[Qemu-devel] [PATCH v7 RFC] block/vxhs: Initial commit to add Veritas HyperScale VxHS block device support

[Ashish Mittal <ashmit602@gmail.com>]

This patch adds support for a new block device type called "vxhs".
Source code for the library that this code loads can be downloaded from:
https://github.com/MittalAshish/libqnio.git

Sample command line using JSON syntax:
./qemu-system-x86_64 -name instance-00000008 -S -vnc 0.0.0.0:0 -k en-us -vga cirrus -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5 -msg timestamp=on 'json:{"driver":"vxhs","vdisk_id":"{c3e9095a-a5ee-4dce-afeb-2a59fb387410}","server":[{"host":"172.172.17.4","port":"9999"},{"host":"172.172.17.2","port":"9999"}]}'

Sample command line using URI syntax:
qemu-img convert -f raw -O raw -n /var/lib/nova/instances/_base/0c5eacd5ebea5ed914b6a3e7b18f1ce734c386ad vxhs://192.168.0.1:9999/%7Bc6718f6b-0401-441d-a8c3-1f0064d75ee0%7D

Signed-off-by: Ashish Mittal <ashish.mittal@veritas.com>
---
v7 changelog:
(1) Got rid of the header file and most of function forward-declarations.
(2) Added wrappers for vxhs_qnio_iio_open() and vxhs_qnio_iio_close()
(3) Fixed a double close attempt of vdisk_rfd.
(4) Changed to pass QEMUIOVector * in a couple of functions instead of
    individual structure members.
(5) Got rid of VXHS_VECTOR_ALIGNED/NOT_ALIGNED.
(6) Got rid of vxhs_calculate_iovec_size().
(7) Changed to use qemu_try_memalign().
(8) Got rid of unnecessary "else" conditions in a couple of places.
(9) Limited the filename case to pass a single URI in vxhs_parse_uri().
    Users will have to use the host/port/vdisk_id syntax to specify
    multiple host information.
(10) Inlined couple of macros including the ones for qemu_spin_unlock.
(11) Other miscellaneous changes.

v6 changelog:
(1) Removed cJSON dependency out of the libqnioshim layer.
(2) Merged libqnioshim code into qemu vxhs driver proper.
    Now qemu-vxhs code only links with libqnio.so.
(3) Replaced use of custom spinlocks with qemu_spin_lock.

v5 changelog:
(1) Removed unused functions.
(2) Changed all qemu_ prefix for functions defined in libqnio and vxhs.c.
(3) Fixed memory leaks in vxhs_qemu_init() and on the close of vxhs device.
(4) Added upper bounds check on num_servers.
(5) Close channel fds whereever necessary.
(6) Changed vdisk_size to int64_t for 32-bit compilations.
(7) Added message to configure file to indicate if vxhs is enabled or not.

v4 changelog:
(1) Reworked QAPI/JSON parsing.
(2) Reworked URI parsing as suggested by Kevin.
(3) Fixes per review comments from Stefan on v1.
(4) Fixes per review comments from Daniel on v3.

v3 changelog:
(1) Implemented QAPI interface for passing VxHS block device parameters.

v2 changelog:
(1) Removed code to dlopen library. We now check if libqnio is installed during
    configure, and directly link with it.
(2) Changed file headers to mention GPLv2-or-later license.
(3) Removed unnecessary type casts and inlines.
(4) Removed custom tokenize function and modified code to use g_strsplit.
(5) Replaced malloc/free with g_new/g_free and removed code that checks for
    memory allocation failure conditions.
(6) Removed some block ops implementations that were place-holders only.
(7) Removed all custom debug messages. Added new messages in block/trace-events
(8) Other miscellaneous corrections.

v1 changelog:
(1) First patch submission for review comments.

 block/Makefile.objs |    2 +
 block/trace-events  |   47 ++
 block/vxhs.c        | 1645 +++++++++++++++++++++++++++++++++++++++++++++++++++
 configure           |   41 ++
 4 files changed, 1735 insertions(+)
 create mode 100644 block/vxhs.c

diff --git a/block/Makefile.objs b/block/Makefile.objs
index 7d4031d..1861bb9 100644
--- a/block/Makefile.objs
+++ b/block/Makefile.objs
@@ -18,6 +18,7 @@ block-obj-$(CONFIG_LIBNFS) += nfs.o
 block-obj-$(CONFIG_CURL) += curl.o
 block-obj-$(CONFIG_RBD) += rbd.o
 block-obj-$(CONFIG_GLUSTERFS) += gluster.o
+block-obj-$(CONFIG_VXHS) += vxhs.o
 block-obj-$(CONFIG_ARCHIPELAGO) += archipelago.o
 block-obj-$(CONFIG_LIBSSH2) += ssh.o
 block-obj-y += accounting.o dirty-bitmap.o
@@ -38,6 +39,7 @@ rbd.o-cflags       := $(RBD_CFLAGS)
 rbd.o-libs         := $(RBD_LIBS)
 gluster.o-cflags   := $(GLUSTERFS_CFLAGS)
 gluster.o-libs     := $(GLUSTERFS_LIBS)
+vxhs.o-libs        := $(VXHS_LIBS)
 ssh.o-cflags       := $(LIBSSH2_CFLAGS)
 ssh.o-libs         := $(LIBSSH2_LIBS)
 archipelago.o-libs := $(ARCHIPELAGO_LIBS)
diff --git a/block/trace-events b/block/trace-events
index 05fa13c..44de452 100644
--- a/block/trace-events
+++ b/block/trace-events
@@ -114,3 +114,50 @@ qed_aio_write_data(void *s, void *acb, int ret, uint64_t offset, size_t len) "s
 qed_aio_write_prefill(void *s, void *acb, uint64_t start, size_t len, uint64_t offset) "s %p acb %p start %"PRIu64" len %zu offset %"PRIu64
 qed_aio_write_postfill(void *s, void *acb, uint64_t start, size_t len, uint64_t offset) "s %p acb %p start %"PRIu64" len %zu offset %"PRIu64
 qed_aio_write_main(void *s, void *acb, int ret, uint64_t offset, size_t len) "s %p acb %p ret %d offset %"PRIu64" len %zu"
+
+# block/vxhs.c
+vxhs_bdrv_init(const char c) "Registering VxHS AIO driver%c"
+vxhs_iio_callback(int error, int reason) "ctx is NULL: error %d, reason %d"
+vxhs_setup_qnio(void *s) "Context to HyperScale IO manager = %p"
+vxhs_setup_qnio_nwerror(char c) "Could not initialize the network channel. Bailing out%c"
+vxhs_iio_callback_iofail(int err, int reason, void *acb, int seg) "Read/Write failed: error %d, reason %d, acb %p, segment %d"
+vxhs_iio_callback_retry(char *guid, void *acb) "vDisk %s, added acb %p to retry queue (5)"
+vxhs_iio_callback_chnlfail(int error) "QNIO channel failed, no i/o (%d)"
+vxhs_iio_callback_fail(int r, void *acb, int seg, uint64_t size, int err) " ALERT: reason = %d , acb = %p, acb->segments = %d, acb->size = %lu Error = %d"
+vxhs_fail_aio(char * guid, void *acb) "vDisk %s, failing acb %p"
+vxhs_iio_callback_ready(char *vd, int err) "async vxhs_iio_callback: IRP_VDISK_CHECK_IO_FAILOVER_READY completed for vdisk %s with error %d"
+vxhs_iio_callback_chnfail(int err, int error) "QNIO channel failed, no i/o %d, %d"
+vxhs_iio_callback_unknwn(int opcode, int err) "unexpected opcode %d, errno %d"
+vxhs_open_fail(int ret) "Could not open the device. Error = %d"
+vxhs_open_epipe(char c) "Could not create a pipe for device. Bailing out%c"
+vxhs_aio_rw(char *guid, int iodir, uint64_t size, uint64_t offset) "vDisk %s, vDisk device is in failed state iodir = %d size = %lu offset = %lu"
+vxhs_aio_rw_retry(char *guid, void *acb, int queue) "vDisk %s, added acb %p to retry queue(%d)"
+vxhs_aio_rw_invalid(int req) "Invalid I/O request iodir %d"
+vxhs_aio_rw_ioerr(char *guid, int iodir, uint64_t size, uint64_t off, void *acb, int seg, int ret, int err) "IO ERROR (vDisk %s) FOR : Read/Write = %d size = %lu offset = %lu ACB = %p Segments = %d. Error = %d, errno = %d"
+vxhs_co_flush(char *guid, int ret, int err) "vDisk (%s) Flush ioctl failed ret = %d errno = %d"
+vxhs_get_vdisk_stat_err(char *guid, int ret, int err) "vDisk (%s) stat ioctl failed, ret = %d, errno = %d"
+vxhs_get_vdisk_stat(char *vdisk_guid, uint64_t vdisk_size) "vDisk %s stat ioctl returned size %lu"
+vxhs_switch_storage_agent(char *ip, char *guid) "Query host %s for vdisk %s"
+vxhs_switch_storage_agent_failed(char *ip, char *guid, int res, int err) "Query to host %s for vdisk %s failed, res = %d, errno = %d"
+vxhs_check_failover_status(char *ip, char *guid) "Switched to storage server host-IP %s for vdisk %s"
+vxhs_check_failover_status_retry(char *guid) "failover_ioctl_cb: keep looking for io target for vdisk %s"
+vxhs_failover_io(char *vdisk) "I/O Failover starting for vDisk %s"
+vxhs_qnio_iio_open(const char *ip) "Failed to connect to storage agent on host-ip %s"
+vxhs_qnio_iio_devopen(const char *fname) "Failed to open vdisk device: %s"
+vxhs_handle_queued_ios(void *acb, int res) "Restarted acb %p res %d"
+vxhs_restart_aio(int dir, int res, int err) "IO ERROR FOR: Read/Write = %d Error = %d, errno = %d"
+vxhs_complete_aio(void *acb, uint64_t ret) "aio failed acb %p ret %ld"
+vxhs_aio_rw_iofail(char *guid) "vDisk %s, I/O operation failed."
