[virtio-dev] Re: [RFC PATCH] docs/interop: define STANDALONE protocol feature for vhost-user

["Alex Bennée" <alex.bennee@linaro.org>]

Alex Bennée <alex.bennee@linaro.org> writes:

> Currently QEMU has to know some details about the back-end to be able
> to setup the guest. While various parts of the setup can be delegated
> to the backend (for example config handling) this is a very piecemeal
> approach.
>
> This patch suggests a new feature flag (VHOST_USER_PROTOCOL_F_STANDALONE)
> which the back-end can advertise which allows a probe message to be
> sent to get all the details QEMU needs to know in one message.
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>
> ---
> Initial RFC for discussion. I intend to prototype this work with QEMU
> and one of the rust-vmm vhost-user daemons.
> ---
>  docs/interop/vhost-user.rst | 37 +++++++++++++++++++++++++++++++++++++
>  hw/virtio/vhost-user.c      |  8 ++++++++
>  2 files changed, 45 insertions(+)
>
> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> index 5a070adbc1..85b1b1583a 100644
> --- a/docs/interop/vhost-user.rst
> +++ b/docs/interop/vhost-user.rst
> @@ -275,6 +275,21 @@ Inflight description
>  
>  :queue size: a 16-bit size of virtqueues
>  
> +Backend specifications
> +^^^^^^^^^^^^^^^^^^^^^^
> +
> ++-----------+-------------+------------+------------+
> +| device id | config size |   min_vqs  |   max_vqs  |
> ++-----------+-------------+------------+------------+
> +
> +:device id: a 32-bit value holding the VirtIO device ID
> +
> +:config size: a 32-bit value holding the config size (see ``VHOST_USER_GET_CONFIG``)
> +
> +:min_vqs: a 32-bit value holding the minimum number of vqs supported
> +
> +:max_vqs: a 32-bit value holding the maximum number of vqs supported, must be >= min_vqs
> +
>  C structure
>  -----------
>  
> @@ -296,6 +311,7 @@ In QEMU the vhost-user message is implemented with the following struct:
>            VhostUserConfig config;
>            VhostUserVringArea area;
>            VhostUserInflight inflight;
> +          VhostUserBackendSpecs specs;
>        };
>    } QEMU_PACKED VhostUserMsg;
>  
> @@ -316,6 +332,7 @@ replies. Here is a list of the ones that do:
>  * ``VHOST_USER_GET_VRING_BASE``
>  * ``VHOST_USER_SET_LOG_BASE`` (if ``VHOST_USER_PROTOCOL_F_LOG_SHMFD``)
>  * ``VHOST_USER_GET_INFLIGHT_FD`` (if ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD``)
> +* ``VHOST_USER_GET_BACKEND_SPECS`` (if ``VHOST_USER_PROTOCOL_F_STANDALONE``)
>  
>  .. seealso::
>  
> @@ -885,6 +902,13 @@ Protocol features
>    #define VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS  15
>    #define VHOST_USER_PROTOCOL_F_STATUS               16
>    #define VHOST_USER_PROTOCOL_F_XEN_MMAP             17
> +  #define VHOST_USER_PROTOCOL_F_STANDALONE           18
> +
> +Some features are only valid in the presence of other supporting
> +features. In the case of ``VHOST_USER_PROTOCOL_F_STANDALONE`` the
> +backend must also support ``VHOST_USER_PROTOCOL_F_CONFIG`` and
> +``VHOST_USER_PROTOCOL_F_STATUS``.
> +

This is too tight a restriction as not all VirtIO backends manage a
config space. So I suggest the following:

  Some features are only valid in the presence of other supporting
  features. In the case of ``VHOST_USER_PROTOCOL_F_STANDALONE`` the
  backend must also support ``VHOST_USER_PROTOCOL_F_STATUS`` and
  optionally ``VHOST_USER_PROTOCOL_F_CONFIG`` (if there is a config space).

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro

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