From: Jason Andryuk <jason.andryuk@amd.com>

This patch set  re-introduces blktap support to libxl.  Unlike earlier
versions, it does not link against any blktap library.  libxl changes
are needed to write to the vbd3 backend XenStore nodes.

blktap has three components.  tapdisk is a daemon implementing the disk
IO, NBD (Network Block Device), and Xen PV interfaces.  tap-ctl is a
tool to control tapdisks - creating, starting, stopping and freeing.
tapback manages the XenStore operations and instructs tapdisk to
connect.

It is notable that tapdisk performs the grant and event channel ops, but
doesn't interact with XenStore.  tapback performs XenStore operations
and notifies tapdisks of values and changes.

The flow is: libxl writes to the "vbd3" XenStore nodes and runs the
block-tap script.  The block-tap script runs tap-ctl to create a tapdisk
instance as the physical device.  tapback then sees the tapdisk and
instructs the tapdisk to connect up the PV blkif interface.

This is expected to work without the kernel blktap driver, so the
block-tap script is modified accordingly to write the UNIX NBD path.
(It works with the kernel blktap driver as well - upstream blktap hasn't
removed the blktap driver requirement yet -
https://github.com/xapi-project/blktap/pull/364)

An example command to attach a vhd:
xl block-attach vm 'vdev=xvdf,backendtype=tap,format=vhd,target=/srv/target.vhd'

VHD support is important for OpenXT since there are lots of existing
VHDs which still need supporting.  tapdisk also supports encrypted VHDs
which is not available in QEMU.

blktap's tapback needs minimal changes to work with libxl:
https://github.com/xapi-project/blktap/pull/394

With a stubdom, the block-tap isn't executed a second time meaning
tap-ctl destroy isn't called.  The stubdom's disk xenstore frontend
entry is removed, but tapback does nothing for this case right now.
Making FE removal transition the BE to closed lets the hotplug script
run properly.

v2 addresses review comments and adds a CHANGELOG entry.

Jason Andryuk (3):
  hotplug: Update block-tap
  libxl: Support blktap with HVM device model
  CHANGELOG: Mention libxl blktap/tapback support

 CHANGELOG.md                  |   1 +
 tools/hotplug/Linux/block-tap | 169 +++++++++++++++++++++++++++++++---
 tools/libs/light/libxl_disk.c |  18 +++-
 tools/libs/light/libxl_dm.c   |   1 -
 4 files changed, 172 insertions(+), 17 deletions(-)

-- 
2.39.2