+vxhs_aio_rw_devfail(char *guid, int dir, uint64_t size, uint64_t off) "vDisk %s, vDisk device failed iodir = %d size = %lu offset = %lu"
+vxhs_parse_uri_filename(const char *filename) "URI passed via bdrv_parse_filename %s"
+vxhs_qemu_init_vdisk(const char *vdisk_id) "vdisk_id from json %s"
+vxhs_qemu_init_numservers(int num_servers) "Number of servers passed = %d"
+vxhs_parse_uri_hostinfo(int num, char *host, int port) "Host %d: IP %s, Port %d"
+vxhs_qemu_init(char *of_vsa_addr, int port) "Adding host %s:%d to BDRVVXHSState"
+vxhs_qemu_init_filename(const char *filename) "Filename passed as %s"
+vxhs_close(char *vdisk_guid) "Closing vdisk %s"
+vxhs_convert_iovector_to_buffer(size_t len) "Could not allocate buffer for size %zu bytes"
+vxhs_qnio_iio_writev(int res) "iio_writev returned %d"
+vxhs_qnio_iio_writev_err(int iter, uint64_t iov_len, int err) "Error for iteration : %d, iov_len = %lu errno = %d"
+vxhs_qnio_iio_readv(void *ctx, int ret, int error) "Error while issuing read to QNIO. ctx %p Error = %d, errno = %d"
+vxhs_qnio_iio_ioctl(uint32_t opcode) "Error while executing IOCTL. Opcode = %u"
diff --git a/block/vxhs.c b/block/vxhs.c
new file mode 100644
index 0000000..90a4343
--- /dev/null
+++ b/block/vxhs.c
@@ -0,0 +1,1645 @@
+/*
+ * QEMU Block driver for Veritas HyperScale (VxHS)
+ *
+ * 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/osdep.h"
+#include "block/block_int.h"
+#include <qnio/qnio_api.h>
+#include "qapi/qmp/qerror.h"
+#include "qapi/qmp/qdict.h"
+#include "qapi/qmp/qstring.h"
+#include "trace.h"
+#include "qemu/uri.h"
+#include "qapi/error.h"
+#include "qemu/error-report.h"
+
+#define QNIO_CONNECT_RETRY_SECS     5
+#define QNIO_CONNECT_TIMOUT_SECS    120
+
+/*
+ * IO specific flags
+ */
+#define IIO_FLAG_ASYNC              0x00000001
+#define IIO_FLAG_DONE               0x00000010
+#define IIO_FLAG_SYNC               0
+
+#define VDISK_FD_READ               0
+#define VDISK_FD_WRITE              1
+#define VXHS_MAX_HOSTS              4
+
+#define VXHS_OPT_FILENAME           "filename"
+#define VXHS_OPT_VDISK_ID           "vdisk_id"
+#define VXHS_OPT_SERVER             "server."
+#define VXHS_OPT_HOST               "host"
+#define VXHS_OPT_PORT               "port"
+
+/* qnio client ioapi_ctx */
+static void *global_qnio_ctx;
+
+/* vdisk prefix to pass to qnio */
+static const char vdisk_prefix[] = "/dev/of/vdisk";
+
+typedef enum {
+    VXHS_IO_INPROGRESS,
+    VXHS_IO_COMPLETED,
+    VXHS_IO_ERROR
+} VXHSIOState;
+
+typedef enum {
+    VDISK_AIO_READ,
+    VDISK_AIO_WRITE,
+    VDISK_STAT,
+    VDISK_TRUNC,
+    VDISK_AIO_FLUSH,
+    VDISK_AIO_RECLAIM,
+    VDISK_GET_GEOMETRY,
+    VDISK_CHECK_IO_FAILOVER_READY,
+    VDISK_AIO_LAST_CMD
+} VDISKAIOCmd;
+
+typedef void (*qnio_callback_t)(ssize_t retval, void *arg);
+
+/*
+ * BDRVVXHSState specific flags
+ */
+#define OF_VDISK_FLAGS_STATE_ACTIVE             0x0000000000000001
+#define OF_VDISK_FLAGS_STATE_FAILED             0x0000000000000002
+#define OF_VDISK_FLAGS_IOFAILOVER_IN_PROGRESS   0x0000000000000004
+
+#define OF_VDISK_ACTIVE(s)                                              \
+        ((s)->vdisk_flags & OF_VDISK_FLAGS_STATE_ACTIVE)
+#define OF_VDISK_SET_ACTIVE(s)                                          \
+        ((s)->vdisk_flags |= OF_VDISK_FLAGS_STATE_ACTIVE)
+#define OF_VDISK_RESET_ACTIVE(s)                                        \
+        ((s)->vdisk_flags &= ~OF_VDISK_FLAGS_STATE_ACTIVE)
+
+#define OF_VDISK_FAILED(s)                                              \
+        ((s)->vdisk_flags & OF_VDISK_FLAGS_STATE_FAILED)
+#define OF_VDISK_SET_FAILED(s)                                          \
+        ((s)->vdisk_flags |= OF_VDISK_FLAGS_STATE_FAILED)
+#define OF_VDISK_RESET_FAILED(s)                                        \
+        ((s)->vdisk_flags &= ~OF_VDISK_FLAGS_STATE_FAILED)
+
+#define OF_VDISK_IOFAILOVER_IN_PROGRESS(s)                              \
+        ((s)->vdisk_flags & OF_VDISK_FLAGS_IOFAILOVER_IN_PROGRESS)
+#define OF_VDISK_SET_IOFAILOVER_IN_PROGRESS(s)                          \
+        ((s)->vdisk_flags |= OF_VDISK_FLAGS_IOFAILOVER_IN_PROGRESS)
+#define OF_VDISK_RESET_IOFAILOVER_IN_PROGRESS(s)                        \
+        ((s)->vdisk_flags &= ~OF_VDISK_FLAGS_IOFAILOVER_IN_PROGRESS)
+
+/*
+ * VXHSAIOCB specific flags
+ */
+#define OF_ACB_QUEUED               0x00000001
+
+#define OF_AIOCB_FLAGS_QUEUED(a)            \
+        ((a)->flags & OF_ACB_QUEUED)
+#define OF_AIOCB_FLAGS_SET_QUEUED(a)        \
+        ((a)->flags |= OF_ACB_QUEUED)
+#define OF_AIOCB_FLAGS_RESET_QUEUED(a)      \
+        ((a)->flags &= ~OF_ACB_QUEUED)
+
+typedef struct qemu2qnio_ctx {
+    uint32_t            qnio_flag;
+    uint64_t            qnio_size;
+    char                *qnio_channel;
+    char                *target;
+    qnio_callback_t     qnio_cb;
