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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>
This commit is contained in:
Kevin Wolf 2022-04-07 15:36:55 +02:00 committed by Michael S. Tsirkin
parent 37cbfcebdd
commit 76b1b64370
1 changed files with 16 additions and 0 deletions
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16
docs/interop/vhost-user.rst
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@ -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