Add a new Linux-specific memory backend, similar to hostmem-file,
except that it doesn't need file path. It also try to enforce memory
sealing if available. It is thus slightly easier and secure, and is
compatible with transparent huge-pages since Linux 4.8.

Since Linux 4.14, memfd supports explicit hugetlb, however without
sealing.

v5:
- add memfd hugetlb support
- add seal and hugeltb options to memfd memory backend
- update qemu-option to explain sharing works with vhost-user for now
- rebased, misc code changes

v4:
- rebased, now that preliminary patches are merged

v3:
- make vhost-user-test use memfd only if possible
- rebased

v1->v2:
- make it linux-specific
- minor changes and commit message tweaks

Marc-André Lureau (6):
  memfd: split qemu_memfd_alloc()
  memfd: remove needless include
  memfd: add error argument, instead of perror()
  memfd: add hugetlb support
  Add memfd based hostmem
  tests: use memfd in vhost-user-test

 include/qemu/memfd.h     |   4 +-
 backends/hostmem-memfd.c | 124 +++++++++++++++++++++++++++++++++++++++++++++++
 hw/virtio/vhost.c        |   8 ++-
 tests/vhost-user-test.c  |  70 ++++++++++++++++----------
 util/memfd.c             | 101 ++++++++++++++++++++++----------------
 backends/Makefile.objs   |   2 +
 qemu-options.hx          |  15 ++++++
 7 files changed, 256 insertions(+), 68 deletions(-)
 create mode 100644 backends/hostmem-memfd.c

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2.15.0.rc0.40.gaefcc5f6f