vhost: Getting Started for SPDK vhost

                         nvme/io_submission.md \

                         nvme/multi_process.md \

                         nvmf/index.md \

                         nvmf/getting_started.md

                         nvmf/getting_started.md \

                         vhost/index.md \

                         vhost/getting_started.md

# This tag can be used to specify the character encoding of the source files

# that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses

    iSCSI target.

 - `app/trace`: A tool for processing trace points output from the iSCSI and

    NVMe-oF targets.

 - `app/vhost`:  A vhost application that presents virtio controllers to

    QEMU-based VMs and process I/O submitted to those controllers.

The application binaries will be in their respective directories after compiling and all

can be run with no arguments to print out their command line arguments. For the iSCSI

- @ref bdev

- @ref blob

- @ref blobfs

- @ref vhost

# vhost Getting Started Guide {#vhost_getting_started}

The Storage Performance Development Kit vhost application is named "vhost".

This application extends SPDK to present virtio-scsi controllers to QEMU-based

VMs and process I/O submitted to devices attached to those controllers.

# Prerequisites {#vhost_prereqs}

The base SPDK build instructions are located README.md in SPDK main directory.

This guide assumes familiarity with building SPDK using the default options.

## Supported Guest Operating Systems

The guest OS must contain virtio drivers. The SPDK vhost target has been tested

with Ubuntu 16.04, Fedora 25, Windows 2012 R2.

# Building

## SPDK

The vhost target is built by default.  To enable/disable building the vhost

target, either modify the following line in the CONFIG file in the root directory:

~~~

    CONFIG_VHOST?=y

~~~

Or specify on the command line:

~~~

    make CONFIG_VHOST=y

~~~

Once built, the binary will be at `app/vhost/vhost`.

## QEMU

Vhost functionality is dependent on QEMU patches to enable vhost-scsi in

userspace - those patches are currently working their way through the QEMU

mailing list, but temporary patches to enable this functionality are available

in the spdk branch at https://github.com/spdk/qemu.

# Configuration {#vhost_config}

## SPDK

A `vhost` specific configuration file is used to configure the SPDK vhost

target.  A fully documented example configuration file is located at

`etc/spdk/vhost.conf.in`.  This file defines the following:

### Storage Backends

Storage backends are block devices which will be exposed as SCSI LUNs on

devices attached to the vhost-scsi controller.  SPDK supports several different

types of storage backends, including NVMe, Linux AIO, malloc ramdisk and Ceph

RBD.  Refer to @ref bdev_getting_started for additional information on

specifying storage backends in the configuration file.

### Mappings Between SCSI Controllers and Storage Backends

The vhost target is exposing SCSI controllers to the virtual machines.

Each device in the vhost controller is associated with an SPDK block device and

configuration file defines those associations.  The block device to Dev mappings

are specified in the configuration file as:

~~~

[VhostScsiX]

  Name vhost.X          # Name of vhost socket

  Dev 0 BackendX        # "BackendX" is block device name from previous

                        # sections in config file

  Dev 1 BackendY

  ...

  Dev n BackendN

  #Cpumask 0x1          # Optional parameter defining which core controller uses

~~~

### Vhost Sockets

Userspace vhost uses UNIX domain sockets for communication between QEMU

and the vhost target.  Each vhost controller is associated with a UNIX domain

socket file with filename equal to the Name argument in configuration file.

Sockets are created at current directory when starting the SPDK vhost target.

### Core Affinity Configuration

Vhost target can be restricted to run on certain cores by specifying a ReactorMask.

Default is to allow vhost target work on core 0. For NUMA systems it is essential

to run vhost with cores on each socket to achieve optimal performance.

To specify which core each controller should use, it can be defined by optional

Cpumask parameter in configuration file.  For NUMA systems the Cpumask should

specify cores on the same CPU socket as its associated VM.

## QEMU

Userspace vhost-scsi adds the following command line option for QEMU:

~~~

       -device vhost-user-scsi-pci,id=scsi0,chardev=char0

~~~

In order to start qemu with vhost you need to specify following options:

 - Socket, which QEMU will use for vhost communication with SPDK:

~~~

       -chardev socket,id=char0,path=/path/to/vhost/socket

~~~

 - Hugepages to share memory between vm and vhost target

~~~

       -object memory-backend-file,id=mem,size=1G,mem-path=/dev/hugepages,share=on

~~~

# Running Vhost Target

To get started, the following example is usually sufficient:

~~~

app/vhost/vhost -c /path/to/vhost.conf

~~~

A full list of command line arguments to vhost can be obtained by:

~~~

app/vhost/vhost -h

~~~

## Example

Assume that qemu and spdk are in respectively `qemu` and `spdk` directories.

~~~

     ./qemu/build/x86_64-softmmu/qemu-system-x86_64 \

             -m 1024 \

             -object memory-backend-file,id=mem,size=1G,mem-path=/dev/hugepages,share=on \

             -numa node,memdev=mem \

             -drive file=$PROJECTS/os.qcow2,if=none,id=disk \

             -device ide-hd,drive=disk,bootindex=0 \

             -chardev socket,id=char0,path=./spdk/vhost.0 \

             -device vhost-user-scsi-pci,id=scsi0,chardev=char0 \

             --enable-kvm

~~~

# Experimental features {#vhost_experimental}

## Multi-Queue Block Layer (blk_mq)

It is possible to use multiqueue feature in vhost.

To enable it on linux it is required to modify kernel options inside

virtual machine.

Instructions below for Ubuntu OS:

1. `vi /etc/default/grub`

2. Make sure mq is enabled:

GRUB_CMDLINE_LINUX="scsi_mod.use_blk_mq=1"

3. `sudo update-grub`

4. Reboot virtual machine

To achieve better performance make sure to increase number of cores

assigned to vm.

# Known bugs and limitations {#vhost_bugs}

## VFIO driver is not supported

Currently, VFIO driver is not supported by vhost library. Please use UIO

driver if running SPDK vhost app. Otherwise any IO to physical device

(eg. IOAT or NVMe) will fail. Support for vhost with VFIO is in progress.

## Hot plug is not supported

Hot plug is not supported in vhost yet. Event queue path doesn't handle that

case. While hot plug will be just ignored, hot removal might cause segmentation

fault.

# vhost {#vhost}

- @ref vhost_getting_started