diff --git a/docs/usb-storage.txt b/docs/usb-storage.txt
new file mode 100644
index 0000000..ff97559
--- /dev/null
+++ b/docs/usb-storage.txt
@@ -0,0 +1,38 @@
+
+qemu usb storage emulation
+--------------------------
+
+Qemu has two emulations for usb storage devices.
+
+Number one emulates the classic bulk-only transport protocol which is
+used by 99% of the usb sticks on the marked today and is called
+"usb-storage".  Usage (hooking up to xhci, other host controllers work
+too):
+
+  qemu ${other_vm_args}                                \
+       -drive if=none,id=stick,file=/path/to/file.img  \
+       -device nec-usb-xhci,id=xhci                    \
+       -device usb-storage,bus=xhci.0,drive=stick
+
+
+Number two is the newer usb attached scsi transport.  This one doesn't
+automagically create a scsi disk, so you have to explicitly attach one
+manually.  Multiple logical units are supported.  Here is an example
+with tree logical units:
+
+  qemu ${other_vm_args}                                                \
+       -drive if=none,id=uas-disk1,file=/path/to/file1.img             \
+       -drive if=none,id=uas-disk2,file=/path/to/file2.img             \
+       -drive if=none,id=uas-cdrom,media=cdrom,file=/path/to/image.iso \
+       -device nec-usb-xhci,id=xhci                                    \
+       -device usb-uas,id=uas,bus=xhci.0                               \
+       -device scsi-hd,bus=uas.0,scsi-id=0,lun=0,drive=uas-disk1       \
+       -device scsi-hd,bus=uas.0,scsi-id=0,lun=1,drive=uas-disk2       \
+       -device scsi-cd,bus=uas.0,scsi-id=0,lun=5,drive=uas-cdrom
+
+
+enjoy,
+  Gerd
+
+--
+Gerd Hoffmann <kraxel@redhat.com>
diff --git a/hw/scsi-bus.c b/hw/scsi-bus.c
index 5ad1013..dc74063 100644
--- a/hw/scsi-bus.c
+++ b/hw/scsi-bus.c
@@ -1354,6 +1354,7 @@
 
 SCSIRequest *scsi_req_ref(SCSIRequest *req)
 {
+    assert(req->refcount > 0);
     req->refcount++;
     return req;
 }
@@ -1362,6 +1363,10 @@
 {
     assert(req->refcount > 0);
     if (--req->refcount == 0) {
+        SCSIBus *bus = DO_UPCAST(SCSIBus, qbus, req->dev->qdev.parent_bus);
+        if (bus->info->free_request && req->hba_private) {
+            bus->info->free_request(bus, req->hba_private);
+        }
         if (req->ops->free_req) {
             req->ops->free_req(req);
         }
diff --git a/hw/scsi.h b/hw/scsi.h
index 76f06d4..367a346 100644
--- a/hw/scsi.h
+++ b/hw/scsi.h
@@ -134,6 +134,7 @@
 
     void (*save_request)(QEMUFile *f, SCSIRequest *req);
     void *(*load_request)(QEMUFile *f, SCSIRequest *req);
+    void (*free_request)(SCSIBus *bus, void *priv);
 };
 
