Hi all,

v11->v12
- Refined VIRTIO Shared Memory Region configuration mem size
  spec description
- Removed unused name in vhost-user-base's vub_device_realize to
  avoid build error
- Initialize shmem in vhost_user_backend_handle_shmem_unmap
  to avoid compiler being confused
- Added stub for memory_region_init_ram_from_fd in memory.h
  for windows builds
- Use memory_region_size for shmem_region handlers instead
  of accessing size member directly in shmem mrs
v10->v11
- Rebase to grab commit fde5930cc37175cfcd0f03a089e26f4458a52311
  and explicitly unset reply_ack on the SHMEM_MAP/UNMAP msg.
- Mention mapping cleanup on device reset in the spec.
v9->v10
- Fix transaction_commit invoked without transaction_begin
  on vhost_user_backend_handle_shmem_map() early errors
- Removed fd tracking on VirtioSharedMemoryMapping, it
  is handled by the RAMBlock
- Reject invalid BAR configurations when VIRTIO Shared Memory
  Regions are in use by vhost-user-test-device
v8->v9
- Fixed vhost-user new handlers to ensure that they always
  reply
- Made MMAP request flags field u64 everywhere
- Fixed double memory_region_del_subregion() on UNMAP
- Add mappings cleaning on virtio_reset()
- Some small typos and fixes
- Fixed virtio pci bar mapping for vhost-user-test-device

This patch series implements dynamic fd-backed memory mapping support
for vhost-user backends, enabling backends to dynamically request memory
mappings and unmappings during runtime through the new
VHOST_USER_BACKEND_SHMEM_MAP/UNMAP protocol messages.

This feature benefits various VIRTIO devices that require dynamic shared
memory management, including virtiofs (for DAX mappings), virtio-gpu
(for resource sharing), and the recently standardized virtio-media.

The implementation introduces a QOM-based architecture for managing
shared memory lifecycle:

- VirtioSharedMemoryMapping: an intermediate object that manages
  individual memory mappings by acting as generic container for regions
  declared in any vhost-user device type
- Dynamic Mapping: backends can request mappings via SHMEM_MAP messages,
  with the frontend creating MemoryRegions from the provided file
  descriptors and adding them as subregions

When a SHMEM_MAP request is received, the frontend:
1. Creates VirtioSharedMemoryMapping to manage the mapping lifecycle
2. Maps the provided fd with memory_region_init_ram_from_fd()
3. Creates a MemoryRegion backed by the mapped memory
4. Adds it as a subregion of the appropiate VIRTIO Shared Memory Region

The QOM reference counting ensures automatic cleanup when mappings are
removed or the device is destroyed.

This patch also includes:
- VHOST_USER_GET_SHMEM_CONFIG: a new frontend request allowing generic
  vhost-user devices to query shared memory configuration from backends
  at device initialization, enabling the generic vhost-user-device
  frontend to work with any backend regardless of specific shared memory
  requirements.

The implementation has been tested with rust-vmm based backends.

Albert Esteve (7):
  vhost-user: Add VirtIO Shared Memory map request
  vhost_user.rst: Align VhostUserMsg excerpt members
  vhost_user.rst: Add SHMEM_MAP/_UNMAP to spec
  vhost_user: Add frontend get_shmem_config command
  vhost_user.rst: Add GET_SHMEM_CONFIG message
  qmp: add shmem feature map
  vhost-user-device: Add shared memory BAR

 docs/interop/vhost-user.rst               | 104 +++++++++
 hw/virtio/vhost-user-base.c               |  46 +++-
 hw/virtio/vhost-user-test-device-pci.c    |  38 ++-
 hw/virtio/vhost-user.c                    | 269 ++++++++++++++++++++++
 hw/virtio/virtio-qmp.c                    |   3 +
 hw/virtio/virtio.c                        | 201 ++++++++++++++++
 include/hw/virtio/vhost-backend.h         |  10 +
 include/hw/virtio/vhost-user.h            |   1 +
 include/hw/virtio/virtio.h                | 137 +++++++++++
 include/system/memory.h                   |  13 ++
 subprojects/libvhost-user/libvhost-user.c |  70 ++++++
 subprojects/libvhost-user/libvhost-user.h |  54 +++++
 12 files changed, 941 insertions(+), 5 deletions(-)

-- 
2.52.0