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Re: [virtio-dev] [RFC PATCH v2] docs/interop: define PROBE feature for v


From: Albert Esteve
Subject: Re: [virtio-dev] [RFC PATCH v2] docs/interop: define PROBE feature for vhost-user VirtIO devices
Date: Tue, 5 Sep 2023 12:02:02 +0200



On Tue, Sep 5, 2023 at 11:43 AM Alex Bennée <alex.bennee@linaro.org> wrote:

Albert Esteve <aesteve@redhat.com> writes:

> This looks great! Thanks for this proposal.
>
> On Fri, Sep 1, 2023 at 1:00 PM Alex Bennée <alex.bennee@linaro.org> wrote:
>
>  Currently QEMU has to know some details about the VirtIO device
>  supported by a vhost-user daemon to be able to setup the guest. This
>  makes it hard for QEMU to add support for additional vhost-user
>  daemons without adding specific stubs for each additional VirtIO
>  device.
>
>  This patch suggests a new feature flag (VHOST_USER_PROTOCOL_F_PROBE)
>  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.
>
>  Together with the existing features VHOST_USER_PROTOCOL_F_STATUS and
>  VHOST_USER_PROTOCOL_F_CONFIG we can create "standalone" vhost-user
>  daemons which are capable of handling all aspects of the VirtIO
>  transactions with only a generic stub on the QEMU side. These daemons
>  can also be used without QEMU in situations where there isn't a full
>  VMM managing their setup.
>
>  Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>
>  ---
>  v2
>    - dropped F_STANDALONE in favour of F_PROBE
>    - split probe details across several messages
>    - probe messages don't automatically imply a standalone daemon
>    - add wording where probe details interact (F_MQ/F_CONFIG)
>    - define VMM and make clear QEMU is only one of many potential VMMs
>    - reword commit message
>  ---
>   docs/interop/vhost-user.rst | 90 ++++++++++++++++++++++++++++++++-----
>   hw/virtio/vhost-user.c      |  8 ++++
>   2 files changed, 88 insertions(+), 10 deletions(-)
>
>  diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
>  index 5a070adbc1..ba3b5e07b7 100644
>  --- a/docs/interop/vhost-user.rst
>  +++ b/docs/interop/vhost-user.rst
>  @@ -7,6 +7,7 @@ Vhost-user Protocol
>   ..
>     Copyright 2014 Virtual Open Systems Sarl.
>     Copyright 2019 Intel Corporation
>  +  Copyright 2023 Linaro Ltd
>     Licence: This work is licensed under the terms of the GNU GPL,
>              version 2 or later. See the COPYING file in the top-level
>              directory.
>  @@ -27,17 +28,31 @@ The protocol defines 2 sides of the communication, *front-end* and
>   *back-end*. The *front-end* is the application that shares its virtqueues, in
>   our case QEMU. The *back-end* is the consumer of the virtqueues.
>
>  -In the current implementation QEMU is the *front-end*, and the *back-end*
>  -is the external process consuming the virtio queues, for example a
>  -software Ethernet switch running in user space, such as Snabbswitch,
>  -or a block device back-end processing read & write to a virtual
>  -disk. In order to facilitate interoperability between various back-end
>  -implementations, it is recommended to follow the :ref:`Backend program
>  -conventions <backend_conventions>`.
>  +In the current implementation a Virtual Machine Manager (VMM) such as
>  +QEMU is the *front-end*, and the *back-end* is the external process
>  +consuming the virtio queues, for example a software Ethernet switch
>  +running in user space, such as Snabbswitch, or a block device back-end
>  +processing read & write to a virtual disk. In order to facilitate
>  +interoperability between various back-end implementations, it is
>  +recommended to follow the :ref:`Backend program conventions
>  +<backend_conventions>`.
>
>   The *front-end* and *back-end* can be either a client (i.e. connecting) or
>   server (listening) in the socket communication.
>
>  +Probing device details
>  +----------------------
>  +
>  +Traditionally the vhost-user daemon *back-end* shares configuration
>  +responsibilities with the VMM *front-end* which needs to know certain
>  +key bits of information about the device. This means the VMM needs to
>  +define at least a minimal stub for each VirtIO device it wants to
>  +support. If the daemon supports the right set of protocol features the
>  +VMM can probe the daemon for the information it needs to setup the
>  +device. See :ref:`Probing features for standalone daemons
>  +<probing_features>` for more details.
>  +
>  +
>   Support for platforms other than Linux
>   --------------------------------------
>
>  @@ -316,6 +331,7 @@ 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::
>
>  @@ -396,9 +412,10 @@ must support changing some configuration aspects on the fly.
>   Multiple queue support
>   ----------------------
>
>  -Many devices have a fixed number of virtqueues.  In this case the front-end
>  -already knows the number of available virtqueues without communicating with the
>  -back-end.
>  +Many devices have a fixed number of virtqueues. In this case the
>  +*front-end* usually already knows the number of available virtqueues
>  +without communicating with the back-end. For standalone daemons this
>  +number can be can be probed with the ``VHOST_USER_GET_MIN_VQ`` message.
>
>   Some devices do not have a fixed number of virtqueues.  Instead the maximum
>   number of virtqueues is chosen by the back-end.  The number can depend on host
>  @@ -885,6 +902,23 @@ 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_PROBE                18
>  +
>  +.. _probing_features:
>  +
>  +Probing features for standalone daemons
>  +---------------------------------------
>  +
>  +The protocol feature ``VHOST_USER_PROTOCOL_F_PROBE`` enables a number
>  +of additional messages which allow the *front-end* to probe details
>  +about the VirtIO device from the *back-end*. However for a *back-end*
>  +to be described as standalone it must also support:
>  +
>  +  * ``VHOST_USER_PROTOCOL_F_STATUS``
>  +  * ``VHOST_USER_PROTOCOL_F_CONFIG`` (if there is a config space)
>  +
>  +which are required to ensure the *back-end* daemon can operate
>  +without the *front-end* managing some aspects of its configuration.
>
>   Front-end message types
>   -----------------------
>  @@ -1440,6 +1474,42 @@ Front-end message types
>     query the back-end for its device status as defined in the Virtio
>     specification.
>
>  +``VHOST_USER_GET_DEVICE_ID``
>  +  :id: 41
>  +  :request payload: N/A
>  +  :reply payload: ``u32``
>  +
>  +  When the ``VHOST_USER_PROTOCOL_F_PROBE`` protocol feature has been
>  +  successfully negotiated, this message is submitted by the front-end
>  +  to query what VirtIO device the back-end support. This is intended
>  +  to remove the need for the front-end to know ahead of time what the
>  +  VirtIO device the backend emulates is.
>  +
>  +``VHOST_USER_GET_CONFIG_SIZE``
>  +  :id: 42
>  +  :request payload: N/A
>  +  :reply payload: ``u32``
>  +
>  +  When the ``VHOST_USER_PROTOCOL_F_PROBE`` protocol feature has been
>  +  successfully negotiated, this message is submitted by the front-end
>  +  to query the size of the VirtIO device's config space. This is
>  +  intended to remove the need for the front-end to know ahead of time
>  +  what the size is. Replying with 0 when
>  +  ``VHOST_USER_PROTOCOL_F_CONFIG`` has been negotiated would indicate
>  +  an bug.
>  +
>  +``VHOST_USER_GET_MIN_VQ``
>  +  :id: 43
>  +  :request payload: N/A
>  +  :reply payload: ``u32``
>  +
>  +  When the ``VHOST_USER_PROTOCOL_F_PROBE`` protocol feature has been
>  +  successfully negotiated, this message is submitted by the front-end to
>  +  query minimum number of VQ's required to support the device. A
>  +  device may support more than this number of VQ's if it advertises
>  +  the ``VHOST_USER_PROTOCOL_F_MQ`` protocol feature. Reporting a
>  +  number greater than the result of ``VHOST_USER_GET_QUEUE_NUM`` would
>  +  indicate a bug.
>
> Maybe I lack some background, but not sure what min_vq is here?

