Re: [PATCH v10 1/7] vhost-user: Add VirtIO Shared Memory map request

[Albert Esteve <aesteve@redhat.com>]

On Thu, Oct 16, 2025 at 4:38 PM Albert Esteve <aesteve@redhat.com> wrote:
>
> Add SHMEM_MAP/UNMAP requests to vhost-user for dynamic management of
> VIRTIO Shared Memory mappings.
>
> This implementation introduces VirtioSharedMemoryMapping as a unified
> QOM object that manages both the mapping metadata and MemoryRegion
> lifecycle. This object provides reference-counted lifecycle management
> with automatic cleanup of file descriptors and memory regions
> through QOM finalization.
>
> This request allows backends to dynamically map file descriptors into a
> VIRTIO Shared Memory Region identified by their shmid. Maps are created
> using memory_region_init_ram_from_fd() with configurable read/write
> permissions, and the resulting MemoryRegions are added as subregions to
> the shmem container region. The mapped memory is then advertised to the
> guest VIRTIO drivers as a base address plus offset for reading and
> writting according to the requested mmap flags.
>
> The backend can unmap memory ranges within a given VIRTIO Shared Memory
> Region to free resources. Upon receiving this message, the frontend
> removes the MemoryRegion as a subregion and automatically unreferences
> the VirtioSharedMemoryMapping object, triggering cleanup if no other
> references exist.
>
> Error handling has been improved to ensure consistent behavior across
> handlers that manage their own vhost_user_send_resp() calls. Since
> these handlers clear the VHOST_USER_NEED_REPLY_MASK flag, explicit
> error checking ensures proper connection closure on failures,
> maintaining the expected error flow.
>
> Note the memory region commit for these operations needs to be delayed
> until after we reply to the backend to avoid deadlocks. Otherwise,
> the MemoryListener would send a VHOST_USER_SET_MEM_TABLE message
> before the reply.
>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Albert Esteve <aesteve@redhat.com>
> ---
>  hw/virtio/vhost-user.c                    | 267 ++++++++++++++++++++++
>  hw/virtio/virtio.c                        | 199 ++++++++++++++++
>  include/hw/virtio/virtio.h                | 135 +++++++++++
>  subprojects/libvhost-user/libvhost-user.c |  70 ++++++
>  subprojects/libvhost-user/libvhost-user.h |  54 +++++
>  5 files changed, 725 insertions(+)
>
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index 36c9c2e04d..890be55937 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -104,6 +104,7 @@ typedef enum VhostUserRequest {
>      VHOST_USER_GET_SHARED_OBJECT = 41,
>      VHOST_USER_SET_DEVICE_STATE_FD = 42,
>      VHOST_USER_CHECK_DEVICE_STATE = 43,
> +    VHOST_USER_GET_SHMEM_CONFIG = 44,
>      VHOST_USER_MAX
>  } VhostUserRequest;
>
> @@ -115,6 +116,8 @@ typedef enum VhostUserBackendRequest {
>      VHOST_USER_BACKEND_SHARED_OBJECT_ADD = 6,
>      VHOST_USER_BACKEND_SHARED_OBJECT_REMOVE = 7,
>      VHOST_USER_BACKEND_SHARED_OBJECT_LOOKUP = 8,
> +    VHOST_USER_BACKEND_SHMEM_MAP = 9,
> +    VHOST_USER_BACKEND_SHMEM_UNMAP = 10,
>      VHOST_USER_BACKEND_MAX
>  }  VhostUserBackendRequest;
>
> @@ -136,6 +139,12 @@ typedef struct VhostUserMemRegMsg {
>      VhostUserMemoryRegion region;
>  } VhostUserMemRegMsg;
>
> +typedef struct VhostUserShMemConfig {
> +    uint32_t nregions;
> +    uint32_t padding;
> +    uint64_t memory_sizes[VIRTIO_MAX_SHMEM_REGIONS];
> +} VhostUserShMemConfig;
> +
>  typedef struct VhostUserLog {
>      uint64_t mmap_size;
>      uint64_t mmap_offset;
> @@ -192,6 +201,23 @@ typedef struct VhostUserShared {
>      unsigned char uuid[16];
>  } VhostUserShared;
>
> +/* For the flags field of VhostUserMMap */
> +#define VHOST_USER_FLAG_MAP_RW (1u << 0)
> +
> +typedef struct {
> +    /* VIRTIO Shared Memory Region ID */
> +    uint8_t shmid;
> +    uint8_t padding[7];
> +    /* File offset */
> +    uint64_t fd_offset;
> +    /* Offset within the VIRTIO Shared Memory Region */
> +    uint64_t shm_offset;
> +    /* Size of the mapping */
> +    uint64_t len;
> +    /* Flags for the mmap operation, from VHOST_USER_FLAG_MAP_* */
> +    uint64_t flags;
> +} VhostUserMMap;
> +
>  typedef struct {
>      VhostUserRequest request;
>
> @@ -224,6 +250,8 @@ typedef union {
>          VhostUserInflight inflight;
>          VhostUserShared object;
>          VhostUserTransferDeviceState transfer_state;
> +        VhostUserMMap mmap;
> +        VhostUserShMemConfig shmem;
>  } VhostUserPayload;
>
>  typedef struct VhostUserMsg {
> @@ -1768,6 +1796,196 @@ vhost_user_backend_handle_shared_object_lookup(struct vhost_user *u,
>      return 0;
>  }
>
> +/**
> + * vhost_user_backend_handle_shmem_map() - Handle SHMEM_MAP backend request
> + * @dev: vhost device
> + * @ioc: QIOChannel for communication
> + * @hdr: vhost-user message header
> + * @payload: message payload containing mapping details
> + * @fd: file descriptor for the shared memory region
> + *
> + * Handles VHOST_USER_BACKEND_SHMEM_MAP requests from the backend. Creates
> + * a VhostUserShmemObject to manage the shared memory mapping and adds it
> + * to the appropriate VirtIO shared memory region. The VhostUserShmemObject
> + * serves as an intermediate parent for the MemoryRegion, ensuring proper
> + * lifecycle management with reference counting.
> + *
> + * Returns: 0 on success, negative errno on failure
> + */
> +static int
> +vhost_user_backend_handle_shmem_map(struct vhost_dev *dev,
> +                                    QIOChannel *ioc,
> +                                    VhostUserHeader *hdr,
> +                                    VhostUserPayload *payload,
> +                                    int fd)
> +{
> +    VirtioSharedMemory *shmem;
> +    VhostUserMMap *vu_mmap = &payload->mmap;
> +    VirtioSharedMemoryMapping *existing;
> +    Error *local_err = NULL;
> +    int ret = 0;
> +
> +    if (fd < 0) {
> +        error_report("Bad fd for map");
> +        ret = -EBADF;
> +        goto send_reply;
> +    }
> +
> +    if (QSIMPLEQ_EMPTY(&dev->vdev->shmem_list)) {
> +        error_report("Device has no VIRTIO Shared Memory Regions. "
> +                     "Requested ID: %d", vu_mmap->shmid);
> +        ret = -EFAULT;
> +        goto send_reply;
> +    }
> +
> +    shmem = virtio_find_shmem_region(dev->vdev, vu_mmap->shmid);
> +    if (!shmem) {
> +        error_report("VIRTIO Shared Memory Region at "
> +                     "ID %d not found or uninitialized", vu_mmap->shmid);
> +        ret = -EFAULT;
> +        goto send_reply;
> +    }
> +
> +    if ((vu_mmap->shm_offset + vu_mmap->len) < vu_mmap->len ||
> +        (vu_mmap->shm_offset + vu_mmap->len) > shmem->mr.size) {
> +        error_report("Bad offset/len for mmap %" PRIx64 "+%" PRIx64,
> +                     vu_mmap->shm_offset, vu_mmap->len);
> +        ret = -EFAULT;
> +        goto send_reply;
> +    }
> +
> +    QTAILQ_FOREACH(existing, &shmem->mmaps, link) {
> +        if (ranges_overlap(existing->offset, existing->len,
> +                           vu_mmap->shm_offset, vu_mmap->len)) {
> +            error_report("VIRTIO Shared Memory mapping overlap");
> +            ret = -EFAULT;
> +            goto send_reply;
> +        }
> +    }
> +
> +    memory_region_transaction_begin();
> +
> +    /* Create VirtioSharedMemoryMapping object */
> +    VirtioSharedMemoryMapping *mapping = virtio_shared_memory_mapping_new(
> +        vu_mmap->shmid, fd, vu_mmap->fd_offset, vu_mmap->shm_offset,
> +        vu_mmap->len, vu_mmap->flags & VHOST_USER_FLAG_MAP_RW);
> +
> +    if (!mapping) {
> +        ret = -EFAULT;
> +        goto send_reply_commit;
> +    }
> +
> +    /* Add the mapping to the shared memory region */
> +    if (virtio_add_shmem_map(shmem, mapping) != 0) {
> +        error_report("Failed to add shared memory mapping");
> +        object_unref(OBJECT(mapping));
> +        ret = -EFAULT;
> +        goto send_reply_commit;
> +    }
> +
> +send_reply_commit:
> +    /* Send reply and commit after transaction started */
> +    if (hdr->flags & VHOST_USER_NEED_REPLY_MASK) {
> +        payload->u64 = !!ret;
> +        hdr->size = sizeof(payload->u64);
> +        if (!vhost_user_send_resp(ioc, hdr, payload, &local_err)) {
> +            error_report_err(local_err);
> +            memory_region_transaction_commit();
> +            return -EFAULT;
> +        }
> +    }
> +    memory_region_transaction_commit();
> +    return 0;
> +
> +send_reply:
> +    if (hdr->flags & VHOST_USER_NEED_REPLY_MASK) {
> +        payload->u64 = !!ret;
> +        hdr->size = sizeof(payload->u64);
> +        if (!vhost_user_send_resp(ioc, hdr, payload, &local_err)) {
> +            error_report_err(local_err);
> +            return -EFAULT;
> +        }
> +    }
> +    return 0;
> +}
> +
> +/**
> + * vhost_user_backend_handle_shmem_unmap() - Handle SHMEM_UNMAP backend request
> + * @dev: vhost device
> + * @ioc: QIOChannel for communication
> + * @hdr: vhost-user message header
> + * @payload: message payload containing unmapping details
> + *
> + * Handles VHOST_USER_BACKEND_SHMEM_UNMAP requests from the backend. Removes
> + * the specified memory mapping from the VirtIO shared memory region. This
> + * automatically unreferences the associated VhostUserShmemObject, which may
> + * trigger its finalization and cleanup (munmap, close fd) if no other
> + * references exist.
> + *
> + * Returns: 0 on success, negative errno on failure
> + */
> +static int
> +vhost_user_backend_handle_shmem_unmap(struct vhost_dev *dev,
> +                                      QIOChannel *ioc,
> +                                      VhostUserHeader *hdr,
> +                                      VhostUserPayload *payload)
> +{
> +    VirtioSharedMemory *shmem;
> +    VirtioSharedMemoryMapping *mmap = NULL;
> +    VhostUserMMap *vu_mmap = &payload->mmap;
> +    Error *local_err = NULL;
> +    int ret = 0;
> +
> +    if (QSIMPLEQ_EMPTY(&dev->vdev->shmem_list)) {
> +        error_report("Device has no VIRTIO Shared Memory Regions. "
> +                     "Requested ID: %d", vu_mmap->shmid);
> +        ret = -EFAULT;
> +        goto send_reply;
> +    }
> +
> +    shmem = virtio_find_shmem_region(dev->vdev, vu_mmap->shmid);
> +    if (!shmem) {
> +        error_report("VIRTIO Shared Memory Region at "
> +                     "ID %d not found or uninitialized", vu_mmap->shmid);
> +        ret = -EFAULT;
> +        goto send_reply;
> +    }
> +
> +    if ((vu_mmap->shm_offset + vu_mmap->len) < vu_mmap->len ||
> +        (vu_mmap->shm_offset + vu_mmap->len) > shmem->mr.size) {
> +        error_report("Bad offset/len for unmmap %" PRIx64 "+%" PRIx64,
> +                     vu_mmap->shm_offset, vu_mmap->len);
> +        ret = -EFAULT;
> +        goto send_reply;
> +    }
> +
> +    mmap = virtio_find_shmem_map(shmem, vu_mmap->shm_offset, vu_mmap->len);
> +    if (!mmap) {
> +        error_report("Shared memory mapping not found at offset %" PRIx64
> +                     " with length %" PRIx64,
> +                     vu_mmap->shm_offset, vu_mmap->len);
> +        ret = -EFAULT;
> +        goto send_reply;
> +    }
> +
> +send_reply:
> +    if (hdr->flags & VHOST_USER_NEED_REPLY_MASK) {
> +        payload->u64 = !!ret;
> +        hdr->size = sizeof(payload->u64);
> +        if (!vhost_user_send_resp(ioc, hdr, payload, &local_err)) {
> +            error_report_err(local_err);
> +            return -EFAULT;
> +        }
> +    }
> +
> +    if (!ret && mmap) {
> +        /* Free the MemoryRegion only after reply */
> +        virtio_del_shmem_map(shmem, vu_mmap->shm_offset, vu_mmap->len);
> +    }
> +
> +    return 0;
> +}
> +
>  static void close_backend_channel(struct vhost_user *u)
>  {
>      g_source_destroy(u->backend_src);
> @@ -1836,6 +2054,19 @@ static gboolean backend_read(QIOChannel *ioc, GIOCondition condition,
>          ret = vhost_user_backend_handle_shared_object_lookup(dev->opaque, ioc,
>                                                               &hdr, &payload);
>          break;
> +    case VHOST_USER_BACKEND_SHMEM_MAP:
> +        /* Handler manages its own response, check error and close connection */
> +        if (vhost_user_backend_handle_shmem_map(dev, ioc, &hdr, &payload,
> +                                                fd ? fd[0] : -1) < 0) {
> +            goto err;
> +        }
> +        break;
> +    case VHOST_USER_BACKEND_SHMEM_UNMAP:
> +        /* Handler manages its own response, check error and close connection */
> +        if (vhost_user_backend_handle_shmem_unmap(dev, ioc, &hdr, &payload) < 0) {
> +            goto err;
> +        }
> +        break;

Once this patch lands:
https://lists.gnu.org/archive/html/qemu-devel/2025-10/msg03932.html
These two handlers will need a `reply_ack = false;` before being
invoked. What's the best way to proceed in this case?

If I can chose, I'd prefer to integrate this one first and then I can
rebase the one I linked and set the reply_ack where needed.

>      default:
>          error_report("Received unexpected msg type: %d.", hdr.request);
>          ret = -EINVAL;
> @@ -3013,6 +3244,41 @@ static int vhost_user_check_device_state(struct vhost_dev *dev, Error **errp)
>      return 0;
>  }
>
> +static int vhost_user_get_shmem_config(struct vhost_dev *dev,
> +                                       int *nregions,
> +                                       uint64_t *memory_sizes,
> +                                       Error **errp)
> +{
> +    int ret;
> +    VhostUserMsg msg = {
> +        .hdr.request = VHOST_USER_GET_SHMEM_CONFIG,
> +        .hdr.flags = VHOST_USER_VERSION,
> +    };
> +
> +    if (!virtio_has_feature(dev->protocol_features,
> +                            VHOST_USER_PROTOCOL_F_SHMEM)) {
> +        *nregions = 0;
> +        return 0;
> +    }
> +
> +    ret = vhost_user_write(dev, &msg, NULL, 0);
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    ret = vhost_user_read(dev, &msg);
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    assert(msg.payload.shmem.nregions <= VIRTIO_MAX_SHMEM_REGIONS);
> +    *nregions = msg.payload.shmem.nregions;
> +    memcpy(memory_sizes,
> +           &msg.payload.shmem.memory_sizes,
> +           sizeof(uint64_t) * VIRTIO_MAX_SHMEM_REGIONS);
> +    return 0;
> +}
> +
>  const VhostOps user_ops = {
>          .backend_type = VHOST_BACKEND_TYPE_USER,
>          .vhost_backend_init = vhost_user_backend_init,
> @@ -3051,4 +3317,5 @@ const VhostOps user_ops = {
>          .vhost_supports_device_state = vhost_user_supports_device_state,
>          .vhost_set_device_state_fd = vhost_user_set_device_state_fd,
>          .vhost_check_device_state = vhost_user_check_device_state,
> +        .vhost_get_shmem_config = vhost_user_get_shmem_config,
>  };
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 153ee0a0cf..f96ed43c18 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -3086,6 +3086,173 @@ int virtio_save(VirtIODevice *vdev, QEMUFile *f)
>      return vmstate_save_state(f, &vmstate_virtio, vdev, NULL, &error_fatal);
>  }
>
> +VirtioSharedMemory *virtio_new_shmem_region(VirtIODevice *vdev, uint8_t shmid, uint64_t size)
> +{
> +    VirtioSharedMemory *elem;
> +    g_autofree char *name = NULL;
> +
> +    elem = g_new0(VirtioSharedMemory, 1);
> +    elem->shmid = shmid;
> +
> +    /* Initialize embedded MemoryRegion as container for shmem mappings */
> +    name = g_strdup_printf("virtio-shmem-%d", shmid);
> +    memory_region_init(&elem->mr, OBJECT(vdev), name, size);
> +    QTAILQ_INIT(&elem->mmaps);
> +    QSIMPLEQ_INSERT_TAIL(&vdev->shmem_list, elem, entry);
> +    return elem;
> +}
> +
> +VirtioSharedMemory *virtio_find_shmem_region(VirtIODevice *vdev, uint8_t shmid)
> +{
> +    VirtioSharedMemory *shmem, *next;
> +    QSIMPLEQ_FOREACH_SAFE(shmem, &vdev->shmem_list, entry, next) {
> +        if (shmem->shmid == shmid) {
> +            return shmem;
> +        }
> +    }
> +    return NULL;
> +}
> +
> +static void virtio_shared_memory_mapping_instance_init(Object *obj)
> +{
> +    VirtioSharedMemoryMapping *mapping = VIRTIO_SHARED_MEMORY_MAPPING(obj);
> +
> +    mapping->shmid = 0;
> +    mapping->offset = 0;
> +    mapping->len = 0;
> +    mapping->mr = NULL;
> +}
> +
> +static void virtio_shared_memory_mapping_instance_finalize(Object *obj)
> +{
> +    VirtioSharedMemoryMapping *mapping = VIRTIO_SHARED_MEMORY_MAPPING(obj);
> +
> +    /* Clean up MemoryRegion if it exists */
> +    if (mapping->mr) {
> +        /* Unparent the MemoryRegion to trigger cleanup */
> +        object_unparent(OBJECT(mapping->mr));
> +        mapping->mr = NULL;
> +    }
> +}
> +
> +VirtioSharedMemoryMapping *virtio_shared_memory_mapping_new(uint8_t shmid,
> +                                                            int fd,
> +                                                            uint64_t fd_offset,
> +                                                            uint64_t shm_offset,
> +                                                            uint64_t len,
> +                                                            bool allow_write)
> +{
> +    VirtioSharedMemoryMapping *mapping;
> +    MemoryRegion *mr;
> +    g_autoptr(GString) mr_name = g_string_new(NULL);
> +    uint32_t ram_flags;
> +    Error *local_err = NULL;
> +
> +    if (len == 0) {
> +        error_report("Shared memory mapping size cannot be zero");
> +        return NULL;
> +    }
> +
> +    fd = dup(fd);
> +    if (fd < 0) {
> +        error_report("Failed to duplicate fd: %s", strerror(errno));
> +        return NULL;
> +    }
> +
> +    /* Determine RAM flags */
> +    ram_flags = RAM_SHARED;
> +    if (!allow_write) {
> +        ram_flags |= RAM_READONLY_FD;
> +    }
> +
> +    /* Create the VirtioSharedMemoryMapping */
> +    mapping = VIRTIO_SHARED_MEMORY_MAPPING(
> +        object_new(TYPE_VIRTIO_SHARED_MEMORY_MAPPING));
> +
> +    /* Set up object properties */
> +    mapping->shmid = shmid;
> +    mapping->offset = shm_offset;
> +    mapping->len = len;
> +
> +    /* Create MemoryRegion as a child of this object */
> +    mr = g_new0(MemoryRegion, 1);
> +    g_string_printf(mr_name, "virtio-shmem-%d-%" PRIx64, shmid, shm_offset);
> +
> +    /* Initialize MemoryRegion with file descriptor */
> +    if (!memory_region_init_ram_from_fd(mr, OBJECT(mapping), mr_name->str,
> +                                        len, ram_flags, fd, fd_offset,
> +                                        &local_err)) {
> +        error_report_err(local_err);
> +        g_free(mr);
> +        close(fd);
> +        object_unref(OBJECT(mapping));
> +        return NULL;
> +    }
> +
> +    mapping->mr = mr;
> +    return mapping;
> +}
> +
> +int virtio_add_shmem_map(VirtioSharedMemory *shmem,
> +                         VirtioSharedMemoryMapping *mapping)
> +{
> +    if (!mapping) {
> +        error_report("VirtioSharedMemoryMapping cannot be NULL");
> +        return -1;
> +    }
> +    if (!mapping->mr) {
> +        error_report("VirtioSharedMemoryMapping has no MemoryRegion");
> +        return -1;
> +    }
> +
> +    /* Validate boundaries against the VIRTIO shared memory region */
> +    if (mapping->offset + mapping->len > shmem->mr.size) {
> +        error_report("Memory exceeds the shared memory boundaries");
> +        return -1;
> +    }
> +
> +    /* Add as subregion to the VIRTIO shared memory */
> +    memory_region_add_subregion(&shmem->mr, mapping->offset, mapping->mr);
> +
> +    /* Add to the mapped regions list */
> +    QTAILQ_INSERT_TAIL(&shmem->mmaps, mapping, link);
> +
> +    return 0;
> +}
> +
> +VirtioSharedMemoryMapping *virtio_find_shmem_map(VirtioSharedMemory *shmem,
> +                                          hwaddr offset, uint64_t size)
> +{
> +    VirtioSharedMemoryMapping *mapping;
> +    QTAILQ_FOREACH(mapping, &shmem->mmaps, link) {
> +        if (mapping->offset == offset && mapping->mr->size == size) {
> +            return mapping;
> +        }
> +    }
> +    return NULL;
> +}
> +
> +void virtio_del_shmem_map(VirtioSharedMemory *shmem, hwaddr offset,
> +                          uint64_t size)
> +{
> +    VirtioSharedMemoryMapping *mapping = virtio_find_shmem_map(shmem, offset, size);
> +    if (mapping == NULL) {
> +        return;
> +    }
> +
> +    /*
> +     * Remove from memory region first
> +     */
> +    memory_region_del_subregion(&shmem->mr, mapping->mr);
> +
> +    /*
> +     * Remove from list and unref the mapping which will trigger automatic cleanup
> +     * when the reference count reaches zero.
> +     */
> +    QTAILQ_REMOVE(&shmem->mmaps, mapping, link);
> +    object_unref(OBJECT(mapping));
> +}
> +
>  /* A wrapper for use as a VMState .put function */
>  static int virtio_device_put(QEMUFile *f, void *opaque, size_t size,
>                                const VMStateField *field, JSONWriter *vmdesc)
> @@ -3212,6 +3379,7 @@ void virtio_reset(void *opaque)
>  {
>      VirtIODevice *vdev = opaque;
>      VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
> +    VirtioSharedMemory *shmem;
>      uint64_t features[VIRTIO_FEATURES_NU64S];
>      int i;
>
> @@ -3251,6 +3419,14 @@ void virtio_reset(void *opaque)
>      for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
>          __virtio_queue_reset(vdev, i);
>      }
> +
> +    /* Mappings are removed to prevent stale fds from remaining open. */
> +    QSIMPLEQ_FOREACH(shmem, &vdev->shmem_list, entry) {
> +        while (!QTAILQ_EMPTY(&shmem->mmaps)) {
> +            VirtioSharedMemoryMapping *mapping = QTAILQ_FIRST(&shmem->mmaps);
> +            virtio_del_shmem_map(shmem, mapping->offset, mapping->mr->size);
> +        }
> +    }
>  }
>
>  static void virtio_device_check_notification_compatibility(VirtIODevice *vdev,
> @@ -3574,6 +3750,7 @@ void virtio_init(VirtIODevice *vdev, uint16_t device_id, size_t config_size)
>              NULL, virtio_vmstate_change, vdev);
>      vdev->device_endian = virtio_default_endian();
>      vdev->use_guest_notifier_mask = true;
> +    QSIMPLEQ_INIT(&vdev->shmem_list);
>  }
>
>  /*
> @@ -4085,11 +4262,24 @@ static void virtio_device_free_virtqueues(VirtIODevice *vdev)
>  static void virtio_device_instance_finalize(Object *obj)
>  {
>      VirtIODevice *vdev = VIRTIO_DEVICE(obj);
> +    VirtioSharedMemory *shmem;
>
>      virtio_device_free_virtqueues(vdev);
>
>      g_free(vdev->config);
>      g_free(vdev->vector_queues);
> +    while (!QSIMPLEQ_EMPTY(&vdev->shmem_list)) {
> +        shmem = QSIMPLEQ_FIRST(&vdev->shmem_list);
> +        while (!QTAILQ_EMPTY(&shmem->mmaps)) {
> +            VirtioSharedMemoryMapping *mapping = QTAILQ_FIRST(&shmem->mmaps);
> +            virtio_del_shmem_map(shmem, mapping->offset, mapping->mr->size);
> +        }
> +
> +        /* Clean up the embedded MemoryRegion */
> +        object_unparent(OBJECT(&shmem->mr));
> +        QSIMPLEQ_REMOVE_HEAD(&vdev->shmem_list, entry);
> +        g_free(shmem);
> +    }
>  }
>
>  static const Property virtio_properties[] = {
> @@ -4455,9 +4645,18 @@ static const TypeInfo virtio_device_info = {
>      .class_size = sizeof(VirtioDeviceClass),
>  };
>
> +static const TypeInfo virtio_shared_memory_mapping_info = {
> +    .name = TYPE_VIRTIO_SHARED_MEMORY_MAPPING,
> +    .parent = TYPE_OBJECT,
> +    .instance_size = sizeof(VirtioSharedMemoryMapping),
> +    .instance_init = virtio_shared_memory_mapping_instance_init,
> +    .instance_finalize = virtio_shared_memory_mapping_instance_finalize,
> +};
> +
>  static void virtio_register_types(void)
>  {
>      type_register_static(&virtio_device_info);
> +    type_register_static(&virtio_shared_memory_mapping_info);
>  }
>
>  type_init(virtio_register_types)
> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> index d97529c3f1..3f6dfba321 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -99,6 +99,45 @@ enum virtio_device_endian {
>      VIRTIO_DEVICE_ENDIAN_BIG,
>  };
>
> +#define TYPE_VIRTIO_SHARED_MEMORY_MAPPING "virtio-shared-memory-mapping"
> +OBJECT_DECLARE_SIMPLE_TYPE(VirtioSharedMemoryMapping, VIRTIO_SHARED_MEMORY_MAPPING)
> +
> +/**
> + * VirtioSharedMemoryMapping:
> + * @parent: Parent QOM object
> + * @shmid: VIRTIO Shared Memory Region ID
> + * @fd: File descriptor for the shared memory region
> + * @offset: Offset within the VIRTIO Shared Memory Region
> + * @len: Size of the mapping
> + * @mr: MemoryRegion associated with this shared memory mapping
> + * @link: List entry for the shared memory region's mapping list
> + *
> + * A QOM object that represents an individual file descriptor-based shared
> + * memory mapping within a VIRTIO Shared Memory Region. It manages the
> + * MemoryRegion lifecycle and file descriptor cleanup through QOM reference
> + * counting. When the object is unreferenced and its reference count drops
> + * to zero, it automatically cleans up the MemoryRegion and closes the file
> + * descriptor.
> + */
> +struct VirtioSharedMemoryMapping {
> +    Object parent;
> +
> +    uint8_t shmid;
> +    hwaddr offset;
> +    uint64_t len;
> +    MemoryRegion *mr;
> +    QTAILQ_ENTRY(VirtioSharedMemoryMapping) link;
> +};
> +
> +struct VirtioSharedMemory {
> +    uint8_t shmid;
> +    MemoryRegion mr;
> +    QTAILQ_HEAD(, VirtioSharedMemoryMapping) mmaps;
> +    QSIMPLEQ_ENTRY(VirtioSharedMemory) entry;
> +};
> +
> +typedef struct VirtioSharedMemory VirtioSharedMemory;
> +
>  /**
>   * struct VirtIODevice - common VirtIO structure
>   * @name: name of the device
> @@ -168,6 +207,8 @@ struct VirtIODevice
>       */
>      EventNotifier config_notifier;
>      bool device_iotlb_enabled;
> +    /* Shared memory region for mappings. */
> +    QSIMPLEQ_HEAD(, VirtioSharedMemory) shmem_list;
>  };
>
>  struct VirtioDeviceClass {
> @@ -298,6 +339,100 @@ void virtio_notify(VirtIODevice *vdev, VirtQueue *vq);
>
>  int virtio_save(VirtIODevice *vdev, QEMUFile *f);
>
> +/**
> + * virtio_new_shmem_region() - Create a new shared memory region
> + * @vdev: VirtIODevice
> + * @shmid: Shared memory ID
> + * @size: Size of the shared memory region
> + *
> + * Creates a new VirtioSharedMemory region for the given device and ID.
> + * The returned VirtioSharedMemory is owned by the VirtIODevice and will
> + * be automatically freed when the device is destroyed. The caller
> + * should not free the returned pointer.
> + *
> + * Returns: Pointer to the new VirtioSharedMemory region, or NULL on failure
> + */
> +VirtioSharedMemory *virtio_new_shmem_region(VirtIODevice *vdev, uint8_t shmid, uint64_t size);
> +
> +/**
> + * virtio_find_shmem_region() - Find an existing shared memory region
> + * @vdev: VirtIODevice
> + * @shmid: Shared memory ID to find
> + *
> + * Finds an existing VirtioSharedMemory region by ID. The returned pointer
> + * is owned by the VirtIODevice and should not be freed by the caller.
> + *
> + * Returns: Pointer to the VirtioSharedMemory region, or NULL if not found
> + */
> +VirtioSharedMemory *virtio_find_shmem_region(VirtIODevice *vdev, uint8_t shmid);
> +
> +/**
> + * virtio_shared_memory_mapping_new() - Create a new VirtioSharedMemoryMapping
> + * @shmid: VIRTIO Shared Memory Region ID
> + * @fd: File descriptor for the shared memory
> + * @fd_offset: Offset within the file descriptor
> + * @shm_offset: Offset within the VIRTIO Shared Memory Region
> + * @len: Size of the mapping
> + * @allow_write: Whether to allow write access to the mapping
> + *
> + * Creates a new VirtioSharedMemoryMapping that manages a shared memory mapping.
> + * The object will create a MemoryRegion using memory_region_init_ram_from_fd()
> + * as a child object. When the object is finalized, it will automatically
> + * clean up the MemoryRegion and close the file descriptor.
> + *
> + * Return: A new VirtioSharedMemoryMapping on success, NULL on error.
> + */
> +VirtioSharedMemoryMapping *virtio_shared_memory_mapping_new(uint8_t shmid,
> +                                                            int fd,
> +                                                            uint64_t fd_offset,
> +                                                            uint64_t shm_offset,
> +                                                            uint64_t len,
> +                                                            bool allow_write);
> +
> +/**
> + * virtio_add_shmem_map() - Add a memory mapping to a shared region
> + * @shmem: VirtioSharedMemory region
> + * @mapping: VirtioSharedMemoryMapping to add (transfers ownership)
> + *
> + * Adds a memory mapping to the shared memory region. The VirtioSharedMemoryMapping
> + * ownership is transferred to the shared memory region and will be automatically
> + * cleaned up through QOM reference counting when virtio_del_shmem_map() is
> + * called or when the shared memory region is destroyed.
> + *
> + * Returns: 0 on success, negative errno on failure
> + */
> +int virtio_add_shmem_map(VirtioSharedMemory *shmem,
> +                         VirtioSharedMemoryMapping *mapping);
> +
> +/**
> + * virtio_find_shmem_map() - Find a memory mapping in a shared region
> + * @shmem: VirtioSharedMemory region
> + * @offset: Offset within the shared memory region
> + * @size: Size of the mapping to find
> + *
> + * Finds an existing memory mapping that covers the specified range.
> + * The returned VirtioSharedMemoryMapping is owned by the VirtioSharedMemory
> + * region and should not be freed by the caller.
> + *
> + * Returns: Pointer to the VirtioSharedMemoryMapping, or NULL if not found
> + */
> +VirtioSharedMemoryMapping *virtio_find_shmem_map(VirtioSharedMemory *shmem,
> +                                          hwaddr offset, uint64_t size);
> +
> +/**
> + * virtio_del_shmem_map() - Remove a memory mapping from a shared region
> + * @shmem: VirtioSharedMemory region
> + * @offset: Offset of the mapping to remove
> + * @size: Size of the mapping to remove
> + *
> + * Removes a memory mapping from the shared memory region. This will
> + * automatically unref the associated VhostUserShmemObject, which may
> + * trigger its finalization and cleanup if no other references exist.
> + * The mapping's MemoryRegion will be properly unmapped and cleaned up.
> + */
> +void virtio_del_shmem_map(VirtioSharedMemory *shmem, hwaddr offset,
> +                          uint64_t size);
> +
>  extern const VMStateInfo virtio_vmstate_info;
>
>  #define VMSTATE_VIRTIO_DEVICE \
> diff --git a/subprojects/libvhost-user/libvhost-user.c b/subprojects/libvhost-user/libvhost-user.c
> index 9c630c2170..034cbfdc3c 100644
> --- a/subprojects/libvhost-user/libvhost-user.c
> +++ b/subprojects/libvhost-user/libvhost-user.c
> @@ -1592,6 +1592,76 @@ vu_rm_shared_object(VuDev *dev, unsigned char uuid[UUID_LEN])
>      return vu_send_message(dev, &msg);
>  }
>
> +bool
> +vu_shmem_map(VuDev *dev, uint8_t shmid, uint64_t fd_offset,
> +             uint64_t shm_offset, uint64_t len, uint64_t flags, int fd)
> +{
> +    VhostUserMsg vmsg = {
> +        .request = VHOST_USER_BACKEND_SHMEM_MAP,
> +        .size = sizeof(vmsg.payload.mmap),
> +        .flags = VHOST_USER_VERSION,
> +        .payload.mmap = {
> +            .shmid = shmid,
> +            .fd_offset = fd_offset,
> +            .shm_offset = shm_offset,
> +            .len = len,
> +            .flags = flags,
> +        },
> +        .fd_num = 1,
> +        .fds[0] = fd,
> +    };
> +
> +    if (!vu_has_protocol_feature(dev, VHOST_USER_PROTOCOL_F_SHMEM)) {
> +        return false;
> +    }
> +
> +    if (vu_has_protocol_feature(dev, VHOST_USER_PROTOCOL_F_REPLY_ACK)) {
> +        vmsg.flags |= VHOST_USER_NEED_REPLY_MASK;
> +    }
> +
> +    pthread_mutex_lock(&dev->backend_mutex);
> +    if (!vu_message_write(dev, dev->backend_fd, &vmsg)) {
> +        pthread_mutex_unlock(&dev->backend_mutex);
> +        return false;
> +    }
> +
> +    /* Also unlocks the backend_mutex */
> +    return vu_process_message_reply(dev, &vmsg);
> +}
> +
> +bool
> +vu_shmem_unmap(VuDev *dev, uint8_t shmid, uint64_t shm_offset, uint64_t len)
> +{
