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Notes on blktap2 and tapdisk
# Some notes on how how tapdevs and associated devices are created. It begins here:libxl:libxl__blktap_devpath calls libblktapctl.so:tap_ctl_create directly.xenops/device.ml:mount calls it indirectly by invoking tap-ctl with the create option (which calls tap-ctl:tap_cli_create) # This function in libblktapctl.so does 4 important things that are listed belowtap_cli_create: tap_ctl_allocate tap_ctl_spawn tap_ctl_attach tap_ctl_open tap_ctl_allocate: # Open the blktap driver # Allocate a ring device and an IO device using the BLKTAP2_IOCTL_ALLOC_TAP ioctl (see below). # tap_ctl_make_device then makes the ring device node /dev/xen/blktap-2/blktapX and an IO device node /devxen/blktap-2/tapdevX tap_ctl_spawn: # Start a new tapdisk2 process that will be associated with the X tap devices. # tapdisk2 creates the listener socket for the new process. # tapdisk2 registers the tapdisk_control_handle_request tap_ctl_attach: # Attach to the listening tapdisk2 process just started so now messages can be passed between libblktapctl.so and that process. tap_ctl_open: # Send the TAPDISK_MESSAGE_OPEN to the new tapdisk2 process. # In the new tapdisk2 process associated with the X devices allocated above.tapdisk_control_handle_request: # Get message TAPDISK_MESSAGE_OPEN and call: tapdisk_control_open_image: # Send the BLKTAP2_IOCTL_CREATE_DEVICE ioctl to blkdev (see below).# Down in the blktap driver, the above activity translates to:blktap_control_ioctl: # BLKTAP2_IOCTL_ALLOC_TAP maps to BLKTAP_IOCTL_ALLOC_TAP # Call blktap_control_create_tap to create the tap ring and IO devices. blktap_ring_ioctl: # BLKTAP2_IOCTL_CREATE_DEVICE maps to BLKTAP_IOCTL_CREATE_DEVICE_COMPAT. Calls: blktap_device_create: # Sets up the /dev/tdx node name (note little x corresponds to a, b, c...) # Creates and adds the actual block device with add_disk() (see include/linux/genhd.h for generic block disk devices). # The rest happens in the standard udev way. NOTE: when destroying a tapdev with tap-ctl the process id is the tapdisk2 process that own the tap devs. The minor number is what is called X above. |