There will be a minimum number of queues you need to support the device.
For example the virtio-sound spec specifies you need four queues:
control, event, tx, rx

> This looks like quering the number of VQs the backend requires/uses.
> Which, in case of MQ, it may be bigger (which is where I assume comes the `min`
> part, if we consider `VHOST_USER_GET_QUEUE_NUM` the `max`).

The MQ extension is currently used by networking but in theory any
device could attempt to parallelism by extending the number of virt
queues needed. So for net you get:

  receiveq1
  transmitq1
  optional controlq

So VHOST_USER_GET_MIN_VQ would report 2 or 3 (if VIRTIO_NET_F_CTRL_VQ is
negotiated). However VHOST_USER_GET_QUEUE_NUM is only usable if
VIRTIO_NET_F_MQ has been negotiated and could report more.


Ah I see, I understand it better now. It is a pity that we cannot multipurpose
the GET_QUEUE_NUM request.

FWIW:
Acked-by: Albert Esteve <aesteve@redhat.com>
 
>
> Couldn't we reuse the `VHOST_USER_GET_QUEUE_NUM` type for this?

>   Back-end message types
>   ----------------------
>  diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
>  index 8dcf049d42..4d433cdf2b 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;

Oops these snuck in, I shall clean them up

>   } VhostUserPayload;
>
>   typedef struct VhostUserMsg {
>  --
>  2.39.2
>
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--
Alex Bennée
Virtualisation Tech Lead @ Linaro


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