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: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
index 3c12607517..a99ba4433c 100644
--- a/docs/interop/vhost-user.rst
+++ b/docs/interop/vhost-user.rst
@@ -1376,6 +1376,14 @@ Front-end message types
   For further details on postcopy, see ``VHOST_USER_SET_MEM_TABLE``.
   They apply to ``VHOST_USER_ADD_MEM_REG`` accordingly.
 
+  Exactly one file descriptor from which the memory is mapped is
+  passed in the ancillary data.
+
+  In postcopy mode (see ``VHOST_USER_POSTCOPY_LISTEN``), the back-end
+  replies with the bases of the memory mapped region to the front-end.
+  For further details on postcopy, see ``VHOST_USER_SET_MEM_TABLE``.
+  They apply to ``VHOST_USER_ADD_MEM_REG`` accordingly.
+
 ``VHOST_USER_REM_MEM_REG``
   :id: 38
   :equivalent ioctl: N/A
@@ -1400,6 +1408,14 @@ Front-end message types
   accept messages with one file descriptor. If a file descriptor is
   passed, the back-end MUST close it without using it otherwise.
 
+  The memory region to be removed is identified by its guest address,
+  user address and size. The mmap offset is ignored.
+
+  No file descriptors SHOULD be passed in the ancillary data. For
+  compatibility with existing incorrect implementations, the back-end MAY
+  accept messages with one file descriptor. If a file descriptor is
+  passed, the back-end MUST close it without using it otherwise.
+
 ``VHOST_USER_SET_STATUS``
   :id: 39
   :equivalent ioctl: VHOST_VDPA_SET_STATUS