+} qemu2qnio_ctx_t;
+
+typedef qemu2qnio_ctx_t qnio2qemu_ctx_t;
+
+typedef struct LibQNIOSymbol {
+        const char *name;
+        gpointer *addr;
+} LibQNIOSymbol;
+
+/*
+ * HyperScale AIO callbacks structure
+ */
+typedef struct VXHSAIOCB {
+    BlockAIOCB          common;
+    size_t              ret;
+    size_t              size;
+    QEMUBH              *bh;
+    int                 aio_done;
+    int                 segments;
+    int                 flags;
+    size_t              io_offset;
+    QEMUIOVector        *qiov;
+    void                *buffer;
+    int                 direction;  /* IO direction (r/w) */
+    QSIMPLEQ_ENTRY(VXHSAIOCB) retry_entry;
+} VXHSAIOCB;
+
+typedef struct VXHSvDiskHostsInfo {
+    int                 qnio_cfd;   /* Channel FD */
+    int                 vdisk_rfd;  /* vDisk remote FD */
+    char                *hostip;    /* Host's IP addresses */
+    int                 port;       /* Host's port number */
+} VXHSvDiskHostsInfo;
+
+/*
+ * Structure per vDisk maintained for state
+ */
+typedef struct BDRVVXHSState {
+    int                     fds[2];
+    int64_t                 vdisk_size;
+    int64_t                 vdisk_blocks;
+    int64_t                 vdisk_flags;
+    int                     vdisk_aio_count;
+    int                     event_reader_pos;
+    VXHSAIOCB               *qnio_event_acb;
+    void                    *qnio_ctx;
+    QemuSpin                vdisk_lock; /* Lock to protect BDRVVXHSState */
+    QemuSpin                vdisk_acb_lock;  /* Protects ACB */
+    VXHSvDiskHostsInfo      vdisk_hostinfo[VXHS_MAX_HOSTS]; /* Per host info */
+    int                     vdisk_nhosts;   /* Total number of hosts */
+    int                     vdisk_cur_host_idx; /* IOs are being shipped to */
+    int                     vdisk_ask_failover_idx; /*asking permsn to ship io*/
+    QSIMPLEQ_HEAD(aio_retryq, VXHSAIOCB) vdisk_aio_retryq;
+    int                     vdisk_aio_retry_qd; /* Currently for debugging */
+    char                    *vdisk_guid;
+} BDRVVXHSState;
+
+static int vxhs_restart_aio(VXHSAIOCB *acb);
+static void vxhs_check_failover_status(int res, void *ctx);
+
+static void vxhs_inc_acb_segment_count(void *ptr, int count)
+{
+    VXHSAIOCB *acb = ptr;
+    BDRVVXHSState *s = acb->common.bs->opaque;
+
+    qemu_spin_lock(&s->vdisk_acb_lock);
+    acb->segments += count;
+    qemu_spin_unlock(&s->vdisk_acb_lock);
+}
+
+static void vxhs_dec_acb_segment_count(void *ptr, int count)
+{
+    VXHSAIOCB *acb = ptr;
+    BDRVVXHSState *s = acb->common.bs->opaque;
+
+    qemu_spin_lock(&s->vdisk_acb_lock);
+    acb->segments -= count;
+    qemu_spin_unlock(&s->vdisk_acb_lock);
+}
+
+static void vxhs_set_acb_buffer(void *ptr, void *buffer)
+{
+    VXHSAIOCB *acb = ptr;
+
+    acb->buffer = buffer;
+}
+
+static void vxhs_inc_vdisk_iocount(void *ptr, uint32_t count)
+{
+    BDRVVXHSState *s = ptr;
+
+    qemu_spin_lock(&s->vdisk_lock);
+    s->vdisk_aio_count += count;
+    qemu_spin_unlock(&s->vdisk_lock);
+}
+
+static void vxhs_dec_vdisk_iocount(void *ptr, uint32_t count)
+{
+    BDRVVXHSState *s = ptr;
+
+    qemu_spin_lock(&s->vdisk_lock);
+    s->vdisk_aio_count -= count;
+    qemu_spin_unlock(&s->vdisk_lock);
+}
+
+static int32_t
+vxhs_qnio_iio_ioctl(void *apictx, uint32_t rfd, uint32_t opcode, int64_t *in,
+                    void *ctx, uint32_t flags)
+{
+    int   ret = 0;
+
+    switch (opcode) {
+    case VDISK_STAT:
+        ret = iio_ioctl(apictx, rfd, IOR_VDISK_STAT,
+                                     in, ctx, flags);
+        break;
+
+    case VDISK_AIO_FLUSH:
+        ret = iio_ioctl(apictx, rfd, IOR_VDISK_FLUSH,
+                                     in, ctx, flags);
+        break;
+
+    case VDISK_CHECK_IO_FAILOVER_READY:
+        ret = iio_ioctl(apictx, rfd, IOR_VDISK_CHECK_IO_FAILOVER_READY,
+                                     in, ctx, flags);
+        break;
+
+    default:
+        ret = -ENOTSUP;
+        break;
+    }
+
+    if (ret) {
+        *in = 0;
+        trace_vxhs_qnio_iio_ioctl(opcode);
+    }
+
+    return ret;
+}
+
+static void vxhs_qnio_iio_close(BDRVVXHSState *s, int idx)
+{
+    /*
+     * Close vDisk device
+     */
+    if (s->vdisk_hostinfo[idx].vdisk_rfd >= 0) {
+        iio_devclose(s->qnio_ctx, 0, s->vdisk_hostinfo[idx].vdisk_rfd);
+        s->vdisk_hostinfo[idx].vdisk_rfd = -1;
+    }
+
+    /*
+     * Close QNIO channel against cached channel-fd
+     */
+    if (s->vdisk_hostinfo[idx].qnio_cfd >= 0) {
+        iio_close(s->qnio_ctx, s->vdisk_hostinfo[idx].qnio_cfd);
+        s->vdisk_hostinfo[idx].qnio_cfd = -1;
+    }
+}
+
+static int vxhs_qnio_iio_open(int *cfd, const char *of_vsa_addr,
+                              int *rfd, const char *file_name)
+{
+    /*
+     * Open qnio channel to storage agent if not opened before.
+     */
+    if (*cfd < 0) {
+        *cfd = iio_open(global_qnio_ctx, of_vsa_addr, 0);
+        if (*cfd < 0) {
+            trace_vxhs_qnio_iio_open(of_vsa_addr);
+            return -ENODEV;
+        }
+    }
+
+    /*
+     * Open vdisk device
+     */
+    *rfd = iio_devopen(global_qnio_ctx, *cfd, file_name, 0);
+
+    if (*rfd < 0) {
+        if (*cfd >= 0) {
+            iio_close(global_qnio_ctx, *cfd);
+            *cfd = -1;
+            *rfd = -1;
+        }
+
+        trace_vxhs_qnio_iio_devopen(file_name);
+        return -ENODEV;
+    }
+
+    return 0;
+}
+
+/*
+ * Try to reopen the vDisk on one of the available hosts
+ * If vDisk reopen is successful on any of the host then
+ * check if that node is ready to accept I/O.
+ */
+static int vxhs_reopen_vdisk(BDRVVXHSState *s, int index)
+{
+    VXHSvDiskHostsInfo hostinfo = s->vdisk_hostinfo[index];
+    char *of_vsa_addr = NULL;
+    char *file_name = NULL;
+    int  res = 0;
+
+    /*
+     * Close stale vdisk device remote-fd and channel-fd
+     * since it could be invalid after a channel disconnect.
+     * We will reopen the vdisk later to get the new fd.
+     */
+    vxhs_qnio_iio_close(s, index);
+
+    /*
+     * Build storage agent address and vdisk device name strings
+     */
+    file_name = g_strdup_printf("%s%s", vdisk_prefix, s->vdisk_guid);
+    of_vsa_addr = g_strdup_printf("of://%s:%d",
+                                  hostinfo.hostip, hostinfo.port);
+
+    res = vxhs_qnio_iio_open(&hostinfo.qnio_cfd, of_vsa_addr,
+                             &hostinfo.vdisk_rfd, file_name);
+
+    g_free(of_vsa_addr);
+    g_free(file_name);
+    return res;
+}
+
+static void vxhs_fail_aio(VXHSAIOCB *acb, int err)
+{
+    BDRVVXHSState *s = NULL;
+    int segcount = 0;
+    int rv = 0;
+
+    s = acb->common.bs->opaque;
+
+    trace_vxhs_fail_aio(s->vdisk_guid, acb);
+    if (!acb->ret) {
+        acb->ret = err;
+    }
+    qemu_spin_lock(&s->vdisk_acb_lock);
+    segcount = acb->segments;
+    qemu_spin_unlock(&s->vdisk_acb_lock);
+    if (segcount == 0) {
+        /*
+         * Complete the io request
+         */
+        rv = qemu_write_full(s->fds[VDISK_FD_WRITE], &acb, sizeof(acb));
+        if (rv != sizeof(acb)) {
+            error_report("VXHS AIO completion failed: %s",
+                         strerror(errno));
+            abort();
+        }
+    }
+}
+
+static int vxhs_handle_queued_ios(BDRVVXHSState *s)
+{
+    VXHSAIOCB *acb = NULL;
+    int res = 0;
+
+    qemu_spin_lock(&s->vdisk_lock);
+    while ((acb = QSIMPLEQ_FIRST(&s->vdisk_aio_retryq)) != NULL) {
+        /*
+         * Before we process the acb, check whether I/O failover
+         * started again due to failback or cascading failure.
+         */
+        if (OF_VDISK_IOFAILOVER_IN_PROGRESS(s)) {
+            qemu_spin_unlock(&s->vdisk_lock);
+            goto out;
+        }
+        QSIMPLEQ_REMOVE_HEAD(&s->vdisk_aio_retryq, retry_entry);
+        s->vdisk_aio_retry_qd--;
+        OF_AIOCB_FLAGS_RESET_QUEUED(acb);
+        if (OF_VDISK_FAILED(s)) {
+            qemu_spin_unlock(&s->vdisk_lock);
+            vxhs_fail_aio(acb, EIO);
+            qemu_spin_lock(&s->vdisk_lock);
+        } else {
+            qemu_spin_unlock(&s->vdisk_lock);
+            res = vxhs_restart_aio(acb);
+            trace_vxhs_handle_queued_ios(acb, res);
+            qemu_spin_lock(&s->vdisk_lock);
+            if (res) {
+                QSIMPLEQ_INSERT_TAIL(&s->vdisk_aio_retryq,
+                                     acb, retry_entry);
+                OF_AIOCB_FLAGS_SET_QUEUED(acb);
+                qemu_spin_unlock(&s->vdisk_lock);
+                goto out;
+            }
+        }
+    }
+    qemu_spin_unlock(&s->vdisk_lock);
+out:
+    return res;
+}
+
+/*
+ * If errors are consistent with storage agent failure:
+ *  - Try to reconnect in case error is transient or storage agent restarted.
+ *  - Currently failover is being triggered on per vDisk basis. There is
+ *    a scope of further optimization where failover can be global (per VM).
+ *  - In case of network (storage agent) failure, for all the vDisks, having
+ *    no redundancy, I/Os will be failed without attempting for I/O failover
+ *    because of stateless nature of vDisk.
