vhost Target

Table of Contents


A vhost target provides a local storage service as a process running on a local machine. It is capable of exposing virtualized block devices to QEMU instances or other arbitrary processes.

The following diagram presents how QEMU-based VM communicates with SPDK Vhost-SCSI device.

QEMU/SPDK vhost data flow

The diagram, and the vhost protocol itself is described in Virtualized I/O with Vhost-user doc.

SPDK provides an accelerated vhost target by applying the same user space and polling techniques as other components in SPDK. Since SPDK is polling for vhost submissions, it can signal the VM to skip notifications on submission. This avoids VMEXITs on I/O submission and can significantly reduce CPU usage in the VM on heavy I/O workloads.


This guide assumes the SPDK has been built according to the instructions in Getting Started. The SPDK vhost target is built with the default configure options.

Vhost Command Line Parameters

Additional command line flags are available for Vhost target.

Param Type Default Description
-S string $PWD directory where UNIX domain sockets will be created

Supported Guest Operating Systems

The guest OS must contain virtio-scsi or virtio-blk drivers. Most Linux and FreeBSD distributions include virtio drivers. Windows virtio drivers must be installed separately. The SPDK vhost target has been tested with recent versions of Ubuntu, Fedora, and Windows


Userspace vhost-scsi target support was added to upstream QEMU in v2.10.0. Run the following command to confirm your QEMU supports userspace vhost-scsi.

qemu-system-x86_64 -device vhost-user-scsi-pci,help

Userspace vhost-blk target support was added to upstream QEMU in v2.12.0. Run the following command to confirm your QEMU supports userspace vhost-blk.

qemu-system-x86_64 -device vhost-user-blk-pci,help

Userspace vhost-nvme target was added as experimental feature for SPDK 18.04 release, patches for QEMU are available in SPDK's QEMU repository only.

Run the following command to confirm your QEMU supports userspace vhost-nvme.

qemu-system-x86_64 -device vhost-user-nvme,help

Starting SPDK vhost target

First, run the SPDK setup.sh script to setup some hugepages for the SPDK vhost target application. This will allocate 4096MiB (4GiB) of hugepages, enough for the SPDK vhost target and the virtual machine.

HUGEMEM=4096 scripts/setup.sh

Next, start the SPDK vhost target application. The following command will start vhost on CPU cores 0 and 1 (cpumask 0x3) with all future socket files placed in /var/tmp. Vhost will fully occupy given CPU cores for I/O polling. Particular vhost devices can be restricted to run on a subset of these CPU cores. See Create a vhost device for details.

build/bin/vhost -S /var/tmp -m 0x3

To list all available vhost options use the following command.

build/bin/vhost -h

SPDK Configuration

Create bdev (block device)

SPDK bdevs are block devices which will be exposed to the guest OS. For vhost-scsi, bdevs are exposed as SCSI LUNs on SCSI devices attached to the vhost-scsi controller in the guest OS. For vhost-blk, bdevs are exposed directly as block devices in the guest OS and are not associated at all with SCSI.

SPDK supports several different types of storage backends, including NVMe, Linux AIO, malloc ramdisk and Ceph RBD. Refer to Block Device User Guide for additional information on configuring SPDK storage backends.

This guide will use a malloc bdev (ramdisk) named Malloc0. The following RPC will create a 64MB malloc bdev with 512-byte block size.

scripts/rpc.py bdev_malloc_create 64 512 -b Malloc0

Create a vhost device


The following RPC will create a vhost-scsi controller which can be accessed by QEMU via /var/tmp/vhost.0. At the time of creation the controller will be bound to a single CPU core with the smallest number of vhost controllers. The optional --cpumask parameter can directly specify which cores should be taken into account - in this case always CPU 0. To achieve optimal performance on NUMA systems, the cpumask should specify cores on the same CPU socket as its associated VM.

scripts/rpc.py vhost_create_scsi_controller --cpumask 0x1 vhost.0

The following RPC will attach the Malloc0 bdev to the vhost.0 vhost-scsi controller. Malloc0 will appear as a single LUN on a SCSI device with target ID 0. SPDK Vhost-SCSI device currently supports only one LUN per SCSI target. Additional LUNs can be added by specifying a different target ID.

