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Subject: Re: [virtio-dev] [RFC PATCH v2] docs/interop: define PROBE feature for vhost-user VirtIO devices
Stefan Hajnoczi <stefanha@gmail.com> writes:
> On Fri, 8 Sept 2023 at 02:43, Alex BennÃe <alex.bennee@linaro.org> wrote:
>>
>>
>> Stefan Hajnoczi <stefanha@redhat.com> writes:
>>
>> > On Tue, Sep 05, 2023 at 10:34:11AM +0100, Alex BennÃe wrote:
>> >>
>> >> Albert Esteve <aesteve@redhat.com> writes:
>> >>
>> >> > This looks great! Thanks for this proposal.
>> >> >
>> >> > On Fri, Sep 1, 2023 at 1:00âPM Alex BennÃe <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 BennÃe <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 BennÃe
Virtualisation Tech Lead @ Linaro
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