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| author | Kevin Wolf <kwolf@redhat.com> | 2022-04-07 15:36:55 +0200 |
|---|---|---|
| committer | Michael S. Tsirkin <mst@redhat.com> | 2022-05-16 16:15:40 -0400 |
| commit | 76b1b64370007234279ea4cc8b09c98cbd2523de (patch) | |
| tree | 0a3747e285d45e91ff703cf716f9f58df8b2baaf /docs/interop | |
| parent | 37cbfcebdd0e7ffbfb8332a8794a91a4a8f1819d (diff) | |
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>
Diffstat (limited to 'docs/interop')
| -rw-r--r-- | docs/interop/vhost-user.rst | 16 |
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 |