scripts/rpc.py vhost_scsi_controller_add_target vhost.0 0 Malloc0

To remove a bdev from a vhost-scsi controller use the following RPC:

scripts/rpc.py vhost_scsi_controller_remove_target vhost.0 0


The following RPC will create a vhost-blk device exposing Malloc0 bdev. The device will be accessible to QEMU via /var/tmp/vhost.1. All the I/O polling will be pinned to the least occupied CPU core within given cpumask - in this case always CPU 0. For NUMA systems, the cpumask should specify cores on the same CPU socket as its associated VM.

scripts/rpc.py vhost_create_blk_controller --cpumask 0x1 vhost.1 Malloc0

It is also possible to create a read-only vhost-blk device by specifying an extra -r or --readonly parameter.

scripts/rpc.py vhost_create_blk_controller --cpumask 0x1 -r vhost.1 Malloc0


Now the virtual machine can be started with QEMU. The following command-line parameters must be added to connect the virtual machine to its vhost controller.

First, specify the memory backend for the virtual machine. Since QEMU must share the virtual machine's memory with the SPDK vhost target, the memory must be specified in this format with share=on.

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

Second, ensure QEMU boots from the virtual machine image and not the SPDK malloc block device by specifying bootindex=0 for the boot image.

-drive file=guest_os_image.qcow2,if=none,id=disk
-device ide-hd,drive=disk,bootindex=0

Finally, specify the SPDK vhost devices:


-chardev socket,id=char0,path=/var/tmp/vhost.0
-device vhost-user-scsi-pci,id=scsi0,chardev=char0


-chardev socket,id=char1,path=/var/tmp/vhost.1
-device vhost-user-blk-pci,id=blk0,chardev=char1

Example output

This example uses an NVMe bdev alongside Mallocs. SPDK vhost application is started on CPU cores 0 and 1, QEMU on cores 2 and 3.

host:~# HUGEMEM=2048 ./scripts/setup.sh
0000:01:00.0 (8086 0953): nvme -> vfio-pci
host:~# ./build/bin/vhost -S /var/tmp -s 1024 -m 0x3 &
Starting DPDK 17.11.0 initialization...
[ DPDK EAL parameters: vhost -c 3 -m 1024 --master-lcore=1 --file-prefix=spdk_pid156014 ]
EAL: Detected 48 lcore(s)
EAL: Probing VFIO support...
EAL: VFIO support initialized
app.c: 369:spdk_app_start: *NOTICE*: Total cores available: 2
reactor.c: 668:spdk_reactors_init: *NOTICE*: Occupied cpu socket mask is 0x1
reactor.c: 424:_spdk_reactor_run: *NOTICE*: Reactor started on core 1 on socket 0
reactor.c: 424:_spdk_reactor_run: *NOTICE*: Reactor started on core 0 on socket 0
host:~# ./scripts/rpc.py bdev_nvme_attach_controller -b Nvme0 -t pcie -a 0000:01:00.0
EAL: PCI device 0000:01:00.0 on NUMA socket 0
EAL: probe driver: 8086:953 spdk_nvme
EAL: using IOMMU type 1 (Type 1)
host:~# ./scripts/rpc.py bdev_malloc_create 128 4096 Malloc0
host:~# ./scripts/rpc.py vhost_create_scsi_controller --cpumask 0x1 vhost.0
VHOST_CONFIG: vhost-user server: socket created, fd: 21
VHOST_CONFIG: bind to /var/tmp/vhost.0
vhost.c: 596:spdk_vhost_dev_construct: *NOTICE*: Controller vhost.0: new controller added
host:~# ./scripts/rpc.py vhost_scsi_controller_add_target vhost.0 0 Nvme0n1
vhost_scsi.c: 840:spdk_vhost_scsi_dev_add_tgt: *NOTICE*: Controller vhost.0: defined target 'Target 0' using lun 'Nvme0'
host:~# ./scripts/rpc.py vhost_scsi_controller_add_target vhost.0 1 Malloc0
vhost_scsi.c: 840:spdk_vhost_scsi_dev_add_tgt: *NOTICE*: Controller vhost.0: defined target 'Target 1' using lun 'Malloc0'
host:~# ./scripts/rpc.py bdev_malloc_create 64 512 -b Malloc1
host:~# ./scripts/rpc.py vhost_create_blk_controller --cpumask 0x2 vhost.1 Malloc1
vhost_blk.c: 719:spdk_vhost_blk_construct: *NOTICE*: Controller vhost.1: using bdev 'Malloc1'
host:~# taskset -c 2,3 qemu-system-x86_64 \
--enable-kvm \
-cpu host -smp 2 \
-m 1G -object memory-backend-file,id=mem0,size=1G,mem-path=/dev/hugepages,share=on -numa node,memdev=mem0 \
-drive file=guest_os_image.qcow2,if=none,id=disk \
-device ide-hd,drive=disk,bootindex=0 \
-chardev socket,id=spdk_vhost_scsi0,path=/var/tmp/vhost.0 \
-device vhost-user-scsi-pci,id=scsi0,chardev=spdk_vhost_scsi0,num_queues=4 \
-chardev socket,id=spdk_vhost_blk0,path=/var/tmp/vhost.1 \
-device vhost-user-blk-pci,chardev=spdk_vhost_blk0,num-queues=4