 #define TYPE_SCSI_BUS "SCSI"
diff --git a/hw/usb/Makefile.objs b/hw/usb/Makefile.objs
index 9c7ddf5..4225136 100644
--- a/hw/usb/Makefile.objs
+++ b/hw/usb/Makefile.objs
@@ -11,3 +11,4 @@
 common-obj-y += host-$(HOST_USB).o dev-bluetooth.o
 common-obj-y += dev-hid.o dev-storage.o dev-wacom.o
 common-obj-y += dev-serial.o dev-network.o dev-audio.o
+common-obj-y += dev-uas.o
diff --git a/hw/usb/dev-uas.c b/hw/usb/dev-uas.c
new file mode 100644
index 0000000..9b02ff4
--- /dev/null
+++ b/hw/usb/dev-uas.c
@@ -0,0 +1,779 @@
+/*
+ * UAS (USB Attached SCSI) emulation
+ *
+ * Copyright Red Hat, Inc. 2012
+ *
+ * Author: Gerd Hoffmann <kraxel@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#include "qemu-common.h"
+#include "qemu-option.h"
+#include "qemu-config.h"
+#include "trace.h"
+
+#include "hw/usb.h"
+#include "hw/usb/desc.h"
+#include "hw/scsi.h"
+#include "hw/scsi-defs.h"
+
+/* --------------------------------------------------------------------- */
+
+#define UAS_UI_COMMAND              0x01
+#define UAS_UI_SENSE                0x03
+#define UAS_UI_RESPONSE             0x04
+#define UAS_UI_TASK_MGMT            0x05
+#define UAS_UI_READ_READY           0x06
+#define UAS_UI_WRITE_READY          0x07
+
+#define UAS_RC_TMF_COMPLETE         0x00
+#define UAS_RC_INVALID_INFO_UNIT    0x02
+#define UAS_RC_TMF_NOT_SUPPORTED    0x04
+#define UAS_RC_TMF_FAILED           0x05
+#define UAS_RC_TMF_SUCCEEDED        0x08
+#define UAS_RC_INCORRECT_LUN        0x09
+#define UAS_RC_OVERLAPPED_TAG       0x0a
+
+#define UAS_TMF_ABORT_TASK          0x01
+#define UAS_TMF_ABORT_TASK_SET      0x02
+#define UAS_TMF_CLEAR_TASK_SET      0x04
+#define UAS_TMF_LOGICAL_UNIT_RESET  0x08
+#define UAS_TMF_I_T_NEXUS_RESET     0x10
+#define UAS_TMF_CLEAR_ACA           0x40
+#define UAS_TMF_QUERY_TASK          0x80
+#define UAS_TMF_QUERY_TASK_SET      0x81
+#define UAS_TMF_QUERY_ASYNC_EVENT   0x82
+
+#define UAS_PIPE_ID_COMMAND         0x01
+#define UAS_PIPE_ID_STATUS          0x02
+#define UAS_PIPE_ID_DATA_IN         0x03
+#define UAS_PIPE_ID_DATA_OUT        0x04
+
+typedef struct {
+    uint8_t    id;
+    uint8_t    reserved;
+    uint16_t   tag;
+} QEMU_PACKED  uas_ui_header;
+
+typedef struct {
+    uint8_t    prio_taskattr;   /* 6:3 priority, 2:0 task attribute   */
+    uint8_t    reserved_1;
+    uint8_t    add_cdb_length;  /* 7:2 additional adb length (dwords) */
+    uint8_t    reserved_2;
+    uint64_t   lun;
+    uint8_t    cdb[16];
+    uint8_t    add_cdb[];
+} QEMU_PACKED  uas_ui_command;
+
+typedef struct {
+    uint16_t   status_qualifier;
+    uint8_t    status;
+    uint8_t    reserved[7];
+    uint16_t   sense_length;
+    uint8_t    sense_data[18];
+} QEMU_PACKED  uas_ui_sense;
+
+typedef struct {
+    uint16_t   add_response_info;
+    uint8_t    response_code;
+} QEMU_PACKED  uas_ui_response;
+
+typedef struct {
+    uint8_t    function;
+    uint8_t    reserved;
+    uint16_t   task_tag;
+    uint64_t   lun;
+} QEMU_PACKED  uas_ui_task_mgmt;
+
+typedef struct {
+    uas_ui_header  hdr;
+    union {
+        uas_ui_command   command;
+        uas_ui_sense     sense;
+        uas_ui_task_mgmt task;
+        uas_ui_response  response;
+    };
+} QEMU_PACKED  uas_ui;
+
+/* --------------------------------------------------------------------- */
+
+typedef struct UASDevice UASDevice;
+typedef struct UASRequest UASRequest;
+typedef struct UASStatus UASStatus;
+
+struct UASDevice {
+    USBDevice                 dev;
+    SCSIBus                   bus;
+    UASRequest                *datain;
+    UASRequest                *dataout;
+    USBPacket                 *status;
+    QEMUBH                    *status_bh;
+    QTAILQ_HEAD(, UASStatus)  results;
+    QTAILQ_HEAD(, UASRequest) requests;
+};
+
+struct UASRequest {
+    uint16_t     tag;
+    uint64_t     lun;
+    UASDevice    *uas;
+    SCSIDevice   *dev;
+    SCSIRequest  *req;
+    USBPacket    *data;
+    bool         data_async;
+    bool         active;
+    bool         complete;
+    uint32_t     buf_off;
+    uint32_t     buf_size;
+    uint32_t     data_off;
+    uint32_t     data_size;
+    QTAILQ_ENTRY(UASRequest)  next;
+};
+
+struct UASStatus {
+    uas_ui                    status;
+    uint32_t                  length;
+    QTAILQ_ENTRY(UASStatus)   next;
+};
+
+/* --------------------------------------------------------------------- */
+
+enum {
+    STR_MANUFACTURER = 1,
+    STR_PRODUCT,
+    STR_SERIALNUMBER,
+    STR_CONFIG_HIGH,
+};
+
+static const USBDescStrings desc_strings = {
+    [STR_MANUFACTURER] = "QEMU",
+    [STR_PRODUCT]      = "USB Attached SCSI HBA",
+    [STR_SERIALNUMBER] = "27842",
+    [STR_CONFIG_HIGH]  = "High speed config (usb 2.0)",
+};
+
+static const USBDescIface desc_iface_high = {
+    .bInterfaceNumber              = 0,
+    .bNumEndpoints                 = 4,
+    .bInterfaceClass               = USB_CLASS_MASS_STORAGE,
+    .bInterfaceSubClass            = 0x06, /* SCSI */
+    .bInterfaceProtocol            = 0x62, /* UAS  */
+    .eps = (USBDescEndpoint[]) {
+        {
+            .bEndpointAddress      = USB_DIR_OUT | UAS_PIPE_ID_COMMAND,
+            .bmAttributes          = USB_ENDPOINT_XFER_BULK,
+            .wMaxPacketSize        = 512,
+            .extra = (uint8_t[]) {
+                0x04,  /*  u8  bLength */
+                0x24,  /*  u8  bDescriptorType */
+                UAS_PIPE_ID_COMMAND,
+                0x00,  /*  u8  bReserved */
+            },
+        },{
+            .bEndpointAddress      = USB_DIR_IN | UAS_PIPE_ID_STATUS,
+            .bmAttributes          = USB_ENDPOINT_XFER_BULK,
+            .wMaxPacketSize        = 512,
+            .extra = (uint8_t[]) {
+                0x04,  /*  u8  bLength */
+                0x24,  /*  u8  bDescriptorType */
+                UAS_PIPE_ID_STATUS,
+                0x00,  /*  u8  bReserved */
+            },
+        },{
