Re: [virtio-dev] [RFC PATCH v2] docs/interop: define PROBE feature for vhost-user VirtIO devices

[Stefan Hajnoczi]

On Tue, Sep 05, 2023 at 10:34:11AM +0100, Alex Bennée 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

I don't understand why the front-end needs to know that? The backend
already reports the number of queues and not all of them need to be
initialized by the driver.

> 
> > 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).

I'm confused. VHOST_USER_GET_MIN_VQ comes before VIRTIO Feature Bit
negotiation (VIRTIO_NET_F_CTRL_VQ).

> However VHOST_USER_GET_QUEUE_NUM is only usable if
> VIRTIO_NET_F_MQ has been negotiated and could report more.

I don't understand. This patch adds a new feature and it can require
VHOST_USER_PROTOCOL_F_MQ. There are no existing back-ends that require
backwards compatibility.

Stefan

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