> +    VhostUserMsg vmsg = {
> +        .request = VHOST_USER_BACKEND_SHMEM_UNMAP,
> +        .size = sizeof(vmsg.payload.mmap),
> +        .flags = VHOST_USER_VERSION,
> +        .payload.mmap = {
> +            .shmid = shmid,
> +            .fd_offset = 0,
> +            .shm_offset = shm_offset,
> +            .len = len,
> +        },
> +    };
> +
> +    if (!vu_has_protocol_feature(dev, VHOST_USER_PROTOCOL_F_SHMEM)) {
> +        return false;
> +    }
> +
> +    if (vu_has_protocol_feature(dev, VHOST_USER_PROTOCOL_F_REPLY_ACK)) {
> +        vmsg.flags |= VHOST_USER_NEED_REPLY_MASK;
> +    }
> +
> +    pthread_mutex_lock(&dev->backend_mutex);
> +    if (!vu_message_write(dev, dev->backend_fd, &vmsg)) {
> +        pthread_mutex_unlock(&dev->backend_mutex);
> +        return false;
> +    }
> +
> +    /* Also unlocks the backend_mutex */
> +    return vu_process_message_reply(dev, &vmsg);
> +}
> +
>  static bool
>  vu_set_vring_call_exec(VuDev *dev, VhostUserMsg *vmsg)
>  {
> diff --git a/subprojects/libvhost-user/libvhost-user.h b/subprojects/libvhost-user/libvhost-user.h
> index 2ffc58c11b..6a2d0f9fae 100644
> --- a/subprojects/libvhost-user/libvhost-user.h
> +++ b/subprojects/libvhost-user/libvhost-user.h
> @@ -69,6 +69,8 @@ enum VhostUserProtocolFeature {
>      /* Feature 16 is reserved for VHOST_USER_PROTOCOL_F_STATUS. */
>      /* Feature 17 reserved for VHOST_USER_PROTOCOL_F_XEN_MMAP. */
>      VHOST_USER_PROTOCOL_F_SHARED_OBJECT = 18,
> +    /* Feature 19 is reserved for VHOST_USER_PROTOCOL_F_DEVICE_STATE */
> +    VHOST_USER_PROTOCOL_F_SHMEM = 20,
>      VHOST_USER_PROTOCOL_F_MAX
>  };
>
> @@ -127,6 +129,8 @@ typedef enum VhostUserBackendRequest {
>      VHOST_USER_BACKEND_SHARED_OBJECT_ADD = 6,
>      VHOST_USER_BACKEND_SHARED_OBJECT_REMOVE = 7,
>      VHOST_USER_BACKEND_SHARED_OBJECT_LOOKUP = 8,
> +    VHOST_USER_BACKEND_SHMEM_MAP = 9,
> +    VHOST_USER_BACKEND_SHMEM_UNMAP = 10,
>      VHOST_USER_BACKEND_MAX
>  }  VhostUserBackendRequest;
>
> @@ -186,6 +190,23 @@ typedef struct VhostUserShared {
>      unsigned char uuid[UUID_LEN];
>  } VhostUserShared;
>
> +/* For the flags field of VhostUserMMap */
> +#define VHOST_USER_FLAG_MAP_RW (1u << 0)
> +
> +typedef struct {
> +    /* VIRTIO Shared Memory Region ID */
> +    uint8_t shmid;
> +    uint8_t padding[7];
> +    /* File offset */
> +    uint64_t fd_offset;
> +    /* Offset within the VIRTIO Shared Memory Region */
> +    uint64_t shm_offset;
> +    /* Size of the mapping */
> +    uint64_t len;
> +    /* Flags for the mmap operation, from VHOST_USER_FLAG_MAP_* */
> +    uint64_t flags;
> +} VhostUserMMap;
> +
>  #define VU_PACKED __attribute__((packed))
>
>  typedef struct VhostUserMsg {
> @@ -210,6 +231,7 @@ typedef struct VhostUserMsg {
>          VhostUserVringArea area;
>          VhostUserInflight inflight;
>          VhostUserShared object;
> +        VhostUserMMap mmap;
>      } payload;
>
>      int fds[VHOST_MEMORY_BASELINE_NREGIONS];
> @@ -593,6 +615,38 @@ bool vu_add_shared_object(VuDev *dev, unsigned char uuid[UUID_LEN]);
>   */
>  bool vu_rm_shared_object(VuDev *dev, unsigned char uuid[UUID_LEN]);
>
> +/**
> + * vu_shmem_map:
> + * @dev: a VuDev context
> + * @shmid: VIRTIO Shared Memory Region ID
> + * @fd_offset: File offset
> + * @shm_offset: Offset within the VIRTIO Shared Memory Region
> + * @len: Size of the mapping
> + * @flags: Flags for the mmap operation
> + * @fd: A file descriptor
> + *
> + * Advertises a new mapping to be made in a given VIRTIO Shared Memory Region.
> + *
> + * Returns: TRUE on success, FALSE on failure.
> + */
> +bool vu_shmem_map(VuDev *dev, uint8_t shmid, uint64_t fd_offset,
> +                  uint64_t shm_offset, uint64_t len, uint64_t flags, int fd);
> +
> +/**
> + * vu_shmem_unmap:
> + * @dev: a VuDev context
> + * @shmid: VIRTIO Shared Memory Region ID
> + * @fd_offset: File offset
> + * @len: Size of the mapping
> + *
> + * The front-end un-mmaps a given range in the VIRTIO Shared Memory Region
> + * with the requested `shmid`.
> + *
> + * Returns: TRUE on success, FALSE on failure.
> + */
> +bool vu_shmem_unmap(VuDev *dev, uint8_t shmid, uint64_t shm_offset,
> +                    uint64_t len);
> +
>  /**
>   * vu_queue_set_notification:
>   * @dev: a VuDev context
> --
> 2.49.0
>