+            .bEndpointAddress      = USB_DIR_IN | UAS_PIPE_ID_DATA_IN,
+            .bmAttributes          = USB_ENDPOINT_XFER_BULK,
+            .wMaxPacketSize        = 512,
+            .extra = (uint8_t[]) {
+                0x04,  /*  u8  bLength */
+                0x24,  /*  u8  bDescriptorType */
+                UAS_PIPE_ID_DATA_IN,
+                0x00,  /*  u8  bReserved */
+            },
+        },{
+            .bEndpointAddress      = USB_DIR_OUT | UAS_PIPE_ID_DATA_OUT,
+            .bmAttributes          = USB_ENDPOINT_XFER_BULK,
+            .wMaxPacketSize        = 512,
+            .extra = (uint8_t[]) {
+                0x04,  /*  u8  bLength */
+                0x24,  /*  u8  bDescriptorType */
+                UAS_PIPE_ID_DATA_OUT,
+                0x00,  /*  u8  bReserved */
+            },
+        },
+    }
+};
+
+static const USBDescDevice desc_device_high = {
+    .bcdUSB                        = 0x0200,
+    .bMaxPacketSize0               = 64,
+    .bNumConfigurations            = 1,
+    .confs = (USBDescConfig[]) {
+        {
+            .bNumInterfaces        = 1,
+            .bConfigurationValue   = 1,
+            .iConfiguration        = STR_CONFIG_HIGH,
+            .bmAttributes          = 0xc0,
+            .nif = 1,
+            .ifs = &desc_iface_high,
+        },
+    },
+};
+
+static const USBDesc desc = {
+    .id = {
+        .idVendor          = 0x46f4, /* CRC16() of "QEMU" */
+        .idProduct         = 0x0002,
+        .bcdDevice         = 0,
+        .iManufacturer     = STR_MANUFACTURER,
+        .iProduct          = STR_PRODUCT,
+        .iSerialNumber     = STR_SERIALNUMBER,
+    },
+    .high = &desc_device_high,
+    .str  = desc_strings,
+};
+
+/* --------------------------------------------------------------------- */
+
+static UASStatus *usb_uas_alloc_status(uint8_t id, uint16_t tag)
+{
+    UASStatus *st = g_new0(UASStatus, 1);
+
+    st->status.hdr.id = id;
+    st->status.hdr.tag = cpu_to_be16(tag);
+    st->length = sizeof(uas_ui_header);
+    return st;
+}
+
+static void usb_uas_send_status_bh(void *opaque)
+{
+    UASDevice *uas = opaque;
+    UASStatus *st = QTAILQ_FIRST(&uas->results);
+    USBPacket *p = uas->status;
+
+    assert(p != NULL);
+    assert(st != NULL);
+
+    uas->status = NULL;
+    usb_packet_copy(p, &st->status, st->length);
+    p->result = st->length;
+    QTAILQ_REMOVE(&uas->results, st, next);
+    g_free(st);
+
+    usb_packet_complete(&uas->dev, p);
+}
+
+static void usb_uas_queue_status(UASDevice *uas, UASStatus *st, int length)
+{
+    st->length += length;
+    QTAILQ_INSERT_TAIL(&uas->results, st, next);
+    if (uas->status) {
+        /*
+         * Just schedule bh make sure any in-flight data transaction
+         * is finished before completing (sending) the status packet.
+         */
+        qemu_bh_schedule(uas->status_bh);
+    } else {
+        USBEndpoint *ep = usb_ep_get(&uas->dev, USB_TOKEN_IN,
+                                     UAS_PIPE_ID_STATUS);
+        usb_wakeup(ep);
+    }
+}
+
+static void usb_uas_queue_response(UASDevice *uas, uint16_t tag,
+                                   uint8_t code, uint16_t add_info)
+{
+    UASStatus *st = usb_uas_alloc_status(UAS_UI_RESPONSE, tag);
+
+    trace_usb_uas_response(uas->dev.addr, tag, code);
+    st->status.response.response_code = code;
+    st->status.response.add_response_info = cpu_to_be16(add_info);
+    usb_uas_queue_status(uas, st, sizeof(uas_ui_response));
+}
+
+static void usb_uas_queue_sense(UASRequest *req, uint8_t status)
+{
+    UASStatus *st = usb_uas_alloc_status(UAS_UI_SENSE, req->tag);
+    int len, slen = 0;
+
+    trace_usb_uas_sense(req->uas->dev.addr, req->tag, status);
+    st->status.sense.status = status;
+    st->status.sense.status_qualifier = cpu_to_be16(0);
+    if (status != GOOD) {
+        slen = scsi_req_get_sense(req->req, st->status.sense.sense_data,
+                                  sizeof(st->status.sense.sense_data));
+        st->status.sense.sense_length = cpu_to_be16(slen);
+    }
+    len = sizeof(uas_ui_sense) - sizeof(st->status.sense.sense_data) + slen;
+    usb_uas_queue_status(req->uas, st, len);
+}
+
+static void usb_uas_queue_read_ready(UASRequest *req)
+{
+    UASStatus *st = usb_uas_alloc_status(UAS_UI_READ_READY, req->tag);
+
+    trace_usb_uas_read_ready(req->uas->dev.addr, req->tag);
+    usb_uas_queue_status(req->uas, st, 0);
+}
+
+static void usb_uas_queue_write_ready(UASRequest *req)
+{
+    UASStatus *st = usb_uas_alloc_status(UAS_UI_WRITE_READY, req->tag);
+
+    trace_usb_uas_write_ready(req->uas->dev.addr, req->tag);
+    usb_uas_queue_status(req->uas, st, 0);
+}
+
+/* --------------------------------------------------------------------- */
+
+static int usb_uas_get_lun(uint64_t lun64)
+{
+    return (lun64 >> 48) & 0xff;
+}
+
+static SCSIDevice *usb_uas_get_dev(UASDevice *uas, uint64_t lun64)
+{
+    if ((lun64 >> 56) != 0x00) {
+        return NULL;
+    }
+    return scsi_device_find(&uas->bus, 0, 0, usb_uas_get_lun(lun64));
+}
+
+static void usb_uas_complete_data_packet(UASRequest *req)
+{
+    USBPacket *p;
+
+    if (!req->data_async) {
+        return;
+    }
+    p = req->data;
+    req->data = NULL;
+    req->data_async = false;
+    usb_packet_complete(&req->uas->dev, p);
+}
+
+static void usb_uas_copy_data(UASRequest *req)
+{
+    uint32_t length;
+
+    length = MIN(req->buf_size - req->buf_off,
+                 req->data->iov.size - req->data->result);
+    trace_usb_uas_xfer_data(req->uas->dev.addr, req->tag, length,
+                            req->data->result, req->data->iov.size,
+                            req->buf_off, req->buf_size);
+    usb_packet_copy(req->data, scsi_req_get_buf(req->req) + req->buf_off,
+                    length);
+    req->buf_off += length;
+    req->data_off += length;
+
+    if (req->data->result == req->data->iov.size) {
+        usb_uas_complete_data_packet(req);
