[RFC patch 0/1] block: vhost-blk backend

[Andrey Zhadchenko]

Although QEMU virtio-blk is quite fast, there is still some room for
improvements. Disk latency can be reduced if we handle virito-blk requests
in host kernel so we avoid a lot of syscalls and context switches.

The biggest disadvantage of this vhost-blk flavor is raw format.
Luckily Kirill Thai proposed device mapper driver for QCOW2 format to attach
files as block devices: https://www.spinics.net/lists/kernel/msg4292965.html

Also by using kernel modules we can bypass iothread limitation and finaly scale
block requests with cpus for high-performance devices. This is planned to be
implemented in next version.

Linux kernel module part:
https://lore.kernel.org/kvm/20220725202753.298725-1-andrey.zhadchenko@virtuozzo.com/

test setups and results:
fio --direct=1 --rw=randread  --bs=4k  --ioengine=libaio --iodepth=128
QEMU drive options: cache=none
filesystem: xfs

SSD:
               | randread, IOPS  | randwrite, IOPS |
Host           |      95.8k      |      85.3k      |
QEMU virtio    |      57.5k      |      79.4k      |
QEMU vhost-blk |      95.6k      |      84.3k      |

RAMDISK (vq == vcpu):
                 | randread, IOPS | randwrite, IOPS |
virtio, 1vcpu    |      123k      |      129k       |
virtio, 2vcpu    |      253k (??) |      250k (??)  |
virtio, 4vcpu    |      158k      |      154k       |
vhost-blk, 1vcpu |      110k      |      113k       |
vhost-blk, 2vcpu |      247k      |      252k       |
vhost-blk, 4vcpu |      576k      |      567k       |

Andrey Zhadchenko (1):
  block: add vhost-blk backend

 configure                     |  13 ++
 hw/block/Kconfig              |   5 +
 hw/block/meson.build          |   1 +
 hw/block/vhost-blk.c          | 395 ++++++++++++++++++++++++++++++++++
 hw/virtio/meson.build         |   1 +
 hw/virtio/vhost-blk-pci.c     | 102 +++++++++
 include/hw/virtio/vhost-blk.h |  44 ++++
 linux-headers/linux/vhost.h   |   3 +
 8 files changed, 564 insertions(+)
 create mode 100644 hw/block/vhost-blk.c
 create mode 100644 hw/virtio/vhost-blk-pci.c
 create mode 100644 include/hw/virtio/vhost-blk.h

-- 
2.31.1