docs/vhost-user: Clarifications for VHOST_USER_ADD/REM_MEM_REG

docs/vhost-user: Clarifications for VHOST_USER_ADD/REM_MEM_REG

The specification for VHOST_USER_ADD/REM_MEM_REG messages is unclear
in several points, which has led to clients having incompatible
implementations. This changes the specification to be more explicit
about them:

* VHOST_USER_ADD_MEM_REG is not specified as receiving a file
  descriptor, though it obviously does need to do so. All
  implementations agree on this one, fix the specification.

* VHOST_USER_REM_MEM_REG is not specified as receiving a file
  descriptor either, and it also has no reason to do so. rust-vmm does
  not send file descriptors for removing a memory region (in agreement
  with the specification), libvhost-user and QEMU do (which is a bug),
  though libvhost-user doesn't actually make any use of it.

  Change the specification so that for compatibility QEMU's behaviour
  becomes legal, even if discouraged, but rust-vmm's behaviour becomes
  the explicitly recommended mode of operation.

* VHOST_USER_ADD_MEM_REG doesn't have a documented return value, which
  is the desired behaviour in the non-postcopy case. It also implemented
  like this in QEMU and rust-vmm, though libvhost-user is buggy and
  sometimes sends an unexpected reply. This will be fixed in a separate
  patch.

  However, in postcopy mode it does reply like VHOST_USER_SET_MEM_TABLE.
  This behaviour is shared between libvhost-user and QEMU; rust-vmm
  doesn't implement postcopy mode yet. Mention it explicitly in the
  spec.

* The specification doesn't mention how VHOST_USER_REM_MEM_REG
  identifies the memory region to be removed. Change it to describe the
  existing behaviour of libvhost-user (guest address, user address and
  size must match).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20220407133657.155281-2-kwolf@redhat.com>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>