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Setup to run SPDK vhost-user devices with Kata Containers

Note: This guide only applies to QEMU, since the vhost-user storage device is only available for QEMU now. The enablement work on other hypervisors is still ongoing.

SPDK vhost-user Target Overview

The Storage Performance Development Kit (SPDK) provides a set of tools and libraries for writing high performance, scalable, user-mode storage applications.

virtio, vhost and vhost-user:

  • virtio is an efficient way to transport data for virtual environments and guests. It is most commonly used in QEMU VMs, where the VM itself exposes a virtual PCI device and the guest OS communicates with it using a specific virtio PCI driver. Its diagram is:
+---------+------+--------+----------+--+
|         +------+-------------------+  |
|         |            +----------+  |  |
| user    |            |          |  |  |
| space   |            |  guest   |  |  |
|         |            |          |  |  |
|    +----+ qemu       | +-+------+  |  |
|    |    |            | | virtio |  |  |
|    |    |            | | driver |  |  |
|    |    |            +-+---++---+  |  |
|    |    +------+-------------------+  |
|    |       ^               |          |
|    |       |               |          |
|    v       |               v          |
+-+------+---+------------+--+-------+--+
| |block |   +------------+ kvm.ko   |  |
| |device|                |          |  |
| +------+                +--+-------+  |
|           host kernel                 |
+---------------------------------------+
  • vhost is a protocol for devices accessible via inter-process communication. It uses the same virtio queue layout as virtio to allow vhost devices to be mapped directly to virtio devices. The initial vhost implementation is a part of the Linux kernel and uses an ioctl interface to communicate with userspace applications. Its diagram is:
+---------+------+--------+----------+--+
|         +------+-------------------+  |
|         |            +----------+  |  |
| user    |            |          |  |  |
| space   |            |  guest   |  |  |
|         |            |          |  |  |
|         | qemu       | +-+------+  |  |
|         |            | | virtio |  |  |
|         |            | | driver |  |  |
|         |            +-+-----++-+  |  |
|         +------+-------------------+  |
|                               |       |
|                               |       |
+-+------+--+-------------+--+--v-------+
| |block |  |vhost-scsi.ko|  | kvm.ko   |
| |device|  |             |  |          |
| +---^--+  +-v---------^-+  +--v-------+
|     |       |   host  |       |       |
|     +-------+  kernel +-------+       |
+---------------------------------------+
  • vhost-user implements the control plane through Unix domain socket to establish virtio queue sharing with a user space process on the same host. SPDK exposes vhost devices via the vhost-user protocol. Its diagram is:
+----------------+------+--+----------+-+
|                +------+-------------+ |
| user           |      +----------+  | |
| space          |      |          |  | |
|                |      |  guest   |  | |
|  +-+-------+   | qemu | +-+------+  | |
|  | vhost   |   |      | | virtio |  | |
|  | backend |   |      | | driver |  | |
|  +-^-^---^-+   |      +-+--+-----+  | |
|    | |   |     |           |        | |
|    | |   |     +--+---+----V------+-+ |
|    | |   |        |        |      |   |
|    | |  ++--------+--+     |      |   |
|    | |  |unix sockets|     |      |   |
|    | |  +------------+     |      |   |
|    | |                     |      |   |
|    | |  +-------------+    |      |   |
|    | +--|shared memory|<---+      |   |
+----+----+-------------+---+--+----+---+
|    |                      |           |
|    +----------------------+ kvm.ko    |
|                           +--+--------+
|           host kernel                 |
+---------------------------------------+

SPDK vhost is a vhost-user slave server. It exposes Unix domain sockets and allows external applications to connect. It is capable of exposing virtualized storage devices to QEMU instances or other arbitrary processes.

Currently, the SPDK vhost-user target can exposes these types of virtualized devices:

  • vhost-user-blk
  • vhost-user-scsi
  • vhost-user-nvme (deprecated from SPDK 21.07 release)

For more information, visit SPDK and SPDK vhost-user target.

Install and setup SPDK vhost-user target

Get source code and build SPDK

Following the SPDK getting started guide.

Run SPDK vhost-user target

First, run the SPDK setup.sh script to setup some hugepages for the SPDK vhost target application. We recommend you use a minimum of 4GiB, enough for the SPDK vhost target and the virtual machine. This will allocate 4096MiB (4GiB) of hugepages, and avoid binding PCI devices:

$ sudo HUGEMEM=4096 PCI_WHITELIST="none" scripts/setup.sh

Then, take directory /var/run/kata-containers/vhost-user as Kata's vhost-user device directory. Make subdirectories for vhost-user sockets and device nodes:

$ sudo mkdir -p /var/run/kata-containers/vhost-user/
$ sudo mkdir -p /var/run/kata-containers/vhost-user/block/
$ sudo mkdir -p /var/run/kata-containers/vhost-user/block/sockets/
$ sudo mkdir -p /var/run/kata-containers/vhost-user/block/devices/

For more details, see section Host setup for vhost-user devices.

Next, start the SPDK vhost target application. The following command will start vhost on the first CPU core with all future socket files placed in /var/run/kata-containers/vhost-user/block/sockets/:

$ sudo app/spdk_tgt/spdk_tgt -S /var/run/kata-containers/vhost-user/block/sockets/ &

To list all available vhost options run the following command:

$ app/spdk_tgt/spdk_tgt -h

Create an experimental vhost-user-blk device based on memory directly:

  • The following RPC will create a 64MB memory block device named Malloc0 with 4096-byte block size:
$ sudo scripts/rpc.py bdev_malloc_create 64 4096 -b Malloc0
  • The following RPC will create a vhost-user-blk device exposing Malloc0 block device. The device will be accessible via /var/run/kata-containers/vhost-user/block/sockets/vhostblk0:
$ sudo scripts/rpc.py vhost_create_blk_controller vhostblk0 Malloc0

Host setup for vhost-user devices

Considering the OCI specification and characteristics of vhost-user device, Kata has chosen to use Linux reserved the block major range 240-254 to map each vhost-user block type to a major. Also a specific directory is used for vhost-user devices.

The base directory for vhost-user device is a configurable value, with the default being /var/run/kata-containers/vhost-user. It can be configured by parameter vhost_user_store_path in Kata TOML configuration file.

Currently, the vhost-user storage device is not enabled by default, so the user should enable it explicitly inside the Kata TOML configuration file by setting enable_vhost_user_store = true. Since SPDK vhost-user target requires hugepages, hugepages should also be enabled inside the Kata TOML configuration file by setting enable_hugepages = true. Here is the conclusion of parameter setting for vhost-user storage device:

enable_hugepages = true
enable_vhost_user_store = true
vhost_user_store_path = "<Path of the base directory for vhost-user device>"

Note: These parameters are under [hypervisor.qemu] section in Kata TOML configuration file. If they are absent, users should still add them under [hypervisor.qemu] section.

For the subdirectories of vhost_user_store_path:

  • block is used for block device;
  • block/sockets is where we expect UNIX domain sockets for vhost-user block devices to live;
  • block/devices is where simulated block device nodes for vhost-user block devices are created.

For example, if using the default directory /var/run/kata-containers/vhost-user, UNIX domain sockets for vhost-user block device are under /var/run/kata-containers/vhost-user/block/sockets/. Device nodes for vhost-user block device are under /var/run/kata-containers/vhost-user/block/devices/.

Currently, Kata has chosen major number 241 to map to vhost-user-blk devices. For vhost-user-blk device named vhostblk0, a UNIX domain socket is already created by SPDK vhost target, and a block device node with major 241 and minor 0 should be created for it, in order to be recognized by Kata runtime:

$ sudo mknod /var/run/kata-containers/vhost-user/block/devices/vhostblk0 b 241 0

Launch a Kata container with SPDK vhost-user block device

To use vhost-user-blk device, use ctr to pass a host vhost-user-blk device to the container. In your config.json, you should use devices to pass a host device to the container.

For example (only vhost-user-blk listed):

{
  "linux": {
    "devices": [
      {
        "path": "/dev/vda",
        "type": "b",
        "major": 241,
        "minor": 0,
        "fileMode": 420,
        "uid": 0,
        "gid": 0
      }
    ]
  }
}

With rootfs provisioned under bundle directory, you can run your SPDK container:

$ sudo ctr run -d --runtime io.containerd.run.kata.v2 --config bundle/config.json spdk_container

Example of performing I/O operations on the vhost-user-blk device inside container:

$ sudo ctr t exec --exec-id 1 -t spdk_container sh
/ # ls -l /dev/vda
brw-r--r--    1 root     root      254,   0 Jan 20 03:54 /dev/vda
/ # dd if=/dev/vda of=/tmp/ddtest bs=4k count=20
20+0 records in
20+0 records out
81920 bytes (80.0KB) copied, 0.002996 seconds, 26.1MB/s