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

[Alex Bennée]

Stefan Hajnoczi <stefanha@gmail.com> writes:

> On Fri, 8 Sept 2023 at 02:43, Alex Bennée <alex.bennee@linaro.org> wrote:
>>
>>
>> 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?
>
> The front-end should prepare to allow the maximum number of virtqueues
> returned by VHOST_USER_GET_QUEUE_NUM (VHOST_USER_PROTOCOL_F_MQ).
>
> VIRTIO Transports have a way to query the maximum number of queues but
> not a way to query the minimum number of queues. Why is the minimum
> necessary?

It seems excessive to automatically create the maximum number of VQs. I
guess for backends that don't support the MQ feature (i.e. a variable
number of VQs) we could just say VHOST_USER_GET_QUEUE_NUM == min. But
now we are overloading a different message originally added for
something else.

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro