Re: [PATCH v11 0/7] vhost-user: Add SHMEM_MAP/UNMAP requests

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Nov 11, 2025 at 10:10:51AM +0100, Albert Esteve wrote:
> Hi all,
> 
> 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
> v7->v8
> - Unified VhostUserShmemObject and VirtioSharedMemoryMapping
> - Refined shmem_obj lifecycle by transferring ownership
> - Other small improvements
> 
> 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.

Looks good, we are in freeze so will merge after 10.2.
If it's not too much trouble, pls ping me after the release
to help make sure it's not forgotten.


> 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               | 102 ++++++++
>  hw/virtio/vhost-user-base.c               |  47 +++-
>  hw/virtio/vhost-user-test-device-pci.c    |  39 +++-
>  hw/virtio/vhost-user.c                    | 269 ++++++++++++++++++++++
>  hw/virtio/virtio-qmp.c                    |   3 +
>  hw/virtio/virtio.c                        | 199 ++++++++++++++++
>  include/hw/virtio/vhost-backend.h         |  10 +
>  include/hw/virtio/vhost-user.h            |   1 +
>  include/hw/virtio/virtio.h                | 137 +++++++++++
>  subprojects/libvhost-user/libvhost-user.c |  70 ++++++
>  subprojects/libvhost-user/libvhost-user.h |  54 +++++
>  11 files changed, 926 insertions(+), 5 deletions(-)
> 
> -- 
> 2.49.0