Please note the following two commands are run on the guest VM.

sda sda QEMU HARDDISK 1:0:0:0 80G ATA block:scsi:pci
sda1 sda1 80G block:scsi:pci
sdb sdb NVMe disk 2:0:0:0 372,6G INTEL block:scsi:virtio:pci
sdc sdc Malloc disk 2:0:1:0 128M INTEL block:scsi:virtio:pci
vda vda 128M 0x1af4 block:virtio:pci
guest:~# poweroff
host:~# fg
<< CTRL + C >>
vhost.c:1006:session_shutdown: *NOTICE*: Exiting

We can see that sdb and sdc are SPDK vhost-scsi LUNs, and vda is SPDK a vhost-blk disk.

Advanced Topics

Multi-Queue Block Layer (blk-mq)

For best performance use the Linux kernel block multi-queue feature with vhost. To enable it on Linux, it is required to modify kernel options inside the 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 the VM and add num_queues parameter to the QEMU device. It should be enough to set num_queues=4 to saturate physical device. Adding too many queues might lead to SPDK vhost performance degradation if many vhost devices are used because each device will require additional num_queues to be polled.


Hotplug/hotremove within a vhost controller is called hot-attach/detach. This is to distinguish it from SPDK bdev hotplug/hotremove. E.g. if an NVMe bdev is attached to a vhost-scsi controller, physically hotremoving the NVMe will trigger vhost-scsi hot-detach. It is also possible to hot-detach a bdev manually via RPC - for example when the bdev is about to be attached to another controller. See the details below.

Please also note that hot-attach/detach is Vhost-SCSI-specific. There are no RPCs to hot-attach/detach the bdev from a Vhost-BLK device. If Vhost-BLK device exposes an NVMe bdev that is hotremoved, all the I/O traffic on that Vhost-BLK device will be aborted - possibly flooding a VM with syslog warnings and errors.


Hot-attach is done by simply attaching a bdev to a vhost controller with a QEMU VM already started. No other extra action is necessary.

scripts/rpc.py vhost_scsi_controller_add_target vhost.0 0 Malloc0


Just like hot-attach, the hot-detach is done by simply removing bdev from a controller when QEMU VM is already started.

scripts/rpc.py vhost_scsi_controller_remove_target vhost.0 0

Removing an entire bdev will hot-detach it from a controller as well.

scripts/rpc.py bdev_malloc_delete Malloc0

Known bugs and limitations

Vhost-NVMe (experimental) can only be supported with latest Linux kernel

Vhost-NVMe target was designed for one new feature of NVMe 1.3 specification, Doorbell Buffer Config Admin command, which is used for emulated NVMe controller only. Linux 4.12 added this feature, so a new Guest kernel later than 4.12 is required to test this feature.

Windows virtio-blk driver before version 0.1.130-1 only works with 512-byte sectors

The Windows viostor driver before version 0.1.130-1 is buggy and does not correctly support vhost-blk devices with non-512-byte block size. See the bug report for more information.

QEMU vhost-user-blk

QEMU vhost-user-blk is supported from version 2.12.