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Subject: Re: [virtio-dev] [RFC PATCH v2] docs/interop: define PROBE feature for vhost-user VirtIO devices
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 > 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). However VHOST_USER_GET_QUEUE_NUM is only usable if VIRTIO_NET_F_MQ has been negotiated and could report more. > > Couldn't we reuse the `VHOST_USER_GET_QUEUE_NUM` type for this? > > Back-end message types > ---------------------- > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c > index 8dcf049d42..4d433cdf2b 100644 > --- a/hw/virtio/vhost-user.c > +++ b/hw/virtio/vhost-user.c > @@ -202,6 +202,13 @@ typedef struct VhostUserInflight { > uint16_t queue_size; > } VhostUserInflight; > > +typedef struct VhostUserBackendSpecs { > + uint32_t device_id; > + uint32_t config_size; > + uint32_t min_vqs; > + uint32_t max_vqs; > +} VhostUserBackendSpecs; > + > typedef struct { > VhostUserRequest request; > > @@ -226,6 +233,7 @@ typedef union { > VhostUserCryptoSession session; > VhostUserVringArea area; > VhostUserInflight inflight; > + VhostUserBackendSpecs specs; Oops these snuck in, I shall clean them up > } VhostUserPayload; > > typedef struct VhostUserMsg { > -- > 2.39.2 > > --------------------------------------------------------------------- > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org -- Alex BennÃe Virtualisation Tech Lead @ Linaro
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