+    }
+    if (req->buf_size && req->buf_off == req->buf_size) {
+        req->buf_off = 0;
+        req->buf_size = 0;
+        scsi_req_continue(req->req);
+    }
+}
+
+static void usb_uas_start_next_transfer(UASDevice *uas)
+{
+    UASRequest *req;
+
+    QTAILQ_FOREACH(req, &uas->requests, next) {
+        if (req->active || req->complete) {
+            continue;
+        }
+        if (req->req->cmd.mode == SCSI_XFER_FROM_DEV && uas->datain == NULL) {
+            uas->datain = req;
+            usb_uas_queue_read_ready(req);
+            req->active = true;
+            return;
+        }
+        if (req->req->cmd.mode == SCSI_XFER_TO_DEV && uas->dataout == NULL) {
+            uas->dataout = req;
+            usb_uas_queue_write_ready(req);
+            req->active = true;
+            return;
+        }
+    }
+}
+
+static UASRequest *usb_uas_alloc_request(UASDevice *uas, uas_ui *ui)
+{
+    UASRequest *req;
+
+    req = g_new0(UASRequest, 1);
+    req->uas = uas;
+    req->tag = be16_to_cpu(ui->hdr.tag);
+    req->lun = be64_to_cpu(ui->command.lun);
+    req->dev = usb_uas_get_dev(req->uas, req->lun);
+    return req;
+}
+
+static void usb_uas_scsi_free_request(SCSIBus *bus, void *priv)
+{
+    UASRequest *req = priv;
+    UASDevice *uas = req->uas;
+
+    if (req == uas->datain) {
+        uas->datain = NULL;
+    }
+    if (req == uas->dataout) {
+        uas->dataout = NULL;
+    }
+    QTAILQ_REMOVE(&uas->requests, req, next);
+    g_free(req);
+}
+
+static UASRequest *usb_uas_find_request(UASDevice *uas, uint16_t tag)
+{
+    UASRequest *req;
+
+    QTAILQ_FOREACH(req, &uas->requests, next) {
+        if (req->tag == tag) {
+            return req;
+        }
+    }
+    return NULL;
+}
+
+static void usb_uas_scsi_transfer_data(SCSIRequest *r, uint32_t len)
+{
+    UASRequest *req = r->hba_private;
+
+    trace_usb_uas_scsi_data(req->uas->dev.addr, req->tag, len);
+    req->buf_off = 0;
+    req->buf_size = len;
+    if (req->data) {
+        usb_uas_copy_data(req);
+    } else {
+        usb_uas_start_next_transfer(req->uas);
+    }
+}
+
+static void usb_uas_scsi_command_complete(SCSIRequest *r,
+                                          uint32_t status, size_t resid)
+{
+    UASRequest *req = r->hba_private;
+    UASDevice *uas = req->uas;
+
+    trace_usb_uas_scsi_complete(req->uas->dev.addr, req->tag, status, resid);
+    req->complete = true;
+    if (req->data) {
+        usb_uas_complete_data_packet(req);
+    }
+    usb_uas_queue_sense(req, status);
+    scsi_req_unref(req->req);
+    usb_uas_start_next_transfer(uas);
+}
+
+static void usb_uas_scsi_request_cancelled(SCSIRequest *r)
+{
+    UASRequest *req = r->hba_private;
+
+    /* FIXME: queue notification to status pipe? */
+    scsi_req_unref(req->req);
+}
+
+static const struct SCSIBusInfo usb_uas_scsi_info = {
+    .tcq = true,
+    .max_target = 0,
+    .max_lun = 255,
+
+    .transfer_data = usb_uas_scsi_transfer_data,
+    .complete = usb_uas_scsi_command_complete,
+    .cancel = usb_uas_scsi_request_cancelled,
+    .free_request = usb_uas_scsi_free_request,
+};
+
+/* --------------------------------------------------------------------- */
+
+static void usb_uas_handle_reset(USBDevice *dev)
+{
+    UASDevice *uas = DO_UPCAST(UASDevice, dev, dev);
+    UASRequest *req, *nreq;
+    UASStatus *st, *nst;
+
+    trace_usb_uas_reset(dev->addr);
+    QTAILQ_FOREACH_SAFE(req, &uas->requests, next, nreq) {
+        scsi_req_cancel(req->req);
+    }
+    QTAILQ_FOREACH_SAFE(st, &uas->results, next, nst) {
+        QTAILQ_REMOVE(&uas->results, st, next);
+        g_free(st);
+    }
+}
+
+static int usb_uas_handle_control(USBDevice *dev, USBPacket *p,
+               int request, int value, int index, int length, uint8_t *data)
+{
+    int ret;
+
+    ret = usb_desc_handle_control(dev, p, request, value, index, length, data);
+    if (ret >= 0) {
+        return ret;
+    }
+    fprintf(stderr, "%s: unhandled control request\n", __func__);
+    return USB_RET_STALL;
+}
+
+static void usb_uas_cancel_io(USBDevice *dev, USBPacket *p)
+{
+    UASDevice *uas = DO_UPCAST(UASDevice, dev, dev);
+    UASRequest *req, *nreq;
+
+    if (uas->status == p) {
+        uas->status = NULL;
+        qemu_bh_cancel(uas->status_bh);
+        return;
+    }
+    QTAILQ_FOREACH_SAFE(req, &uas->requests, next, nreq) {
+        if (req->data == p) {
+            req->data = NULL;
+            return;
+        }
+    }
+    assert(!"canceled usb packet not found");
+}
+
+static void usb_uas_command(UASDevice *uas, uas_ui *ui)
+{
+    UASRequest *req;
+    uint32_t len;
+
+    req = usb_uas_find_request(uas, be16_to_cpu(ui->hdr.tag));
+    if (req) {
+        goto overlapped_tag;
+    }
+    req = usb_uas_alloc_request(uas, ui);
+    if (req->dev == NULL) {
+        goto bad_target;
+    }
+
+    trace_usb_uas_command(uas->dev.addr, req->tag,
+                          usb_uas_get_lun(req->lun),
+                          req->lun >> 32, req->lun & 0xffffffff);
+    QTAILQ_INSERT_TAIL(&uas->requests, req, next);
+    req->req = scsi_req_new(req->dev, req->tag,
+                            usb_uas_get_lun(req->lun),
+                            ui->command.cdb, req);
+    len = scsi_req_enqueue(req->req);
+    if (len) {
+        req->data_size = len;
+        scsi_req_continue(req->req);
+    }
+    return;
+
+overlapped_tag:
+    usb_uas_queue_response(uas, req->tag, UAS_RC_OVERLAPPED_TAG, 0);
+    return;
+
+bad_target:
+    /*
+     * FIXME: Seems to upset linux, is this wrong?
+     * NOTE: Happens only with no scsi devices at the bus, not sure
+     *       this is a valid UAS setup in the first place.
+     */
+    usb_uas_queue_response(uas, req->tag, UAS_RC_INVALID_INFO_UNIT, 0);
+    g_free(req);
+    return;
+}
+
+static void usb_uas_task(UASDevice *uas, uas_ui *ui)
+{
+    uint16_t tag = be16_to_cpu(ui->hdr.tag);
+    uint64_t lun64 = be64_to_cpu(ui->task.lun);
