[PATCH v1 0/4] virtio: fix IO request length in virtio SCSI/block

[Denis Plotnikov <dplotnikov@virtuozzo.com>]

v1:
  * make seg_max size dependent on virtuqueue size
  * don't expose seg_max as property
  * add new machine types with increased queue size
  * add test to check the new machine types
  * check queue size for non-modern virtio devices
---
From: "Denis V. Lunev" <den@openvz.org>

Linux guests submit IO requests no longer than PAGE_SIZE * max_seg
field reported by SCSI controler. Thus typical sequential read with
1 MB size results in the following pattern of the IO from the guest:
  8,16   1    15754     2.766095122  2071  D   R 2095104 + 1008 [dd]
  8,16   1    15755     2.766108785  2071  D   R 2096112 + 1008 [dd]
  8,16   1    15756     2.766113486  2071  D   R 2097120 + 32 [dd]
  8,16   1    15757     2.767668961     0  C   R 2095104 + 1008 [0]
  8,16   1    15758     2.768534315     0  C   R 2096112 + 1008 [0]
  8,16   1    15759     2.768539782     0  C   R 2097120 + 32 [0]
The IO was generated by
  dd if=/dev/sda of=/dev/null bs=1024 iflag=direct

This effectively means that on rotational disks we will observe 3 IOPS
for each 2 MBs processed. This definitely negatively affects both
guest and host IO performance.

The cure is relatively simple - we should report lengthy scatter-gather
ability of the SCSI controller. Fortunately the situation here is very
good. VirtIO transport layer can accomodate 1024 items in one request
while we are using only 128. This situation is present since almost
very beginning. 2 items are dedicated for request metadata thus we
should publish VIRTQUEUE_MAX_SIZE - 2 as max_seg.

The following pattern is observed after the patch:
  8,16   1     9921     2.662721340  2063  D   R 2095104 + 1024 [dd]
  8,16   1     9922     2.662737585  2063  D   R 2096128 + 1024 [dd]
  8,16   1     9923     2.665188167     0  C   R 2095104 + 1024 [0]
  8,16   1     9924     2.665198777     0  C   R 2096128 + 1024 [0]
which is much better.

The dark side of this patch is that we are tweaking guest visible
parameter, though this should be relatively safe as above transport
layer support is present in QEMU/host Linux for a very long time.
The patch adds configurable property for VirtIO SCSI with a new default
and hardcode option for VirtBlock which does not provide good
configurable framework.

Unfortunately the commit can not be applied as is. For the real cure we
need guest to be fixed to accomodate that queue length, which is done
only in the latest 4.14 kernel. Thus we are going to expose the property
and tweak it on machine type level.

The problem with the old kernels is that they have
max_segments <= virtqueue_size restriction which cause the guest
crashing in the case of violation.
To fix the case described above in the old kernels we can increase
virtqueue_size to 256 and max_segments to 254. The pitfall here is
that seabios allows the virtqueue_size-s < 128, however, the seabios
patch extending that value to 256 is pending.

Denis Plotnikov (4):
  virtio: protect non-modern devices from too big virtqueue size setting
  virtio: make seg_max virtqueue size dependent
  virtio: increase virtuqueue sizes in new machine types
  iotests: add test for virtio-scsi and virtio-blk machine type settings

 hw/block/virtio-blk.c       |   2 +-
 hw/core/machine.c           |  14 ++++
 hw/i386/pc_piix.c           |  16 +++-
 hw/i386/pc_q35.c            |  14 +++-
 hw/scsi/virtio-scsi.c       |   2 +-
 hw/virtio/virtio-blk-pci.c  |   9 +++
 hw/virtio/virtio-scsi-pci.c |  10 +++
 include/hw/boards.h         |   6 ++
 tests/qemu-iotests/267      | 154 ++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/267.out  |   1 +
 tests/qemu-iotests/group    |   1 +
 11 files changed, 222 insertions(+), 7 deletions(-)
 create mode 100755 tests/qemu-iotests/267
 create mode 100644 tests/qemu-iotests/267.out

-- 
2.17.0