+ *  - If local or source storage agent is down then send an ioctl to remote
+ *    storage agent to check if remote storage agent in a state to accept
+ *    application I/Os.
+ *  - Once remote storage agent is ready to accept I/O, start I/O shipping.
+ *  - If I/Os cannot be serviced then vDisk will be marked failed so that
+ *    new incoming I/Os are returned with failure immediately.
+ *  - If vDisk I/O failover is in progress then all new/inflight I/Os will
+ *    queued and will be restarted or failed based on failover operation
+ *    is successful or not.
+ *  - I/O failover can be started either in I/O forward or I/O backward
+ *    path.
+ *  - I/O failover will be started as soon as all the pending acb(s)
+ *    are queued and there is no pending I/O count.
+ *  - If I/O failover couldn't be completed within QNIO_CONNECT_TIMOUT_SECS
+ *    then vDisk will be marked failed and all I/Os will be completed with
+ *    error.
+ */
+
+static int vxhs_switch_storage_agent(BDRVVXHSState *s)
+{
+    int res = 0;
+    int flags = (IIO_FLAG_ASYNC | IIO_FLAG_DONE);
+
+    trace_vxhs_switch_storage_agent(
+              s->vdisk_hostinfo[s->vdisk_ask_failover_idx].hostip,
+              s->vdisk_guid);
+
+    res = vxhs_reopen_vdisk(s, s->vdisk_ask_failover_idx);
+    if (res == 0) {
+        res = vxhs_qnio_iio_ioctl(s->qnio_ctx,
+                  s->vdisk_hostinfo[s->vdisk_ask_failover_idx].vdisk_rfd,
+                  VDISK_CHECK_IO_FAILOVER_READY, NULL, s, flags);
+    } else {
+        trace_vxhs_switch_storage_agent_failed(
+                  s->vdisk_hostinfo[s->vdisk_ask_failover_idx].hostip,
+                  s->vdisk_guid, res, errno);
+        /*
+         * Try the next host.
+         * Calling vxhs_check_failover_status from here ties up the qnio
+         * epoll loop if vxhs_qnio_iio_ioctl fails synchronously (-1)
+         * for all the hosts in the IO target list.
+         */
+
+        vxhs_check_failover_status(res, s);
+    }
+    return res;
+}
+
+static void vxhs_check_failover_status(int res, void *ctx)
+{
+    BDRVVXHSState *s = ctx;
+
+    if (res == 0) {
+        /* found failover target */
+        s->vdisk_cur_host_idx = s->vdisk_ask_failover_idx;
+        s->vdisk_ask_failover_idx = 0;
+        trace_vxhs_check_failover_status(
+                   s->vdisk_hostinfo[s->vdisk_cur_host_idx].hostip,
+                   s->vdisk_guid);
+        qemu_spin_lock(&s->vdisk_lock);
+        OF_VDISK_RESET_IOFAILOVER_IN_PROGRESS(s);
+        qemu_spin_unlock(&s->vdisk_lock);
+        vxhs_handle_queued_ios(s);
+    } else {
+        /* keep looking */
+        trace_vxhs_check_failover_status_retry(s->vdisk_guid);
+        s->vdisk_ask_failover_idx++;
+        if (s->vdisk_ask_failover_idx == s->vdisk_nhosts) {
+            /* pause and cycle through list again */
+            sleep(QNIO_CONNECT_RETRY_SECS);
+            s->vdisk_ask_failover_idx = 0;
+        }
+        res = vxhs_switch_storage_agent(s);
+    }
+}
+
+static int vxhs_failover_io(BDRVVXHSState *s)
+{
+    int res = 0;
+
+    trace_vxhs_failover_io(s->vdisk_guid);
+
+    s->vdisk_ask_failover_idx = 0;
+    res = vxhs_switch_storage_agent(s);
+
+    return res;
+}
+
+static void vxhs_iio_callback(int32_t rfd, uint32_t reason, void *ctx,
+                       uint32_t error, uint32_t opcode)
+{
+    VXHSAIOCB *acb = NULL;
+    BDRVVXHSState *s = NULL;
+    int rv = 0;
+    int segcount = 0;
+
+    switch (opcode) {
+    case IRP_READ_REQUEST:
+    case IRP_WRITE_REQUEST:
+
+    /*
+     * ctx is VXHSAIOCB*
+     * ctx is NULL if error is QNIOERROR_CHANNEL_HUP or reason is IIO_REASON_HUP
+     */
+    if (ctx) {
+        acb = ctx;
+        s = acb->common.bs->opaque;
+    } else {
+        trace_vxhs_iio_callback(error, reason);
+        goto out;
+    }
+
+    if (error) {
+        trace_vxhs_iio_callback_iofail(error, reason, acb, acb->segments);
+
+        if (reason == IIO_REASON_DONE || reason == IIO_REASON_EVENT) {
+            /*
+             * Storage agent failed while I/O was in progress
+             * Fail over only if the qnio channel dropped, indicating
+             * storage agent failure. Don't fail over in response to other
+             * I/O errors such as disk failure.
+             */
+            if (error == QNIOERROR_RETRY_ON_SOURCE || error == QNIOERROR_HUP ||
+                error == QNIOERROR_CHANNEL_HUP || error == -1) {
+                /*
+                 * Start vDisk IO failover once callback is
+                 * called against all the pending IOs.
+                 * If vDisk has no redundancy enabled
+                 * then IO failover routine will mark
+                 * the vDisk failed and fail all the
+                 * AIOs without retry (stateless vDisk)
+                 */
+                qemu_spin_lock(&s->vdisk_lock);
+                if (!OF_VDISK_IOFAILOVER_IN_PROGRESS(s)) {
+                    OF_VDISK_SET_IOFAILOVER_IN_PROGRESS(s);
+                }
+                /*
+                 * Check if this acb is already queued before.
