While many iSCSI implementations are inside the kernel the one implemented in OpenBSD is written in userland similar to the NetBSD initiator. But unlike NetBSD's, OpenBSD's version does not use a userland filesystem to hook the iSCSI disks into the kernel. Instead access is performed via vscsi(4) -- a virtual scsibus -- which allows userland to talk directly to the kernel SCSI midlayer. Vscsi is implemented in roughly 600 lines of code exporting the scsibus to userland via 6 ioctl(2) plus the possibility to use poll(2) or kqueue(2) to allow simple I/O multiplexing. The vscsi device uses these ioctls to issue commands and send or receive data from the kernel. The use of this interface it is fairly simple and makes it possible to write an iSCSI initiator in userland. The userland process can offload all the complex SCSI protocol handling to vscsi and the kernel and "only" needs to implement the session establishment and error recovery.
