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

[Alex Bennée]

Stefan Hajnoczi <stefanha@redhat.com> writes:

> 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.

But how many don't need to be initialised? We can't just skip:

    /* Allocate queues */
    vub->vqs = g_ptr_array_sized_new(vub->num_vqs);
    for (int i = 0; i < vub->num_vqs; i++) {
        g_ptr_array_add(vub->vqs,
                        virtio_add_queue(vdev, vub->vq_size, 
vub_handle_output));
    }

Or are you saying just require probe-able backends to support
VHOST_USER_PROTOCOL_F_MQ and have it always report the minimmum number
of queues if it is not a MQ capable device?

>> > 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


-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro