Current memory operations like pinning may take a lot of time at the
destination.  Currently they are done after the source of the migration is
stopped, and before the workload is resumed at the destination.  This is a
period where neigher traffic can flow, nor the VM workload can continue
(downtime).

We can do better as we know the memory layout of the guest RAM at the
destination from the moment the migration starts.  Moving that operation allows
QEMU to communicate the kernel the maps while the workload is still running in
the source, so Linux can start mapping them.  Ideally, all IOMMU is configured,
but if the vDPA parent driver uses on-chip IOMMU and .set_map we're still
saving all the pinning time.

Note that further devices setup at the end of the migration may alter the guest
memory layout. But same as the previous point, many operations are still done
incrementally, like memory pinning, so we're saving time anyway.

The first bunch of patches just reorganizes the code, so memory related
operation parameters are shared between all vhost_vdpa devices.  This is
because the destination does not know what vhost_vdpa struct will have the
registered listener member, so it is easier to place them in a shared struct
rather to keep them in vhost_vdpa struct.  Future version may squash or omit
these patches.

Only tested with vdpa_sim. I'm sending this before full benchmark, as some work
like [1] can be based on it, and Si-Wei agreed on benchmark this series with
his experience.

Future directions on top of this series may include:
* Iterative migration of virtio-net devices, as it may reduce downtime per [1].
  vhost-vdpa net can apply the configuration through CVQ in the destination
  while the source is still migrating.
* Move more things ahead of migration time, like DRIVER_OK.
* Check that the devices of the destination are valid, and cancel the migration
  in case it is not.

[1] https://lore.kernel.org/qemu-devel/6c8ebb97-d546-3f1c-4cdd-54e23a566f61@nvidia.com/T/

Eugenio Pérez (18):
  vdpa: add VhostVDPAShared
  vdpa: move iova tree to the shared struct
  vdpa: move iova_range to vhost_vdpa_shared
  vdpa: move shadow_data to vhost_vdpa_shared
  vdpa: use vdpa shared for tracing
  vdpa: move file descriptor to vhost_vdpa_shared
  vdpa: move iotlb_batch_begin_sent to vhost_vdpa_shared
  vdpa: move backend_cap to vhost_vdpa_shared
  vdpa: remove msg type of vhost_vdpa
  vdpa: move iommu_list to vhost_vdpa_shared
  vdpa: use VhostVDPAShared in vdpa_dma_map and unmap
  vdpa: use dev_shared in vdpa_iommu
  vdpa: move memory listener to vhost_vdpa_shared
  vdpa: do not set virtio status bits if unneeded
  vdpa: add vhost_vdpa_load_setup
  vdpa: add vhost_vdpa_net_load_setup NetClient callback
  vdpa: use shadow_data instead of first device v->shadow_vqs_enabled
  virtio_net: register incremental migration handlers

 include/hw/virtio/vhost-vdpa.h |  43 +++++---
 include/net/net.h              |   4 +
 hw/net/virtio-net.c            |  23 +++++
 hw/virtio/vdpa-dev.c           |   7 +-
 hw/virtio/vhost-vdpa.c         | 183 ++++++++++++++++++---------------
 net/vhost-vdpa.c               | 127 ++++++++++++-----------
 hw/virtio/trace-events         |  14 +--
 7 files changed, 239 insertions(+), 162 deletions(-)

-- 
2.39.3