On Tue, Sep 05, 2023 at 10:34:11AM +0100, Alex Bennée wrote: > > Albert Esteve <aest...@redhat.com> writes: > > > This looks great! Thanks for this proposal. > > > > On Fri, Sep 1, 2023 at 1:00 PM Alex Bennée <alex.ben...@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.ben...@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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