+                 * It is possible in case if I/Os are submitted
+                 * in multiple segments (QNIO_MAX_IO_SIZE).
+                 */
+                qemu_spin_lock(&s->vdisk_acb_lock);
+                if (!OF_AIOCB_FLAGS_QUEUED(acb)) {
+                    QSIMPLEQ_INSERT_TAIL(&s->vdisk_aio_retryq,
+                                         acb, retry_entry);
+                    OF_AIOCB_FLAGS_SET_QUEUED(acb);
+                    s->vdisk_aio_retry_qd++;
+                    trace_vxhs_iio_callback_retry(s->vdisk_guid, acb);
+                }
+                segcount = --acb->segments;
+                qemu_spin_unlock(&s->vdisk_acb_lock);
+                /*
+                 * Decrement AIO count only when callback is called
+                 * against all the segments of aiocb.
+                 */
+                if (segcount == 0 && --s->vdisk_aio_count == 0) {
+                    /*
+                     * Start vDisk I/O failover
+                     */
+                    qemu_spin_unlock(&s->vdisk_lock);
+                    /*
+                     * TODO:
+                     * Need to explore further if it is possible to optimize
+                     * the failover operation on Virtual-Machine (global)
+                     * specific rather vDisk specific.
+                     */
+                    vxhs_failover_io(s);
+                    goto out;
+                }
+                qemu_spin_unlock(&s->vdisk_lock);
+                goto out;
+            }
+        } else if (reason == IIO_REASON_HUP) {
+            /*
+             * Channel failed, spontaneous notification,
+             * not in response to I/O
+             */
+            trace_vxhs_iio_callback_chnlfail(error);
+            /*
+             * TODO: Start channel failover when no I/O is outstanding
+             */
+            goto out;
+        } else {
+            trace_vxhs_iio_callback_fail(reason, acb, acb->segments,
+                                         acb->size, error);
+        }
+    }
+    /*
+     * Set error into acb if not set. In case if acb is being
+     * submitted in multiple segments then need to set the error
+     * only once.
+     *
+     * Once acb done callback is called for the last segment
+     * then acb->ret return status will be sent back to the
+     * caller.
+     */
+    qemu_spin_lock(&s->vdisk_acb_lock);
+    if (error && !acb->ret) {
+        acb->ret = error;
+    }
+    --acb->segments;
+    segcount = acb->segments;
+    assert(segcount >= 0);
+    qemu_spin_unlock(&s->vdisk_acb_lock);
+    /*
+     * Check if all the outstanding I/Os are done against acb.
+     * If yes then send signal for AIO completion.
+     */
+    if (segcount == 0) {
+        rv = qemu_write_full(s->fds[VDISK_FD_WRITE], &acb, sizeof(acb));
+        if (rv != sizeof(acb)) {
+            error_report("VXHS AIO completion failed: %s", strerror(errno));
+            abort();
+        }
+    }
+    break;
+
+    case IRP_VDISK_CHECK_IO_FAILOVER_READY:
+        /* ctx is BDRVVXHSState* */
+        assert(ctx);
+        trace_vxhs_iio_callback_ready(((BDRVVXHSState *)ctx)->vdisk_guid,
+                                      error);
+        vxhs_check_failover_status(error, ctx);
+        break;
+
+    default:
+        if (reason == IIO_REASON_HUP) {
+            /*
+             * Channel failed, spontaneous notification,
+             * not in response to I/O
+             */
+            trace_vxhs_iio_callback_chnfail(error, errno);
+            /*
+             * TODO: Start channel failover when no I/O is outstanding
+             */
+        } else {
+            trace_vxhs_iio_callback_unknwn(opcode, error);
+        }
+        break;
+    }
+out:
+    return;
+}
+
+static void vxhs_complete_aio(VXHSAIOCB *acb, BDRVVXHSState *s)
+{
+    BlockCompletionFunc *cb = acb->common.cb;
+    void *opaque = acb->common.opaque;
+    int ret = 0;
+
+    if (acb->ret != 0) {
+        trace_vxhs_complete_aio(acb, acb->ret);
+        /*
+         * We mask all the IO errors generically as EIO for upper layers
+         * Right now our IO Manager uses non standard error codes. Instead
+         * of confusing upper layers with incorrect interpretation we are
+         * doing this workaround.
+         */
+        ret = (-EIO);
+    }
+    /*
+     * Copy back contents from stablization buffer into original iovector
+     * before returning the IO
+     */
+    if (acb->buffer != NULL) {
+        qemu_iovec_from_buf(acb->qiov, 0, acb->buffer, acb->qiov->size);
+        qemu_vfree(acb->buffer);
+        acb->buffer = NULL;
+    }
+    vxhs_dec_vdisk_iocount(s, 1);
+    acb->aio_done = VXHS_IO_COMPLETED;
+    qemu_aio_unref(acb);
+    cb(opaque, ret);
+}
+
+/*
+ * This is the HyperScale event handler registered to QEMU.
+ * It is invoked when any IO gets completed and written on pipe
+ * by callback called from QNIO thread context. Then it marks
+ * the AIO as completed, and releases HyperScale AIO callbacks.
