diff --git a/hw/vfio/user-protocol.h b/hw/vfio/user-protocol.h
new file mode 100644
index 0000000..d23877c
--- /dev/null
+++ b/hw/vfio/user-protocol.h
@@ -0,0 +1,54 @@
+#ifndef VFIO_USER_PROTOCOL_H
+#define VFIO_USER_PROTOCOL_H
+
+/*
+ * vfio protocol over a UNIX socket.
+ *
+ * Copyright © 2018, 2021 Oracle and/or its affiliates.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.  See
+ * the COPYING file in the top-level directory.
+ *
+ * Each message has a standard header that describes the command
+ * being sent, which is almost always a VFIO ioctl().
+ *
+ * The header may be followed by command-specific data, such as the
+ * region and offset info for read and write commands.
+ */
+
+typedef struct {
+    uint16_t id;
+    uint16_t command;
+    uint32_t size;
+    uint32_t flags;
+    uint32_t error_reply;
+} VFIOUserHdr;
+
+/* VFIOUserHdr commands */
+enum vfio_user_command {
+    VFIO_USER_VERSION                   = 1,
+    VFIO_USER_DMA_MAP                   = 2,
+    VFIO_USER_DMA_UNMAP                 = 3,
+    VFIO_USER_DEVICE_GET_INFO           = 4,
+    VFIO_USER_DEVICE_GET_REGION_INFO    = 5,
+    VFIO_USER_DEVICE_GET_REGION_IO_FDS  = 6,
+    VFIO_USER_DEVICE_GET_IRQ_INFO       = 7,
+    VFIO_USER_DEVICE_SET_IRQS           = 8,
+    VFIO_USER_REGION_READ               = 9,
+    VFIO_USER_REGION_WRITE              = 10,
+    VFIO_USER_DMA_READ                  = 11,
+    VFIO_USER_DMA_WRITE                 = 12,
+    VFIO_USER_DEVICE_RESET              = 13,
+    VFIO_USER_DIRTY_PAGES               = 14,
+    VFIO_USER_MAX,
+};
+
+/* VFIOUserHdr flags */
+#define VFIO_USER_REQUEST       0x0
+#define VFIO_USER_REPLY         0x1
+#define VFIO_USER_TYPE          0xF
+
+#define VFIO_USER_NO_REPLY      0x10
+#define VFIO_USER_ERROR         0x20
+
+#endif /* VFIO_USER_PROTOCOL_H */
diff --git a/hw/vfio/user.h b/hw/vfio/user.h
index da92862..68a1080 100644
--- a/hw/vfio/user.h
+++ b/hw/vfio/user.h
@@ -11,6 +11,8 @@
  *
  */
 
+#include "user-protocol.h"
+
 typedef struct {
     int send_fds;
     int recv_fds;
@@ -27,6 +29,7 @@ enum msg_type {
 
 typedef struct VFIOUserMsg {
     QTAILQ_ENTRY(VFIOUserMsg) next;
+    VFIOUserHdr *hdr;
     VFIOUserFDs *fds;
     uint32_t rsize;
     uint32_t id;
@@ -66,6 +69,8 @@ typedef struct VFIOProxy {
     VFIOUserMsgQ incoming;
     VFIOUserMsgQ outgoing;
     VFIOUserMsg *last_nowait;
+    VFIOUserMsg *part_recv;
+    size_t recv_left;
     enum proxy_state state;
 } VFIOProxy;
 
@@ -74,5 +79,8 @@ typedef struct VFIOProxy {
 
 VFIOProxy *vfio_user_connect_dev(SocketAddress *addr, Error **errp);
 void vfio_user_disconnect(VFIOProxy *proxy);
+void vfio_user_set_handler(VFIODevice *vbasedev,
+                           void (*handler)(void *opaque, VFIOUserMsg *msg),
+                           void *reqarg);
 
 #endif /* VFIO_USER_H */
diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
index 8f65074..7ef11c0 100644
--- a/hw/vfio/pci.c
+++ b/hw/vfio/pci.c
@@ -3387,6 +3387,11 @@ type_init(register_vfio_pci_dev_type)
  * vfio-user routines.
  */
 