+    SCSIDevice *dev = usb_uas_get_dev(uas, lun64);
+    int lun = usb_uas_get_lun(lun64);
+    UASRequest *req;
+    uint16_t task_tag;
+
+    req = usb_uas_find_request(uas, be16_to_cpu(ui->hdr.tag));
+    if (req) {
+        goto overlapped_tag;
+    }
+
+    switch (ui->task.function) {
+    case UAS_TMF_ABORT_TASK:
+        task_tag = be16_to_cpu(ui->task.task_tag);
+        trace_usb_uas_tmf_abort_task(uas->dev.addr, tag, task_tag);
+        if (dev == NULL) {
+            goto bad_target;
+        }
+        if (dev->lun != lun) {
+            goto incorrect_lun;
+        }
+        req = usb_uas_find_request(uas, task_tag);
+        if (req && req->dev == dev) {
+            scsi_req_cancel(req->req);
+        }
+        usb_uas_queue_response(uas, tag, UAS_RC_TMF_COMPLETE, 0);
+        break;
+
+    case UAS_TMF_LOGICAL_UNIT_RESET:
+        trace_usb_uas_tmf_logical_unit_reset(uas->dev.addr, tag, lun);
+        if (dev == NULL) {
+            goto bad_target;
+        }
+        if (dev->lun != lun) {
+            goto incorrect_lun;
+        }
+        qdev_reset_all(&dev->qdev);
+        usb_uas_queue_response(uas, tag, UAS_RC_TMF_COMPLETE, 0);
+        break;
+
+    default:
+        trace_usb_uas_tmf_unsupported(uas->dev.addr, tag, ui->task.function);
+        usb_uas_queue_response(uas, tag, UAS_RC_TMF_NOT_SUPPORTED, 0);
+        break;
+    }
+    return;
+
+overlapped_tag:
+    usb_uas_queue_response(uas, req->tag, UAS_RC_OVERLAPPED_TAG, 0);
+    return;
+
+bad_target:
+    /* FIXME: correct?  [see long comment in usb_uas_command()] */
+    usb_uas_queue_response(uas, tag, UAS_RC_INVALID_INFO_UNIT, 0);
+    return;
+
+incorrect_lun:
+    usb_uas_queue_response(uas, tag, UAS_RC_INCORRECT_LUN, 0);
+    return;
+}
+
+static int usb_uas_handle_data(USBDevice *dev, USBPacket *p)
+{
+    UASDevice *uas = DO_UPCAST(UASDevice, dev, dev);
+    uas_ui ui;
+    UASStatus *st;
+    UASRequest *req;
+    int length, ret = 0;
+
+    switch (p->ep->nr) {
+    case UAS_PIPE_ID_COMMAND:
+        length = MIN(sizeof(ui), p->iov.size);
+        usb_packet_copy(p, &ui, length);
+        switch (ui.hdr.id) {
+        case UAS_UI_COMMAND:
+            usb_uas_command(uas, &ui);
+            ret = length;
+            break;
+        case UAS_UI_TASK_MGMT:
+            usb_uas_task(uas, &ui);
+            ret = length;
+            break;
+        default:
+            fprintf(stderr, "%s: unknown command ui: id 0x%x\n",
+                    __func__, ui.hdr.id);
+            ret = USB_RET_STALL;
+            break;
+        }
+        break;
+    case UAS_PIPE_ID_STATUS:
+        st = QTAILQ_FIRST(&uas->results);
+        if (st == NULL) {
+            assert(uas->status == NULL);
+            uas->status = p;
+            ret = USB_RET_ASYNC;
+            break;
+        }
+        usb_packet_copy(p, &st->status, st->length);
+        ret = st->length;
+        QTAILQ_REMOVE(&uas->results, st, next);
+        g_free(st);
+        break;
+    case UAS_PIPE_ID_DATA_IN:
+    case UAS_PIPE_ID_DATA_OUT:
+        req = (p->ep->nr == UAS_PIPE_ID_DATA_IN) ? uas->datain : uas->dataout;
+        if (req == NULL) {
+            fprintf(stderr, "%s: no inflight request\n", __func__);
+            ret = USB_RET_STALL;
+            break;
+        }
+        scsi_req_ref(req->req);
+        req->data = p;
+        usb_uas_copy_data(req);
+        if (p->result == p->iov.size || req->complete) {
+            req->data = NULL;
+            ret = p->result;
+        } else {
+            req->data_async = true;
+            ret = USB_RET_ASYNC;
+        }
+        scsi_req_unref(req->req);
+        usb_uas_start_next_transfer(uas);
+        break;
+    default:
+        fprintf(stderr, "%s: invalid endpoint %d\n", __func__, p->ep->nr);
+        ret = USB_RET_STALL;
+        break;
+    }
+    return ret;
+}
+
+static void usb_uas_handle_destroy(USBDevice *dev)
+{
+    UASDevice *uas = DO_UPCAST(UASDevice, dev, dev);
+
+    qemu_bh_delete(uas->status_bh);
+}
+
+static int usb_uas_init(USBDevice *dev)
+{
+    UASDevice *uas = DO_UPCAST(UASDevice, dev, dev);
+
+    usb_desc_create_serial(dev);
+    usb_desc_init(dev);
+
+    QTAILQ_INIT(&uas->results);
+    QTAILQ_INIT(&uas->requests);
+    uas->status_bh = qemu_bh_new(usb_uas_send_status_bh, uas);
+
+    scsi_bus_new(&uas->bus, &uas->dev.qdev, &usb_uas_scsi_info);
+
+    return 0;
+}
+
+static const VMStateDescription vmstate_usb_uas = {
+    .name = "usb-uas",
+    .unmigratable = 1,
+    .fields = (VMStateField[]) {
+        VMSTATE_USB_DEVICE(dev, UASDevice),
+        VMSTATE_END_OF_LIST()
+    }
+};
+
+static void usb_uas_class_initfn(ObjectClass *klass, void *data)
+{
+    DeviceClass *dc = DEVICE_CLASS(klass);
+    USBDeviceClass *uc = USB_DEVICE_CLASS(klass);
+
+    uc->init           = usb_uas_init;
+    uc->product_desc   = desc_strings[STR_PRODUCT];
+    uc->usb_desc       = &desc;
+    uc->cancel_packet  = usb_uas_cancel_io;
+    uc->handle_attach  = usb_desc_attach;
+    uc->handle_reset   = usb_uas_handle_reset;
+    uc->handle_control = usb_uas_handle_control;
+    uc->handle_data    = usb_uas_handle_data;
+    uc->handle_destroy = usb_uas_handle_destroy;
+    dc->fw_name = "storage";
+    dc->vmsd = &vmstate_usb_uas;
+}
+
+static TypeInfo uas_info = {
+    .name          = "usb-uas",
+    .parent        = TYPE_USB_DEVICE,
+    .instance_size = sizeof(UASDevice),
+    .class_init    = usb_uas_class_initfn,
+};
+
+static void usb_uas_register_types(void)
+{
+    type_register_static(&uas_info);
+}
+
+type_init(usb_uas_register_types)
diff --git a/hw/usb/hcd-ehci.c b/hw/usb/hcd-ehci.c
index 080f62c..b043e7c 100644
--- a/hw/usb/hcd-ehci.c
+++ b/hw/usb/hcd-ehci.c
@@ -420,6 +420,7 @@
     USBPort ports[NB_PORTS];
     USBPort *companion_ports[NB_PORTS];
     uint32_t usbsts_pending;
+    uint32_t usbsts_frindex;
     EHCIQueueHead aqueues;
     EHCIQueueHead pqueues;
 
