diff --git a/hw/vfio/pci.h b/hw/vfio/pci.h
index ec9f345..643ff75 100644
--- a/hw/vfio/pci.h
+++ b/hw/vfio/pci.h
@@ -194,6 +194,7 @@ struct VFIOUserPCIDevice {
     VFIOPCIDevice device;
     char *sock_name;
     bool send_queued;   /* all sends are queued */
+    bool no_post;       /* all regions write are sync */
 };
 
 /* Use uin32_t for vendor & device so PCI_ANY_ID expands and cannot match hw */
diff --git a/hw/vfio/user-protocol.h b/hw/vfio/user-protocol.h
index 104bf4f..56904cf 100644
--- a/hw/vfio/user-protocol.h
+++ b/hw/vfio/user-protocol.h
@@ -109,4 +109,16 @@ typedef struct {
     uint64_t offset;
 } VFIOUserRegionInfo;
 
+/*
+ * VFIO_USER_REGION_READ
+ * VFIO_USER_REGION_WRITE
+ */
+typedef struct {
+    VFIOUserHdr hdr;
+    uint64_t offset;
+    uint32_t region;
+    uint32_t count;
+    char data[];
+} VFIOUserRegionRW;
+
 #endif /* VFIO_USER_PROTOCOL_H */
diff --git a/hw/vfio/user.h b/hw/vfio/user.h
index 19edd84..f2098f2 100644
--- a/hw/vfio/user.h
+++ b/hw/vfio/user.h
@@ -75,6 +75,7 @@ typedef struct VFIOProxy {
 /* VFIOProxy flags */
 #define VFIO_PROXY_CLIENT        0x1
 #define VFIO_PROXY_FORCE_QUEUED  0x4
+#define VFIO_PROXY_NO_POST       0x8
 
 VFIOProxy *vfio_user_connect_dev(SocketAddress *addr, Error **errp);
 void vfio_user_disconnect(VFIOProxy *proxy);
diff --git a/include/hw/vfio/vfio-common.h b/include/hw/vfio/vfio-common.h
index e2d7ee1..b498964 100644
--- a/include/hw/vfio/vfio-common.h
+++ b/include/hw/vfio/vfio-common.h
@@ -56,6 +56,7 @@ typedef struct VFIORegion {
     uint32_t nr_mmaps;
     VFIOMmap *mmaps;
     uint8_t nr; /* cache the region number for debug */
+    bool post_wr; /* writes can be posted */
     int remfd; /* fd if exported from remote process */
 } VFIORegion;
 
diff --git a/hw/vfio/common.c b/hw/vfio/common.c
index 47ec28f..e19f321 100644
--- a/hw/vfio/common.c
+++ b/hw/vfio/common.c
@@ -213,6 +213,7 @@ void vfio_region_write(void *opaque, hwaddr addr,
         uint32_t dword;
         uint64_t qword;
     } buf;
+    bool post = region->post_wr;
     int ret;
 
     switch (size) {
@@ -233,7 +234,11 @@ void vfio_region_write(void *opaque, hwaddr addr,
         break;
     }
 
-    ret = VDEV_REGION_WRITE(vbasedev, region->nr, addr, size, &buf, false);
+    /* read-after-write hazard if guest can directly access region */
+    if (region->nr_mmaps) {
+        post = false;
+    }
+    ret = VDEV_REGION_WRITE(vbasedev, region->nr, addr, size, &buf, post);
     if (ret != size) {
         const char *err = ret < 0 ? strerror(-ret) : "short write";
 
diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
index 40eb9e6..d5f9987 100644
--- a/hw/vfio/pci.c
+++ b/hw/vfio/pci.c
@@ -1665,6 +1665,9 @@ static void vfio_bar_prepare(VFIOPCIDevice *vdev, int nr)
     bar->type = pci_bar & (bar->ioport ? ~PCI_BASE_ADDRESS_IO_MASK :
                                          ~PCI_BASE_ADDRESS_MEM_MASK);
     bar->size = bar->region.size;
+
+    /* IO regions are sync, memory can be async */
+    bar->region.post_wr = (bar->ioport == 0);
 }
 
 static void vfio_bars_prepare(VFIOPCIDevice *vdev)
@@ -3513,6 +3516,9 @@ static void vfio_user_pci_realize(PCIDevice *pdev, Error **errp)
     if (udev->send_queued) {
         proxy->flags |= VFIO_PROXY_FORCE_QUEUED;
     }
+    if (udev->no_post) {
+        proxy->flags |= VFIO_PROXY_NO_POST;
+    }
 
     vfio_user_validate_version(vbasedev, &err);
     if (err != NULL) {
@@ -3565,6 +3571,7 @@ static void vfio_user_instance_finalize(Object *obj)
 static Property vfio_user_pci_dev_properties[] = {
     DEFINE_PROP_STRING("socket", VFIOUserPCIDevice, sock_name),
     DEFINE_PROP_BOOL("x-send-queued", VFIOUserPCIDevice, send_queued, false),
+    DEFINE_PROP_BOOL("x-no-posted-writes", VFIOUserPCIDevice, no_post, false),
     DEFINE_PROP_END_OF_LIST(),
 };
 