+static void vfio_user_pci_process_req(void *opaque, VFIOUserMsg *msg)
+{
+
+}
+
 /*
  * Emulated devices don't use host hot reset
  */
@@ -3433,6 +3438,7 @@ static void vfio_user_pci_realize(PCIDevice *pdev, Error **errp)
         return;
     }
     vbasedev->proxy = proxy;
+    vfio_user_set_handler(vbasedev, vfio_user_pci_process_req, vdev);
 
     vbasedev->name = g_strdup_printf("VFIO user <%s>", udev->sock_name);
     vbasedev->dev = DEVICE(vdev);
diff --git a/hw/vfio/user.c b/hw/vfio/user.c
index c843f90..16b37cb 100644
--- a/hw/vfio/user.c
+++ b/hw/vfio/user.c
@@ -25,10 +25,26 @@
 #include "sysemu/iothread.h"
 #include "user.h"
 
+static uint64_t max_xfer_size;
 static IOThread *vfio_user_iothread;
 
 static void vfio_user_shutdown(VFIOProxy *proxy);
+static VFIOUserMsg *vfio_user_getmsg(VFIOProxy *proxy, VFIOUserHdr *hdr,
+                                     VFIOUserFDs *fds);
+static VFIOUserFDs *vfio_user_getfds(int numfds);
+static void vfio_user_recycle(VFIOProxy *proxy, VFIOUserMsg *msg);
 
+static void vfio_user_recv(void *opaque);
+static int vfio_user_recv_one(VFIOProxy *proxy);
+static void vfio_user_cb(void *opaque);
+
+static void vfio_user_request(void *opaque);
+
+static inline void vfio_user_set_error(VFIOUserHdr *hdr, uint32_t err)
+{
+    hdr->flags |= VFIO_USER_ERROR;
+    hdr->error_reply = err;
+}
 
 /*
  * Functions called by main, CPU, or iothread threads
@@ -40,10 +56,338 @@ static void vfio_user_shutdown(VFIOProxy *proxy)
     qio_channel_set_aio_fd_handler(proxy->ioc, proxy->ctx, NULL, NULL, NULL);
 }
 
+static VFIOUserMsg *vfio_user_getmsg(VFIOProxy *proxy, VFIOUserHdr *hdr,
+                                     VFIOUserFDs *fds)
+{
+    VFIOUserMsg *msg;
+
+    msg = QTAILQ_FIRST(&proxy->free);
+    if (msg != NULL) {
+        QTAILQ_REMOVE(&proxy->free, msg, next);
+    } else {
+        msg = g_malloc0(sizeof(*msg));
+        qemu_cond_init(&msg->cv);
+    }
+
+    msg->hdr = hdr;
+    msg->fds = fds;
+    return msg;
+}
+
+/*
+ * Recycle a message list entry to the free list.
+ */
+static void vfio_user_recycle(VFIOProxy *proxy, VFIOUserMsg *msg)
+{
+    if (msg->type == VFIO_MSG_NONE) {
+        error_printf("vfio_user_recycle - freeing free msg\n");
+        return;
+    }
+
+    /* free msg buffer if no one is waiting to consume the reply */
+    if (msg->type == VFIO_MSG_NOWAIT || msg->type == VFIO_MSG_ASYNC) {
+        g_free(msg->hdr);
+        if (msg->fds != NULL) {
+            g_free(msg->fds);
+        }
+    }
+
+    msg->type = VFIO_MSG_NONE;
+    msg->hdr = NULL;
+    msg->fds = NULL;
+    msg->complete = false;
+    QTAILQ_INSERT_HEAD(&proxy->free, msg, next);
+}
+
+static VFIOUserFDs *vfio_user_getfds(int numfds)
+{
+    VFIOUserFDs *fds = g_malloc0(sizeof(*fds) + (numfds * sizeof(int)));
+
+    fds->fds = (int *)((char *)fds + sizeof(*fds));
+
+    return fds;
+}
+
 /*
  * Functions only called by iothread
  */
 
+/*
+ * Process a received message.
+ */
+static void vfio_user_process(VFIOProxy *proxy, VFIOUserMsg *msg, bool isreply)
+{
+
+    /*
+     * Replies signal a waiter, if none just check for errors
+     * and free the message buffer.
+     *
+     * Requests get queued for the BH.
+     */
+    if (isreply) {
+        msg->complete = true;
+        if (msg->type == VFIO_MSG_WAIT) {
+            qemu_cond_signal(&msg->cv);
+        } else {
+            if (msg->hdr->flags & VFIO_USER_ERROR) {
+                error_printf("vfio_user_rcv:  error reply on async request ");
+                error_printf("command %x error %s\n", msg->hdr->command,
+                             strerror(msg->hdr->error_reply));
+            }
+            /* youngest nowait msg has been ack'd */
+            if (proxy->last_nowait == msg) {
+                proxy->last_nowait = NULL;
+            }
+            vfio_user_recycle(proxy, msg);
+        }
+    } else {
+        QTAILQ_INSERT_TAIL(&proxy->incoming, msg, next);
+        qemu_bh_schedule(proxy->req_bh);
+    }
+}
+
+/*
+ * Complete a partial message read
+ */
+static int vfio_user_complete(VFIOProxy *proxy, Error **errp)
+{
+    VFIOUserMsg *msg = proxy->part_recv;
+    size_t msgleft = proxy->recv_left;
+    bool isreply;
+    char *data;
+    int ret;
+
+    data = (char *)msg->hdr + (msg->hdr->size - msgleft);
+    while (msgleft > 0) {
+        ret = qio_channel_read(proxy->ioc, data, msgleft, errp);
+
+        /* error or would block */
+        if (ret <= 0) {
+            /* try for rest on next iternation */
+            if (ret == QIO_CHANNEL_ERR_BLOCK) {
+                proxy->recv_left = msgleft;
+            }
+            return ret;
+        }
+
+        msgleft -= ret;
+        data += ret;
+    }
+
+    /*
+     * Read complete message, process it.
+     */
+    proxy->part_recv = NULL;
+    proxy->recv_left = 0;
+    isreply = (msg->hdr->flags & VFIO_USER_TYPE) == VFIO_USER_REPLY;
+    vfio_user_process(proxy, msg, isreply);
+
+    /* return positive value */
+    return 1;
+}
+
+static void vfio_user_recv(void *opaque)
+{
+    VFIOProxy *proxy = opaque;
+
+    QEMU_LOCK_GUARD(&proxy->lock);
+
+    if (proxy->state == VFIO_PROXY_CONNECTED) {
+        while (vfio_user_recv_one(proxy) == 0) {
+            ;
+        }
+    }
+}
+
+/*
+ * Receive and process one incoming message.
+ *
+ * For replies, find matching outgoing request and wake any waiters.
+ * For requests, queue in incoming list and run request BH.
+ */
+static int vfio_user_recv_one(VFIOProxy *proxy)
+{
+    VFIOUserMsg *msg = NULL;
+    g_autofree int *fdp = NULL;
+    VFIOUserFDs *reqfds;
+    VFIOUserHdr hdr;
+    struct iovec iov = {
+        .iov_base = &hdr,
+        .iov_len = sizeof(hdr),
+    };
+    bool isreply = false;
+    int i, ret;
+    size_t msgleft, numfds = 0;
+    char *data = NULL;
+    char *buf = NULL;
+    Error *local_err = NULL;
+
+    /*
+     * Complete any partial reads
+     */
+    if (proxy->part_recv != NULL) {
+        ret = vfio_user_complete(proxy, &local_err);
+
+        /* still not complete, try later */
+        if (ret == QIO_CHANNEL_ERR_BLOCK) {
+            return ret;
+        }
+
+        if (ret <= 0) {
+            goto fatal;
+        }
+        /* else fall into reading another msg */
+    }
+
+    /*
+     * Read header
+     */
+    ret = qio_channel_readv_full(proxy->ioc, &iov, 1, &fdp, &numfds,
+                                 &local_err);
+    if (ret == QIO_CHANNEL_ERR_BLOCK) {
+        return ret;
+    }
+
+    /* read error or other side closed connection */
+    if (ret <= 0) {
+        goto fatal;
+    }
+
+    if (ret < sizeof(msg)) {
+        error_setg(&local_err, "short read of header");
+        goto fatal;
+    }
+
+    /*
+     * Validate header
+     */
+    if (hdr.size < sizeof(VFIOUserHdr)) {
+        error_setg(&local_err, "bad header size");
+        goto fatal;
+    }
+    switch (hdr.flags & VFIO_USER_TYPE) {
+    case VFIO_USER_REQUEST:
+        isreply = false;
+        break;
+    case VFIO_USER_REPLY:
+        isreply = true;
+        break;
+    default:
+        error_setg(&local_err, "unknown message type");
+        goto fatal;
+    }
+
+    /*
+     * For replies, find the matching pending request.
+     * For requests, reap incoming FDs.
+     */
+    if (isreply) {
+        QTAILQ_FOREACH(msg, &proxy->pending, next) {
+            if (hdr.id == msg->id) {
+                break;
+            }
+        }
+        if (msg == NULL) {
+            error_setg(&local_err, "unexpected reply");
+            goto err;
+        }
+        QTAILQ_REMOVE(&proxy->pending, msg, next);
+
+        /*
+         * Process any received FDs
+         */
+        if (numfds != 0) {
+            if (msg->fds == NULL || msg->fds->recv_fds < numfds) {
+                error_setg(&local_err, "unexpected FDs");
+                goto err;
+            }
+            msg->fds->recv_fds = numfds;
+            memcpy(msg->fds->fds, fdp, numfds * sizeof(int));
+        }
+    } else {
+        if (numfds != 0) {
+            reqfds = vfio_user_getfds(numfds);
+            memcpy(reqfds->fds, fdp, numfds * sizeof(int));
+        } else {
+            reqfds = NULL;
+        }
+    }
+
+    /*
+     * Put the whole message into a single buffer.
+     */
+    if (isreply) {
+        if (hdr.size > msg->rsize) {
+            error_setg(&local_err, "reply larger than recv buffer");
+            goto err;
+        }
+        *msg->hdr = hdr;
+        data = (char *)msg->hdr + sizeof(hdr);
+    } else {
+        if (hdr.size > max_xfer_size) {
+            error_setg(&local_err, "vfio_user_recv request larger than max");
+            goto err;
+        }
+        buf = g_malloc0(hdr.size);
+        memcpy(buf, &hdr, sizeof(hdr));
+        data = buf + sizeof(hdr);
+        msg = vfio_user_getmsg(proxy, (VFIOUserHdr *)buf, reqfds);
+        msg->type = VFIO_MSG_REQ;
+    }
+
+    /*
+     * Read rest of message.
+     */
+    msgleft = hdr.size - sizeof(hdr);
+    while (msgleft > 0) {
+        ret = qio_channel_read(proxy->ioc, data, msgleft, &local_err);
+
+        /* prepare to complete read on next iternation */
+        if (ret == QIO_CHANNEL_ERR_BLOCK) {
+            proxy->part_recv = msg;
+            proxy->recv_left = msgleft;
+            return ret;
+        }
+
+        if (ret <= 0) {
+            goto fatal;
+        }
+
+        msgleft -= ret;
+        data += ret;
+    }
+
+    vfio_user_process(proxy, msg, isreply);
+    return 0;
+
+    /*
+     * fatal means the other side closed or we don't trust the stream
+     * err means this message is corrupt
+     */
+fatal:
+    vfio_user_shutdown(proxy);
+    proxy->state = VFIO_PROXY_ERROR;
+
+    /* set error if server side closed */
+    if (ret == 0) {
+        error_setg(&local_err, "server closed socket");
+    }
+
+err:
+    for (i = 0; i < numfds; i++) {
+        close(fdp[i]);
+    }
+    if (isreply && msg != NULL) {
+        /* force an error to keep sending thread from hanging */
+        vfio_user_set_error(msg->hdr, EINVAL);
+        msg->complete = true;
+        qemu_cond_signal(&msg->cv);
+    }
+    error_prepend(&local_err, "vfio_user_recv: ");
+    error_report_err(local_err);
+    return -1;
+}
+
 static void vfio_user_cb(void *opaque)
 {
     VFIOProxy *proxy = opaque;
@@ -59,6 +403,51 @@ static void vfio_user_cb(void *opaque)
  * Functions called by main or CPU threads
  */
 
+/*
+ * Process incoming requests.
+ *
+ * The bus-specific callback has the form:
+ *    request(opaque, msg)
+ * where 'opaque' was specified in vfio_user_set_handler
+ * and 'msg' is the inbound message.
+ *
+ * The callback is responsible for disposing of the message buffer,
+ * usually by re-using it when calling vfio_send_reply or vfio_send_error,
+ * both of which free their message buffer when the reply is sent.
+ *
+ * If the callback uses a new buffer, it needs to free the old one.
+ */
+static void vfio_user_request(void *opaque)
+{
+    VFIOProxy *proxy = opaque;
+    VFIOUserMsgQ new, free;
+    VFIOUserMsg *msg, *m1;
+
+    /* reap all incoming */
+    QTAILQ_INIT(&new);
+    WITH_QEMU_LOCK_GUARD(&proxy->lock) {
+        QTAILQ_FOREACH_SAFE(msg, &proxy->incoming, next, m1) {
+            QTAILQ_REMOVE(&proxy->pending, msg, next);
+            QTAILQ_INSERT_TAIL(&new, msg, next);
+        }
+    }
+
+    /* process list */
+    QTAILQ_INIT(&free);
+    QTAILQ_FOREACH_SAFE(msg, &new, next, m1) {
+        QTAILQ_REMOVE(&new, msg, next);
+        proxy->request(proxy->req_arg, msg);
+        QTAILQ_INSERT_HEAD(&free, msg, next);
+    }
+
+    /* free list */
+    WITH_QEMU_LOCK_GUARD(&proxy->lock) {
+        QTAILQ_FOREACH_SAFE(msg, &free, next, m1) {
+            vfio_user_recycle(proxy, msg);
+        }
+    }
+}
+
 static QLIST_HEAD(, VFIOProxy) vfio_user_sockets =
     QLIST_HEAD_INITIALIZER(vfio_user_sockets);
 