@@ -558,34 +559,45 @@
     s->usbsts &= ~mask;
 }
 
-static inline void ehci_set_interrupt(EHCIState *s, int intr)
+/* update irq line */
+static inline void ehci_update_irq(EHCIState *s)
 {
     int level = 0;
 
-    // TODO honour interrupt threshold requests
-
-    ehci_set_usbsts(s, intr);
-
     if ((s->usbsts & USBINTR_MASK) & s->usbintr) {
         level = 1;
     }
 
-    trace_usb_ehci_interrupt(level, s->usbsts, s->usbintr);
+    trace_usb_ehci_irq(level, s->frindex, s->usbsts, s->usbintr);
     qemu_set_irq(s->irq, level);
 }
 
-static inline void ehci_record_interrupt(EHCIState *s, int intr)
+/* flag interrupt condition */
+static inline void ehci_raise_irq(EHCIState *s, int intr)
 {
     s->usbsts_pending |= intr;
 }
 
-static inline void ehci_commit_interrupt(EHCIState *s)
+/*
+ * Commit pending interrupts (added via ehci_raise_irq),
+ * at the rate allowed by "Interrupt Threshold Control".
+ */
+static inline void ehci_commit_irq(EHCIState *s)
 {
+    uint32_t itc;
+
     if (!s->usbsts_pending) {
         return;
     }
-    ehci_set_interrupt(s, s->usbsts_pending);
+    if (s->usbsts_frindex > s->frindex) {
+        return;
+    }
+
+    itc = (s->usbcmd >> 16) & 0xff;
+    s->usbsts |= s->usbsts_pending;
     s->usbsts_pending = 0;
+    s->usbsts_frindex = s->frindex + itc;
+    ehci_update_irq(s);
 }
 
 static void ehci_update_halt(EHCIState *s)
@@ -849,7 +861,8 @@
     *portsc |= PORTSC_CONNECT;
     *portsc |= PORTSC_CSC;
 
-    ehci_set_interrupt(s, USBSTS_PCD);
+    ehci_raise_irq(s, USBSTS_PCD);
+    ehci_commit_irq(s);
 }
 
 static void ehci_detach(USBPort *port)
@@ -878,7 +891,8 @@
     *portsc &= ~(PORTSC_CONNECT|PORTSC_PED);
     *portsc |= PORTSC_CSC;
 
