From: "Alex Bennée" <alex.bennee@linaro.org>
To: slp@redhat.com, mst@redhat.com, marcandre.lureau@redhat.com,
stefanha@redhat.com, viresh.kumar@linaro.org,
sgarzare@redhat.com, takahiro.akashi@linaro.org,
erik.schilling@linaro.org, manos.pitsidianakis@linaro.org,
mathieu.poirier@linaro.org
Cc: qemu-devel@nongnu.org, virtio-dev@lists.oasis-open.org,
virtio-comment@lists.oasis-open.org,
"Alex Bennée" <alex.bennee@linaro.org>
Subject: [RFC PATCH v2] docs/interop: define PROBE feature for vhost-user VirtIO devices
Date: Fri, 1 Sep 2023 12:00:18 +0100 [thread overview]
Message-ID: <20230901110018.3704459-1-alex.bennee@linaro.org> (raw)
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.
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;
} VhostUserPayload;
typedef struct VhostUserMsg {
--
2.39.2
next reply other threads:[~2023-09-01 11:01 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-09-01 11:00 Alex Bennée [this message]
2023-09-01 11:59 ` [virtio-dev] [RFC PATCH v2] docs/interop: define PROBE feature for vhost-user VirtIO devices Albert Esteve
2023-09-05 9:34 ` Alex Bennée
2023-09-05 10:02 ` Albert Esteve
2023-09-07 19:29 ` Stefan Hajnoczi
2023-09-08 6:41 ` Alex Bennée
2023-09-08 10:11 ` Stefan Hajnoczi
2023-09-08 11:59 ` Alex Bennée
2023-09-08 12:38 ` Stefan Hajnoczi
2023-09-07 19:22 ` Stefan Hajnoczi
2023-09-08 12:03 ` Alex Bennée
2023-09-08 13:01 ` Stefan Hajnoczi
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