@@ -97,6 +486,7 @@ VFIOProxy *vfio_user_connect_dev(SocketAddress *addr, Error **errp)
     }
 
     proxy->ctx = iothread_get_aio_context(vfio_user_iothread);
+    proxy->req_bh = qemu_bh_new(vfio_user_request, proxy);
 
     QTAILQ_INIT(&proxy->outgoing);
     QTAILQ_INIT(&proxy->incoming);
@@ -107,6 +497,18 @@ VFIOProxy *vfio_user_connect_dev(SocketAddress *addr, Error **errp)
     return proxy;
 }
 
+void vfio_user_set_handler(VFIODevice *vbasedev,
+                           void (*handler)(void *opaque, VFIOUserMsg *msg),
+                           void *req_arg)
+{
+    VFIOProxy *proxy = vbasedev->proxy;
+
+    proxy->request = handler;
+    proxy->req_arg = req_arg;
+    qio_channel_set_aio_fd_handler(proxy->ioc, proxy->ctx,
+                                   vfio_user_recv, NULL, proxy);
+}
+
 void vfio_user_disconnect(VFIOProxy *proxy)
 {
     VFIOUserMsg *r1, *r2;
@@ -122,6 +524,8 @@ void vfio_user_disconnect(VFIOProxy *proxy)
     }
     object_unref(OBJECT(proxy->ioc));
     proxy->ioc = NULL;
+    qemu_bh_delete(proxy->req_bh);
+    proxy->req_bh = NULL;
 
     proxy->state = VFIO_PROXY_CLOSING;
     QTAILQ_FOREACH_SAFE(r1, &proxy->outgoing, next, r2) {
diff --git a/MAINTAINERS b/MAINTAINERS
index cd44f91..c81f8b6 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1964,6 +1964,7 @@ S: Supported
 F: docs/devel/vfio-user.rst
 F: hw/vfio/user.c
 F: hw/vfio/user.h
+F: hw/vfio/user-protocol.h
 
 vhost
 M: Michael S. Tsirkin <mst@redhat.com>
