration-supported, without general > phrases about "devices that support" and "usual". We'll have to > update it when more devices added (my next step is vhost-user-blk), > but at least, it would be absolutely correct at the moment.
How about.... # @local: Permit the use of optimizations for local migration. # This must only be set when both the source and destination # QEMU processes are on the same OS and directly connected # with a UNIX domain socket as the migration channel to enable # use of file descriptor passing. Individual device backends # may need additional configuration flags set to enable local # migration optimizations. This will be documented against the # device backends where it applies. (Since 11.2) With regards, Daniel -- |: https://berrange.com ~~ https://hachyderm.io/@berrange :| |: https://libvirt.org ~~ https://entangle-photo.org :| |: https://pixelfed.art/berrange ~~ https://fstop138.berrange.com :|