-    ehci_set_interrupt(s, USBSTS_PCD);
+    ehci_raise_irq(s, USBSTS_PCD);
+    ehci_commit_irq(s);
 }
 
 static void ehci_child_detach(USBPort *port, USBDevice *child)
@@ -997,6 +1011,8 @@
 
     s->usbcmd = NB_MAXINTRATE << USBCMD_ITC_SH;
     s->usbsts = USBSTS_HALT;
+    s->usbsts_pending = 0;
+    s->usbsts_frindex = 0;
 
     s->astate = EST_INACTIVE;
     s->pstate = EST_INACTIVE;
@@ -1188,7 +1204,7 @@
         val &= USBSTS_RO_MASK;              // bits 6 through 31 are RO
         ehci_clear_usbsts(s, val);          // bits 0 through 5 are R/WC
         val = s->usbsts;
-        ehci_set_interrupt(s, 0);
+        ehci_update_irq(s);
         break;
 
     case USBINTR:
@@ -1419,18 +1435,18 @@
         case USB_RET_NODEV:
             q->qh.token |= (QTD_TOKEN_HALT | QTD_TOKEN_XACTERR);
             set_field(&q->qh.token, 0, QTD_TOKEN_CERR);
-            ehci_record_interrupt(q->ehci, USBSTS_ERRINT);
+            ehci_raise_irq(q->ehci, USBSTS_ERRINT);
             break;
         case USB_RET_STALL:
             q->qh.token |= QTD_TOKEN_HALT;
-            ehci_record_interrupt(q->ehci, USBSTS_ERRINT);
+            ehci_raise_irq(q->ehci, USBSTS_ERRINT);
             break;
         case USB_RET_NAK:
             set_field(&q->qh.altnext_qtd, 0, QH_ALTNEXT_NAKCNT);
             return; /* We're not done yet with this transaction */
         case USB_RET_BABBLE:
             q->qh.token |= (QTD_TOKEN_HALT | QTD_TOKEN_BABBLE);
-            ehci_record_interrupt(q->ehci, USBSTS_ERRINT);
+            ehci_raise_irq(q->ehci, USBSTS_ERRINT);
             break;
         default:
             /* should not be triggerable */
@@ -1441,7 +1457,7 @@
     } else if ((p->usb_status > p->tbytes) && (p->pid == USB_TOKEN_IN)) {
         p->usb_status = USB_RET_BABBLE;
         q->qh.token |= (QTD_TOKEN_HALT | QTD_TOKEN_BABBLE);
-        ehci_record_interrupt(q->ehci, USBSTS_ERRINT);
+        ehci_raise_irq(q->ehci, USBSTS_ERRINT);
     } else {
         // TODO check 4.12 for splits
 
@@ -1462,7 +1478,7 @@
     q->qh.token &= ~QTD_TOKEN_ACTIVE;
 
     if (q->qh.token & QTD_TOKEN_IOC) {
-        ehci_record_interrupt(q->ehci, USBSTS_INT);
+        ehci_raise_irq(q->ehci, USBSTS_INT);
     }
 }
 
@@ -1597,12 +1613,12 @@
                     /* 3.3.2: XACTERR is only allowed on IN transactions */
                     if (dir) {
                         itd->transact[i] |= ITD_XACT_XACTERR;
-                        ehci_record_interrupt(ehci, USBSTS_ERRINT);
+                        ehci_raise_irq(ehci, USBSTS_ERRINT);
                     }
                     break;
                 case USB_RET_BABBLE:
                     itd->transact[i] |= ITD_XACT_BABBLE;
-                    ehci_record_interrupt(ehci, USBSTS_ERRINT);
+                    ehci_raise_irq(ehci, USBSTS_ERRINT);
                     break;
                 case USB_RET_NAK:
                     /* no data for us, so do a zero-length transfer */
@@ -1620,7 +1636,7 @@
                 }
             }
             if (itd->transact[i] & ITD_XACT_IOC) {
-                ehci_record_interrupt(ehci, USBSTS_INT);
+                ehci_raise_irq(ehci, USBSTS_INT);
             }
             itd->transact[i] &= ~ITD_XACT_ACTIVE;
         }
@@ -2208,8 +2224,6 @@
         }
     }
     while (again);
-
-    ehci_commit_interrupt(ehci);
 }
 
 static void ehci_advance_async_state(EHCIState *ehci)
@@ -2255,7 +2269,7 @@
             ehci_queues_tag_unused_async(ehci);
             DPRINTF("ASYNC: doorbell request acknowledged\n");
             ehci->usbcmd &= ~USBCMD_IAAD;
-            ehci_set_interrupt(ehci, USBSTS_IAA);
+            ehci_raise_irq(ehci, USBSTS_IAA);
         }
         break;
 
@@ -2328,12 +2342,17 @@
         ehci->frindex += 8;
 
         if (ehci->frindex == 0x00002000) {
-            ehci_set_interrupt(ehci, USBSTS_FLR);
+            ehci_raise_irq(ehci, USBSTS_FLR);
         }
 
         if (ehci->frindex == 0x00004000) {
-            ehci_set_interrupt(ehci, USBSTS_FLR);
+            ehci_raise_irq(ehci, USBSTS_FLR);
             ehci->frindex = 0;
+            if (ehci->usbsts_frindex > 0x00004000) {
+                ehci->usbsts_frindex -= 0x00004000;
+            } else {
+                ehci->usbsts_frindex = 0;
+            }
         }
     }
 }
@@ -2341,7 +2360,7 @@
 static void ehci_frame_timer(void *opaque)
 {
     EHCIState *ehci = opaque;
-    int schedules = 0;
+    int need_timer = 0;
     int64_t expire_time, t_now;
     uint64_t ns_elapsed;
     int frames, skipped_frames;
@@ -2352,8 +2371,8 @@
     frames = ns_elapsed / FRAME_TIMER_NS;
 
