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Subject: Re: [RFC PATCH v2] docs/interop: define PROBE feature for vhost-user VirtIO devices
Stefan Hajnoczi <stefanha@redhat.com> writes: > On Fri, Sep 01, 2023 at 12:00:18PM +0100, Alex BennÃe 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> > > I think the mindset for this change should be "vhost-user is becoming a > VIRTIO Transport". VIRTIO Transports have a reasonably well-defined > feature set in the VIRTIO specification. The goal should be to cover > every VIRTIO Transport operation via vhost-user protocol messages so > that the VIRTIO device model can be fully conveyed over vhost-user. Is it though? The transport is a guest visible construct whereas vhost-user is purely a backend implementation detail that should be invisible to the guest. Also the various backends do things a different set of ways. The differences between MMIO and PCI are mostly around where config space is and how IRQs are handled. For CCW we do actually have a set of commands we can look at: #define CCW_CMD_SET_VQ 0x13 #define CCW_CMD_VDEV_RESET 0x33 #define CCW_CMD_SET_IND 0x43 #define CCW_CMD_SET_CONF_IND 0x53 #define CCW_CMD_SET_IND_ADAPTER 0x73 #define CCW_CMD_READ_FEAT 0x12 #define CCW_CMD_WRITE_FEAT 0x11 #define CCW_CMD_READ_CONF 0x22 #define CCW_CMD_WRITE_CONF 0x21 #define CCW_CMD_WRITE_STATUS 0x31 #define CCW_CMD_READ_VQ_CONF 0x32 #define CCW_CMD_SET_VIRTIO_REV 0x83 #define CCW_CMD_READ_STATUS 0x72 which I think we already have mappings for. > Anything less is yet another ad-hoc protocol extension that will lead to > more bugs and hacks when it turns out some VIRTIO devices cannot be > expressed due to limitations in the protocol. I agree we want to do this right. > This requires going through the VIRTIO spec to find a correspondence > between virtio-pci/virtio-mmio/virtio-ccw's interfaces and vhost-user > protocol messages. In most cases vhost-user already offers messages and > your patch adds more of what is missing. I think this effort is already > very close but missing the final check that it really matches the VIRTIO > spec. > > Please do the comparison against the VIRTIO Transports and then adjust > this patch to make it clear that the back-end is becoming a full-fledged > VIRTIO Transport: > - The name of the patch series should reflect that. > - The vhost-user protocol feature should be named F_TRANSPORT. > - The messages added in this patch should have a 1:1 correspondence with > the VIRTIO spec including using the same terminology for consistency. > > Sorry for the hassle, but I think this is a really crucial point where > we have the chance to make vhost-user work smoothly in the future...but > only if we can faithfully expose VIRTIO Transport semantics. I wonder if first be handled by cleaning up the VirtIO spec to make it clear what capabilities each transport needs to support? >> --- >> 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. > > "... without a per-device stub in the VMM" > > This makes it clear that this sentence is describing an alternative > to the per-device stub in the VMM. > >> See :ref:`Probing features for standalone daemons >> +<probing_features>` for more details. > > The current section is named "Probing device details" and one being > reference is called "Probing features for standalone daemons". Are > "features" or "device details" two terms for the same thing? Why > "daemons" and not "back-end"? > > I suggest calling this section "Standalone back-ends" and the other > section "Probing standalone back-ends" to keep the terminology > consistent. > >> + >> + >> 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``) > > F_STANDALONE vs F_PROBE > > "SPECS" vs "features" vs "details". > > Please be consistent. > >> >> .. 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 > > "Usually" is vague. It's possible to be precise: > > In this case a front-end that is aware of the device type already > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > knows the number of available virtqueues without communicating with > the back-end. > >> +number can be can be probed with the ``VHOST_USER_GET_MIN_VQ`` message. > > Then this sentence can be adjusted to: > > When the front-end is not aware of the device type, the number 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. > > "... VIRTIO device type that the backend emulates is." > > "Device type" is the name used in the VIRTIO spec. > >> + >> +``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. > > "a bug" > > What is the harm in returning 0 when the device has an empty > Configuration Space like the Entropy device, the I2C Adapter, the SCMI > device, etc? > >> + >> +``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. > > What is the purpose of this message? I don't see an equivalent in the > VIRTIO specification. > >> >> 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; > > This message is undocumented? I think it may be outdated and you split > it up into individual messages. > >> + >> 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 >> -- Alex BennÃe Virtualisation Tech Lead @ Linaro
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