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Migration of back-end state happens while the device is suspended (i.e all vrings are stopped). To resume normal operation on the destination, the vrings need to be started again with a kick (either a write on the FD or the VHOST_USER_VRING_KICK in-band message if negotiated). While these notifications are typically sent by the driver, it has no reason to send them in the destination if it already sent them in the source as the driver is unaware that a migration took place. Therefore it should be the responsibility of the vhost-user front-end to ensure these vrings are started. This is particularly necessary for queues where data only flows from device to driver, such as those used by the vsock and input devices. This behavior is already used by some qemu vhost-user front-ends (e.g vhost-user-blk) and by front-ends implemented on other VMMs(e.g CrosVm). Adding it to the vhost-user documentation makes it explicit that this strategy is permitted and suggest it to vhost-user front-end authors. Explicitly documenting it is necessary because vring kicks appear designed to originate in the driver, so having some originate in the front-end can be counterintuitive and cause developers to waste time looking for other alternatives or face pushback during code review. Signed-off-by: Jorge E. Moreira <jemoreira@google.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Message-Id: <20260411021205.3592118-1-jemoreira@google.com>