From: Kevin Wolf <kw...@redhat.com> 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 <kw...@redhat.com> Message-Id: <20220407133657.155281-2-kw...@redhat.com> Reviewed-by: Raphael Norwitz <raphael.norw...@nutanix.com> Reviewed-by: Michael S. Tsirkin <m...@redhat.com> Signed-off-by: Michael S. Tsirkin <m...@redhat.com> Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com> --- docs/interop/vhost-user.rst | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) 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 -- MST
