Re: [PATCH 1/3] docs/vhost-user: Clarifications for VHOST_USER_ADD/REM_MEM_REG

[Raphael Norwitz]

On Thu, Apr 07, 2022 at 03:36:55PM +0200, Kevin Wolf wrote:
> 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>

Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>

> ---
>  docs/interop/vhost-user.rst | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> index 4dbc84fd00..f9e721ba5f 100644
> --- a/docs/interop/vhost-user.rst
> +++ b/docs/interop/vhost-user.rst
> @@ -308,6 +308,7 @@ replies. Here is a list of the ones that do:
>  There are several messages that the master sends with file descriptors passed
>  in the ancillary data:
>  
> +* ``VHOST_USER_ADD_MEM_REG``
>  * ``VHOST_USER_SET_MEM_TABLE``
>  * ``VHOST_USER_SET_LOG_BASE`` (if ``VHOST_USER_PROTOCOL_F_LOG_SHMFD``)
>  * ``VHOST_USER_SET_LOG_FD``
> @@ -1334,6 +1335,14 @@ Master message types
>    ``VHOST_USER_REM_MEM_REG`` message, this message is used to set and
>    update the memory tables of the slave device.
>  
> +  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 slave
> +  replies with the bases of the memory mapped region to the master.
> +  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
> @@ -1349,6 +1358,14 @@ Master message types
>    ``VHOST_USER_ADD_MEM_REG`` message, this message is used to set and
>    update the memory tables of the slave device.
>  
> +  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 slave MAY
> +  accept messages with one file descriptor. If a file descriptor is
> +  passed, the slave MUST close it without using it otherwise.
> +
>  ``VHOST_USER_SET_STATUS``
>    :id: 39
>    :equivalent ioctl: VHOST_VDPA_SET_STATUS
> -- 
> 2.35.1
>