v2:
 * Only enable this feauture for Linux for eventfd is a Linux-specific
   feature

This patch series is an implementation of vhost-user-backend server, thanks to Stefan's guidance after reviewing the draft version and the instructions on https://wiki.qemu.org/Google_Summer_of_Code_2019#vhost-user-blk_device_backend,

Vhost-user-backend server is a UserCreatable object and can be started using object_add,

 (qemu) object_add vhost-user-server,id=ID,unix_socket=/tmp/vhost-user-blk_vhost.socket,name=DRIVE_NAME,writable=off
 (qemu) object_del ID

or appending the "-object" option when starting QEMU,

  $ -object vhost-user-server,id=disk,unix_socket=/tmp/vhost-user-blk_vhost.socket,name=disk,writable=off

Then vhost-user client can connect to the server backend. For example, QEMU could act as a client,
  $ -m 256 -object memory-backend-memfd,id=mem,size=256M,share=on -numa node,memdev=mem -chardev socket,id=char1,path=/tmp/vhost-user-blk_vhost.socket -device vhost-user-blk-pci,id=blk0,chardev=char1

And guest OS could access this vhost-user block device after mouting it.

patches are against commit 035eed4c0d257c905a556fa0f4865a0c077b4e7f.

Coiby Xu (5):
  vhost-user block device backend
  extend libvhost to support IOThread
  a standone-alone tool to directly share disk image file via vhost-user
    protocol
  new qTest case for the vhost-user-blk device backend
  building configuration files changes

 Makefile                              |    1 +
 Makefile.objs                         |    2 +-
 Makefile.target                       |    1 +
 blockdev-vu.c                         | 1008 +++++++++++++++++++++++++
 configure                             |    2 +-
 contrib/libvhost-user/libvhost-user.c |   64 +-
 contrib/libvhost-user/libvhost-user.h |   36 +-
 include/block/vhost-user.h            |   46 ++
 qemu-vu.c                             |  264 +++++++
 tests/Makefile.include                |    5 +-
 tests/libqos/vhost-user-blk.c         |  125 +++
 tests/libqos/vhost-user-blk.h         |   44 ++
 tests/vhost-user-blk-test.c           |  691 +++++++++++++++++
 vl.c                                  |    4 +
 14 files changed, 2277 insertions(+), 16 deletions(-)
 create mode 100644 blockdev-vu.c
 create mode 100644 include/block/vhost-user.h
 create mode 100644 qemu-vu.c
 create mode 100644 tests/libqos/vhost-user-blk.c
 create mode 100644 tests/libqos/vhost-user-blk.h
 create mode 100644 tests/vhost-user-blk-test.c

--
2.24.1