diff --git a/hw/vfio/user.c b/hw/vfio/user.c
index b40c4ed..781cbfd 100644
--- a/hw/vfio/user.c
+++ b/hw/vfio/user.c
@@ -50,6 +50,8 @@ static void vfio_user_cb(void *opaque);
 
 static void vfio_user_request(void *opaque);
 static int vfio_user_send_queued(VFIOProxy *proxy, VFIOUserMsg *msg);
+static void vfio_user_send_async(VFIOProxy *proxy, VFIOUserHdr *hdr,
+                                 VFIOUserFDs *fds);
 static void vfio_user_send_wait(VFIOProxy *proxy, VFIOUserHdr *hdr,
                                 VFIOUserFDs *fds, int rsize, bool nobql);
 static void vfio_user_request_msg(VFIOUserHdr *hdr, uint16_t cmd,
@@ -533,6 +535,33 @@ static int vfio_user_send_queued(VFIOProxy *proxy, VFIOUserMsg *msg)
     return 0;
 }
 
+/*
+ * async send - msg can be queued, but will be freed when sent
+ */
+static void vfio_user_send_async(VFIOProxy *proxy, VFIOUserHdr *hdr,
+                                 VFIOUserFDs *fds)
+{
+    VFIOUserMsg *msg;
+    int ret;
+
+    if (!(hdr->flags & (VFIO_USER_NO_REPLY|VFIO_USER_REPLY))) {
+        error_printf("vfio_user_send_async on sync message\n");
+        return;
+    }
+
+    QEMU_LOCK_GUARD(&proxy->lock);
+
+    msg = vfio_user_getmsg(proxy, hdr, fds);
+    msg->id = hdr->id;
+    msg->rsize = 0;
+    msg->type = VFIO_MSG_ASYNC;
+
+    ret = vfio_user_send_queued(proxy, msg);
+    if (ret < 0) {
+        vfio_user_recycle(proxy, msg);
+    }
+}
+
 static void vfio_user_send_wait(VFIOProxy *proxy, VFIOUserHdr *hdr,
                                 VFIOUserFDs *fds, int rsize, bool nobql)
 {
@@ -957,6 +986,62 @@ static int vfio_user_get_region_info(VFIOProxy *proxy,
     return 0;
 }
 
+static int vfio_user_region_read(VFIOProxy *proxy, uint8_t index, off_t offset,
+                                 uint32_t count, void *data)
+{
+    g_autofree VFIOUserRegionRW *msgp = NULL;
+    int size = sizeof(*msgp) + count;
+
+    msgp = g_malloc0(size);
+    vfio_user_request_msg(&msgp->hdr, VFIO_USER_REGION_READ, sizeof(*msgp), 0);
+    msgp->offset = offset;
+    msgp->region = index;
+    msgp->count = count;
+
+    vfio_user_send_wait(proxy, &msgp->hdr, NULL, size, false);
+    if (msgp->hdr.flags & VFIO_USER_ERROR) {
+        return -msgp->hdr.error_reply;
+    } else if (msgp->count > count) {
+        return -E2BIG;
+    } else {
+        memcpy(data, &msgp->data, msgp->count);
+    }
+
+    return msgp->count;
+}
+
+static int vfio_user_region_write(VFIOProxy *proxy, uint8_t index, off_t offset,
+                                  uint32_t count, void *data, bool post)
+{
+    VFIOUserRegionRW *msgp = NULL;
+    int flags = post ? VFIO_USER_NO_REPLY : 0;
+    int size = sizeof(*msgp) + count;
+    int ret;
+
+    msgp = g_malloc0(size);
+    vfio_user_request_msg(&msgp->hdr, VFIO_USER_REGION_WRITE, size, flags);
+    msgp->offset = offset;
+    msgp->region = index;
+    msgp->count = count;
+    memcpy(&msgp->data, data, count);
+
+    /* async send will free msg after it's sent */
+    if (post && !(proxy->flags & VFIO_PROXY_NO_POST)) {
+        vfio_user_send_async(proxy, &msgp->hdr, NULL);
+        return count;
+    }
+
+    vfio_user_send_wait(proxy, &msgp->hdr, NULL, 0, false);
+    if (msgp->hdr.flags & VFIO_USER_ERROR) {
+        ret = -msgp->hdr.error_reply;
+    } else {
+        ret = count;
+    }
+
+    g_free(msgp);
+    return ret;
+}
+
 
 /*
  * Socket-based io_ops
@@ -990,8 +1075,24 @@ static int vfio_user_io_get_region_info(VFIODevice *vbasedev,
     return VDEV_VALID_REGION_INFO(vbasedev, info, fd);
 }
 
+static int vfio_user_io_region_read(VFIODevice *vbasedev, uint8_t index,
+                                    off_t off, uint32_t size, void *data)
+{
+    return vfio_user_region_read(vbasedev->proxy, index, off, size, data);
+}
+
+static int vfio_user_io_region_write(VFIODevice *vbasedev, uint8_t index,
+                                     off_t off, unsigned size, void *data,
+                                     bool post)
+{
+    return vfio_user_region_write(vbasedev->proxy, index, off, size, data,
+                                  post);
+}
+
 VFIODevIO vfio_dev_io_sock = {
     .get_info = vfio_user_io_get_info,
     .get_region_info = vfio_user_io_get_region_info,
+    .region_read = vfio_user_io_region_read,
+    .region_write = vfio_user_io_region_write,
 };
 
