Re: [PATCH v9 1/7] vhost-user: Add VirtIO Shared Memory map request

[Albert Esteve <aesteve@redhat.com>]

On Wed, Oct 15, 2025 at 2:51 PM Manos Pitsidianakis
<manos.pitsidianakis@linaro.org> wrote:
>
> On Wed, 15 Oct 2025 15:29, Albert Esteve <aesteve@redhat.com> wrote:
> >On Wed, Oct 15, 2025 at 11:52 AM Manos Pitsidianakis
> ><manos.pitsidianakis@linaro.org> wrote:
> >>
> >> On Wed, 15 Oct 2025 11:59, 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.
> >> >
> >> >Signed-off-by: Albert Esteve <aesteve@redhat.com>
> >> >---
> >> > hw/virtio/vhost-user.c                    | 213 ++++++++++++++++++++++
> >> > hw/virtio/virtio.c                        | 207 +++++++++++++++++++++
> >> > include/hw/virtio/virtio.h                | 136 ++++++++++++++
> >> > subprojects/libvhost-user/libvhost-user.c |  70 +++++++
> >> > subprojects/libvhost-user/libvhost-user.h |  54 ++++++
> >> > 5 files changed, 680 insertions(+)
> >> >
> >> >diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> >> >index 36c9c2e04d..8a4385a5ad 100644
> >> >--- a/hw/virtio/vhost-user.c
> >> >+++ b/hw/virtio/vhost-user.c
> >> >@@ -115,6 +115,8 @@ 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;
> >> >
> >> >@@ -192,6 +194,23 @@ 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 +243,7 @@ typedef union {
> >> >         VhostUserInflight inflight;
> >> >         VhostUserShared object;
> >> >         VhostUserTransferDeviceState transfer_state;
> >> >+        VhostUserMMap mmap;
> >> > } VhostUserPayload;
> >> >
> >> > typedef struct VhostUserMsg {
> >> >@@ -1768,6 +1788,186 @@ 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;
> >>
> >> Apologies if I missed something obvious, but won't going to send_reply
> >> at this point cause memory_region_transaction_commit() to be called?
> >> Maybe we should have two cleanup labels, depending on whether
> >> memory_region_transaction_begin() has been called yet?
> >
> >You are right, this is not good, at transaction_depth 0 it would fail
> >an assertion within memory_region_transaction_commit().
> >
> >Not sure another cleanup label will work, though, as the transaction
> >commit has to go after the reply. Maybe we can make the reply within
> >the handler optional (leveraging the REPLY_MASK flag). But that will
> >need to be tuned when this patch lands:
> >https://lists.gnu.org/archive/html/qemu-devel/2025-10/msg03632.html
> >
> >Or just a simpler solution like this would do?
>
>
> Yes this looks like enough. The another cleanup label alternative I
> meant would be to do:
>
> +send_reply_no_commit:
> +    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;
> +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);
> +            memory_region_transaction_commit();
> +            return -EFAULT;
> +        }
> +    }
> +
> +    memory_region_transaction_commit();
> +
> +    return 0;
> +}
>
> But this is not much prettier. (In rust this would be trivial with a MR
> transaction RAII object...)

I tried this solution too and it fails with `Illegal instruction (core
dumped)`. I am not sure why, the logic should be basically the same.
It has me puzzled.

So... I'll use the boolean variable solution. That one seems to work fine.

>
> >
> >diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> >index c39cf52dbc..46126eed3b 100644
> >--- a/hw/virtio/vhost-user.c
> >+++ b/hw/virtio/vhost-user.c
> >@@ -1824,6 +1824,7 @@ vhost_user_backend_handle_shmem_map(struct vhost_dev *dev,
> >     VirtioSharedMemoryMapping *existing;
> >     Error *local_err = NULL;
> >     int ret = 0;
> >+    bool transaction_started = false;
> >
> >     if (fd < 0) {
> >         error_report("Bad fd for map");
> >@@ -1864,6 +1865,7 @@ vhost_user_backend_handle_shmem_map(struct vhost_dev *dev,
> >     }
> >
> >     memory_region_transaction_begin();
> >+    transaction_started = true;
> >
> >     /* Create VirtioSharedMemoryMapping object */
> >     VirtioSharedMemoryMapping *mapping = virtio_shared_memory_mapping_new(
> >@@ -1889,14 +1891,18 @@ send_reply:
> >         hdr->size = sizeof(payload->u64);
> >         if (!vhost_user_send_resp(ioc, hdr, payload, &local_err)) {
> >             error_report_err(local_err);
> >-            memory_region_transaction_commit();
> >+            if (transaction_started) {
> >+                memory_region_transaction_commit();
> >+            }
> >             return -EFAULT;
> >         }
> >     }
> >
> >-    memory_region_transaction_commit();
> >-
> >-    return 0;
> >+    /* Only commit if we started a transaction */
> >+    if (transaction_started) {
> >+        memory_region_transaction_commit();
> >+    }
> >+    return ret;
> > }
> >
> >>
> >> >+    }
> >> >+
> >> >+    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;
> >> >+    }
> >> >+
> >> >+    /* 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;
> >> >+    }
> >> >+
> >> >+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);
> >> >+            memory_region_transaction_commit();
> >> >+            return -EFAULT;
> >> >+        }
> >> >+    }
> >> >+
> >> >+    memory_region_transaction_commit();
> >> >+
> >> >+    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 +2036,19 @@ 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;
> >> >     default:
> >> >         error_report("Received unexpected msg type: %d.", hdr.request);
> >> >         ret = -EINVAL;
> >> >diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> >> >index 153ee0a0cf..37070e7951 100644
> >> >--- a/hw/virtio/virtio.c
> >> >+++ b/hw/virtio/virtio.c
> >> >@@ -3086,6 +3086,181 @@ 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->fd = -1;
> >> >+    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;
> >> >+    }
> >> >+
> >> >+    /* Close file descriptor */
> >> >+    if (mapping->fd >= 0) {
> >> >+        close(mapping->fd);
> >> >+        mapping->fd = -1;
> >> >+    }
> >> >+}
> >> >+
> >> >+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->fd = fd;
> >> >+    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 +3387,7 @@ 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 +3427,14 @@ 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 +3758,7 @@ 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 +4270,24 @@ 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 +4653,18 @@ 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..cee807e236 100644
> >> >--- a/include/hw/virtio/virtio.h
> >> >+++ b/include/hw/virtio/virtio.h
> >> >@@ -99,6 +99,46 @@ 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;
> >> >+    int fd;
> >> >+    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 +208,8 @@ struct VirtIODevice
> >> >      */
> >> >     EventNotifier config_notifier;
> >> >     bool device_iotlb_enabled;
> >> >+    /* Shared memory region for mappings. */
> >> >+    QSIMPLEQ_HEAD(, VirtioSharedMemory) shmem_list;
> >> > };
> >> >
> >> > struct VirtioDeviceClass {
> >> >@@ -298,6 +340,100 @@ 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
> >> >
> >>
> >
>