Commit ba7331a7 authored by John Kariuki's avatar John Kariuki Committed by Tomasz Zawadzki
Browse files

doc/vhost: update Vhost-user link and terminology 



Fix broken link to Vhost-user protocol documentation.
Update terminology to match protocol documentation.

Change-Id: Id1aeb0fc8bc99990577efb767ab3dde2411bd664
Signed-off-by: default avatarJohn Kariuki <John.K.Kariuki@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/13849


Tested-by: default avatarSPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: default avatarBen Walker <benjamin.walker@intel.com>
Reviewed-by: default avatarChangpeng Liu <changpeng.liu@intel.com>
parent 17948bca

doc/vhost_processing.md

+14 −12
Original line number Diff line number Diff line
@@ -42,7 +42,7 @@ The initial vhost implementation is a part of the Linux kernel and uses ioctl 
interface to communicate with userspace applications. What makes it possible for

SPDK to expose a vhost device is Vhost-user protocol.



The [Vhost-user specification](https://git.qemu.org/?p=qemu.git;a=blob_plain;f=docs/interop/vhost-user.txt;hb=HEAD)

The [Vhost-user specification](https://qemu-project.gitlab.io/qemu/interop/vhost-user.html)

describes the protocol as follows:



> [Vhost-user protocol] is aiming to complement the ioctl interface used to
@@ -51,17 +51,19 @@ describes the protocol as follows: 
> same host. It uses communication over a Unix domain socket to share file

> descriptors in the ancillary data of the message.

>

> The protocol defines 2 sides of the communication, master and slave. Master is

> the application that shares its virtqueues, in our case QEMU. Slave is the

> consumer of the virtqueues.

> 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 Master, and the Slave is intended to

> be a software Ethernet switch running in user space, such as Snabbswitch.

> 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 and write to a virtual disk.

>

> Master and slave can be either a client (i.e. connecting) or server (listening)

> in the socket communication.

> The front-end and back-end can be either a client (i.e. connecting) or

> server (listening) in the socket communication.



SPDK vhost is a Vhost-user slave server. It exposes Unix domain sockets and

SPDK vhost is a Vhost-user back-end server. It exposes Unix domain sockets and

allows external applications to connect.



## QEMU {#vhost_processing_qemu}
@@ -76,7 +78,7 @@ communicates with SPDK Vhost-SCSI device. 


All initialization and management information is exchanged using Vhost-user

messages. The connection always starts with the feature negotiation. Both

the Master and the Slave exposes a list of their implemented features and

the front-end and the back-end expose a list of their implemented features and

upon negotiation they choose a common set of those. Most of these features are

implementation-related, but also regard e.g. multiqueue support or live migration.
@@ -94,7 +96,7 @@ the following data: 
- user offset - positive offset for the mmap

- size



The Master will send new memory regions after each memory change - usually

The front-end will send new memory regions after each memory change - usually

hotplug/hotremove. The previous mappings will be removed.



Drivers may also request a device config, consisting of e.g. disk geometry.
@@ -116,7 +118,7 @@ polled as soon as they're initialized. 


## I/O path {#vhost_processing_io_path}



The Master sends I/O by allocating proper buffers in shared memory, filling

The front-end sends I/O by allocating proper buffers in shared memory, filling

the request data, and putting guest addresses of those buffers into virtqueues.



A Virtio-Block request looks as follows.