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Subject: Re: [virtio-dev] [RFC PATCH v2] docs/interop: define PROBE feature for vhost-user VirtIO devices
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?
>> > 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 BennÃe
Virtualisation Tech Lead @ Linaro
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