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

[Stefano Garzarella <sgarzare@redhat.com>]

On Tue, Jul 04, 2023 at 01:36:00PM +0100, Alex Bennée wrote:
> 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.

Thanks for starting this discussion!

I'm comparing with vhost-vdpa IOCTLs, so my questions may be
superficial, but they help me understand the differences.

> ---
> 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

Why do we need the minimum?

> +
> +:max_vqs: a 32-bit value holding the maximum number of vqs supported, must be >= min_vqs

Is this overlap with VHOST_USER_GET_QUEUE_NUM?

> +
> 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``.
> +

What about adding a new section where we will describe what we mean
with "standalone" devices?

For example that the entire virtio device is emulated in the backend,
etc.

By the way, I was thinking more about F_FULL_DEVICE, but I'm not good
with names, so I'll just throw out an idea :-)

Thanks,
Stefano

> Front-end message types
> -----------------------
> @@ -1440,6 +1464,19 @@ Front-end message types
>   query the back-end for its device status as defined in the Virtio
>   specification.
>
> +``VHOST_USER_GET_BACKEND_SPECS``
> +  :id: 41
> +  :request payload: N/A
> +  :reply payload: ``Backend specifications``
> +
> +  When the ``VHOST_USER_PROTOCOL_F_STANDALONE`` protocol feature has been
> +  successfully negotiated, this message is submitted by the front-end to
> +  query the back-end for its capabilities. This is intended to remove
> +  the need for the front-end to know ahead of time what the VirtIO
> +  device the backend emulates is.
> +
> +  The reply contains the device id, size of the config space and the
> +  range of VirtQueues the backend supports.
>
> Back-end message types
> ----------------------
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index c4e0cbd702..28b021d5d3 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -202,6 +202,13 @@ typedef struct VhostUserInflight {
>     uint16_t queue_size;
> } VhostUserInflight;
>
> +typedef struct VhostUserBackendSpecs {
> +    uint32_t device_id;
> +    uint32_t config_size;
> +    uint32_t min_vqs;
> +    uint32_t max_vqs;
> +} VhostUserBackendSpecs;
> +
> typedef struct {
>     VhostUserRequest request;
>
> @@ -226,6 +233,7 @@ typedef union {
>         VhostUserCryptoSession session;
>         VhostUserVringArea area;
>         VhostUserInflight inflight;
> +        VhostUserBackendSpecs specs;
> } VhostUserPayload;
>
> typedef struct VhostUserMsg {
> --
> 2.39.2
>
>
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