virtio-blk and virtio-scsi devices need a way to specify the mapping between
IOThreads and virtqueues. At the moment all virtqueues are assigned to a single
IOThread or the main loop. This single thread can be a CPU bottleneck, so it is
necessary to allow finer-grained assignment to spread the load. With this
series applied, "pidstat -t 1" shows that guests with -smp 2 or higher are able
to exploit multiple IOThreads.

This series introduces command-line syntax for the new iothread-vq-mapping
property is as follows:

  --device '{"driver":"virtio-blk-pci","iothread-vq-mapping":[{"iothread":"iothread0","vqs":[0,1,2]},...]},...'

IOThreads are specified by name and virtqueues are specified by 0-based
index.

It will be common to simply assign virtqueues round-robin across a set
of IOThreads. A convenient syntax that does not require specifying
individual virtqueue indices is available:

  --device '{"driver":"virtio-blk-pci","iothread-vq-mapping":[{"iothread":"iothread0"},{"iothread":"iothread1"},...]},...'

There is no way to reassign virtqueues at runtime and I expect that to be a
very rare requirement.

Note that JSON --device syntax is required for the iothread-vq-mapping
parameter because it's non-scalar.

Based-on: 20230912231037.826804-1-stefanha@redhat.com ("[PATCH v3 0/5] block-backend: process I/O in the current AioContext")

Stefan Hajnoczi (2):
  qdev: add IOThreadVirtQueueMappingList property type
  virtio-blk: add iothread-vq-mapping parameter

 qapi/virtio.json                    |  30 +++++
 hw/block/dataplane/virtio-blk.h     |   3 +
 include/hw/qdev-properties-system.h |   4 +
 include/hw/virtio/virtio-blk.h      |   2 +
 hw/block/dataplane/virtio-blk.c     | 163 +++++++++++++++++++++-------
 hw/block/virtio-blk.c               |  92 ++++++++++++++--
 hw/core/qdev-properties-system.c    |  47 ++++++++
 7 files changed, 287 insertions(+), 54 deletions(-)

-- 
2.41.0