     if (ehci_periodic_enabled(ehci) || ehci->pstate != EST_INACTIVE) {
-        schedules++;
-        expire_time = t_now + (get_ticks_per_sec() / FRAME_TIMER_FREQ);
+        need_timer++;
+        ehci->async_stepdown = 0;
 
         if (frames > ehci->maxframes) {
             skipped_frames = frames - ehci->maxframes;
@@ -2372,8 +2391,6 @@
         if (ehci->async_stepdown < ehci->maxframes / 2) {
             ehci->async_stepdown++;
         }
-        expire_time = t_now + (get_ticks_per_sec()
-                               * ehci->async_stepdown / FRAME_TIMER_FREQ);
         ehci_update_frindex(ehci, frames);
         ehci->last_run_ns += FRAME_TIMER_NS * frames;
     }
@@ -2382,11 +2399,19 @@
      *  called
      */
     if (ehci_async_enabled(ehci) || ehci->astate != EST_INACTIVE) {
-        schedules++;
-        qemu_bh_schedule(ehci->async_bh);
+        need_timer++;
+        ehci_advance_async_state(ehci);
     }
 
-    if (schedules) {
+    ehci_commit_irq(ehci);
+    if (ehci->usbsts_pending) {
+        need_timer++;
+        ehci->async_stepdown = 0;
+    }
+
+    if (need_timer) {
+        expire_time = t_now + (get_ticks_per_sec()
+                               * (ehci->async_stepdown+1) / FRAME_TIMER_FREQ);
         qemu_mod_timer(ehci->frame_timer, expire_time);
     }
 }
diff --git a/hw/usb/hcd-uhci.c b/hw/usb/hcd-uhci.c
index 8f652d2..2aac8a2 100644
--- a/hw/usb/hcd-uhci.c
+++ b/hw/usb/hcd-uhci.c
@@ -388,11 +388,23 @@
     }
 };
 
+static int uhci_post_load(void *opaque, int version_id)
+{
+    UHCIState *s = opaque;
+
+    if (version_id < 2) {
+        s->expire_time = qemu_get_clock_ns(vm_clock) +
+            (get_ticks_per_sec() / FRAME_TIMER_FREQ);
+    }
+    return 0;
+}
+
 static const VMStateDescription vmstate_uhci = {
     .name = "uhci",
     .version_id = 2,
     .minimum_version_id = 1,
     .minimum_version_id_old = 1,
+    .post_load = uhci_post_load,
     .fields      = (VMStateField []) {
         VMSTATE_PCI_DEVICE(dev, UHCIState),
         VMSTATE_UINT8_EQUAL(num_ports_vmstate, UHCIState),
diff --git a/trace-events b/trace-events
index fc32bc6..fa0c883 100644
--- a/trace-events
+++ b/trace-events
@@ -258,7 +258,7 @@
 usb_ehci_data(int rw, uint32_t cpage, uint32_t offset, uint32_t addr, uint32_t len, uint32_t bufpos) "write %d, cpage %d, offset 0x%03x, addr 0x%08x, len %d, bufpos %d"
 usb_ehci_queue_action(void *q, const char *action) "q %p: %s"
 usb_ehci_packet_action(void *q, void *p, const char *action) "q %p p %p: %s"
-usb_ehci_interrupt(uint32_t level, uint32_t sts, uint32_t mask) "level %d, sts 0x%x, mask 0x%x"
+usb_ehci_irq(uint32_t level, uint32_t frindex, uint32_t sts, uint32_t mask) "level %d, frindex 0x%04x, sts 0x%x, mask 0x%x"
 
 # hw/usb/hcd-uhci.c
 usb_uhci_reset(void) "=== RESET ==="
@@ -347,6 +347,20 @@
 usb_hub_attach(int addr, int nr) "dev %d, port %d"
 usb_hub_detach(int addr, int nr) "dev %d, port %d"
 
+# hw/usb/dev-uas.c
+usb_uas_reset(int addr) "dev %d"
+usb_uas_command(int addr, uint16_t tag, int lun, uint32_t lun64_1, uint32_t lun64_2) "dev %d, tag 0x%x, lun %d, lun64 %08x-%08x"
+usb_uas_response(int addr, uint16_t tag, uint8_t code) "dev %d, tag 0x%x, code 0x%x"
+usb_uas_sense(int addr, uint16_t tag, uint8_t status) "dev %d, tag 0x%x, status 0x%x"
+usb_uas_read_ready(int addr, uint16_t tag) "dev %d, tag 0x%x"
+usb_uas_write_ready(int addr, uint16_t tag) "dev %d, tag 0x%x"
+usb_uas_xfer_data(int addr, uint16_t tag, uint32_t copy, uint32_t uoff, uint32_t usize, uint32_t soff, uint32_t ssize) "dev %d, tag 0x%x, copy %d, usb-pkt %d/%d, scsi-buf %d/%d"
+usb_uas_scsi_data(int addr, uint16_t tag, uint32_t bytes) "dev %d, tag 0x%x, bytes %d"
+usb_uas_scsi_complete(int addr, uint16_t tag, uint32_t status, uint32_t resid) "dev %d, tag 0x%x, status 0x%x, residue %d"
+usb_uas_tmf_abort_task(int addr, uint16_t tag, uint16_t task_tag) "dev %d, tag 0x%x, task-tag 0x%x"
+usb_uas_tmf_logical_unit_reset(int addr, uint16_t tag, int lun) "dev %d, tag 0x%x, lun %d"
+usb_uas_tmf_unsupported(int addr, uint16_t tag, uint32_t function) "dev %d, tag 0x%x, function 0x%x"
+
 # hw/usb/host-linux.c
 usb_host_open_started(int bus, int addr) "dev %d:%d"
 usb_host_open_success(int bus, int addr) "dev %d:%d"