[RFC PATCH v2 0/1] block: vhost-blk backend

[Andrey Zhadchenko]

Sending second version of this patchset as @stefanha requested.
The main difference from the previous version is added vhost
multithreading support.

Also I must note currently there are several problems which
I intend to reconsider/fix later:
 - vmsd is present but migration is not supported
 - BlockConf is used but block layer is actually bypassed
 - posix BlockDriver internal state guessing

---

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:
v2: 
https://lore.kernel.org/kvm/20221013151839.689700-1-andrey.zhadchenko@virtuozzo.com/

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

SSD:
               | randread, IOPS  | randwrite, IOPS |
Host           |      95.8k      |      85.3k      |
QEMU virtio    |      61.5k      |      79.9k      |
QEMU vhost-blk |      95.6k      |      84.3k      |

RAMDISK (vq == vcpu == numjobs):
                 | randread, IOPS | randwrite, IOPS |
virtio, 1vcpu    |      133k      |      133k       |
virtio, 2vcpu    |      305k      |      306k       |
virtio, 4vcpu    |      310k      |      298k       |
virtio, 8vcpu    |      271k      |      252k       |
vhost-blk, 1vcpu |      110k      |      113k       |
vhost-blk, 2vcpu |      247k      |      252k       |
vhost-blk, 4vcpu |      558k      |      556k       |
vhost-blk, 8vcpu |      576k      |      575k       | *single kernel thread
vhost-blk, 8vcpu |      803k      |      779k       | *two kernel threads

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

 hw/block/Kconfig              |   5 +
 hw/block/meson.build          |   4 +
 hw/block/vhost-blk.c          | 403 ++++++++++++++++++++++++++++++++++
 hw/virtio/meson.build         |   3 +
 hw/virtio/vhost-blk-pci.c     | 102 +++++++++
 include/hw/virtio/vhost-blk.h |  52 +++++
 meson.build                   |   6 +
 meson_options.txt             |   2 +
 scripts/meson-buildoptions.sh |   3 +
 9 files changed, 580 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