+ */
+static void vxhs_aio_event_reader(void *opaque)
+{
+    BDRVVXHSState *s = opaque;
+    ssize_t ret;
+
+    do {
+        char *p = (char *)&s->qnio_event_acb;
+
+        ret = read(s->fds[VDISK_FD_READ], p + s->event_reader_pos,
+                   sizeof(s->qnio_event_acb) - s->event_reader_pos);
+        if (ret > 0) {
+            s->event_reader_pos += ret;
+            if (s->event_reader_pos == sizeof(s->qnio_event_acb)) {
+                s->event_reader_pos = 0;
+                vxhs_complete_aio(s->qnio_event_acb, s);
+            }
+        }
+    } while (ret < 0 && errno == EINTR);
+}
+
+/*
+ * Call QNIO operation to create channels to do IO on vDisk.
+ */
+
+static void *vxhs_setup_qnio(void)
+{
+    void *qnio_ctx = NULL;
+
+    qnio_ctx = iio_init(vxhs_iio_callback);
+
+    if (qnio_ctx != NULL) {
+        trace_vxhs_setup_qnio(qnio_ctx);
+    } else {
+        trace_vxhs_setup_qnio_nwerror('.');
+    }
+
+    return qnio_ctx;
+}
+
+/*
+ * This helper function converts an array of iovectors into a flat buffer.
+ */
+
+static void *vxhs_convert_iovector_to_buffer(QEMUIOVector *qiov)
+{
+    void *buf = NULL;
+    size_t size = 0;
+
+    if (qiov->niov == 0) {
+        return buf;
+    }
+
+    size = qiov->size;
+    buf = qemu_try_memalign(BDRV_SECTOR_SIZE, size);
+    if (!buf) {
+        trace_vxhs_convert_iovector_to_buffer(size);
+        errno = -ENOMEM;
+        return NULL;
+    }
+    return buf;
+}
+
+/*
+ * This helper function iterates over the iovector and checks
+ * if the length of every element is an integral multiple
+ * of the sector size.
+ * Return Value:
+ *      On Success : true
+ *      On Failure : false
+ */
+static int vxhs_is_iovector_read_aligned(QEMUIOVector *qiov, size_t sector)
+{
+    struct iovec *iov = qiov->iov;
+    int niov = qiov->niov;
+    int i;
+
+    for (i = 0; i < niov; i++) {
+        if (iov[i].iov_len % sector != 0) {
+            return false;
+        }
+    }
+    return true;
+}
+
+static int32_t
+vxhs_qnio_iio_writev(void *qnio_ctx, uint32_t rfd, QEMUIOVector *qiov,
+                     uint64_t offset, void *ctx, uint32_t flags)
+{
+    struct iovec cur;
+    uint64_t cur_offset = 0;
+    uint64_t cur_write_len = 0;
+    int segcount = 0;
+    int ret = 0;
+    int i, nsio = 0;
+    int iovcnt = qiov->niov;
+    struct iovec *iov = qiov->iov;
+
+    errno = 0;
+    cur.iov_base = 0;
+    cur.iov_len = 0;
+
+    ret = iio_writev(qnio_ctx, rfd, iov, iovcnt, offset, ctx, flags);
+
+    if (ret == -1 && errno == EFBIG) {
+        trace_vxhs_qnio_iio_writev(ret);
+        /*
+         * IO size is larger than IIO_IO_BUF_SIZE hence need to
+         * split the I/O at IIO_IO_BUF_SIZE boundary
+         * There are two cases here:
+         *  1. iovcnt is 1 and IO size is greater than IIO_IO_BUF_SIZE
+         *  2. iovcnt is greater than 1 and IO size is greater than
+         *     IIO_IO_BUF_SIZE.
+         *
+         * Need to adjust the segment count, for that we need to compute
+         * the segment count and increase the segment count in one shot
+         * instead of setting iteratively in for loop. It is required to
+         * prevent any race between the splitted IO submission and IO
+         * completion.
+         */
+        cur_offset = offset;
+        for (i = 0; i < iovcnt; i++) {
+            if (iov[i].iov_len <= IIO_IO_BUF_SIZE && iov[i].iov_len > 0) {
+                cur_offset += iov[i].iov_len;
+                nsio++;
+            } else if (iov[i].iov_len > 0) {
+                cur.iov_base = iov[i].iov_base;
+                cur.iov_len = IIO_IO_BUF_SIZE;
+                cur_write_len = 0;
+                while (1) {
+                    nsio++;
+                    cur_write_len += cur.iov_len;
+                    if (cur_write_len == iov[i].iov_len) {
+                        break;
+                    }
+                    cur_offset += cur.iov_len;
+                    cur.iov_base += cur.iov_len;
+                    if ((iov[i].iov_len - cur_write_len) > IIO_IO_BUF_SIZE) {
+                        cur.iov_len = IIO_IO_BUF_SIZE;
+                    } else {
+                        cur.iov_len = (iov[i].iov_len - cur_write_len);
+                    }
+                }
+            }
+        }
+
+        segcount = nsio - 1;
+        vxhs_inc_acb_segment_count(ctx, segcount);
+        /*
+         * Split the IO and submit it to QNIO.
+         * Reset the cur_offset before splitting the IO.
+         */
+        cur_offset = offset;
+        nsio = 0;
+        for (i = 0; i < iovcnt; i++) {
+            if (iov[i].iov_len <= IIO_IO_BUF_SIZE && iov[i].iov_len > 0) {
+                errno = 0;
+                ret = iio_writev(qnio_ctx, rfd, &iov[i], 1, cur_offset, ctx,
+                                 flags);
+                if (ret == -1) {
+                    trace_vxhs_qnio_iio_writev_err(i, iov[i].iov_len, errno);
+                    /*
+                     * Need to adjust the AIOCB segment count to prevent
+                     * blocking of AIOCB completion within QEMU block driver.
+                     */
+                    if (segcount > 0 && (segcount - nsio) > 0) {
+                        vxhs_dec_acb_segment_count(ctx, segcount - nsio);
+                    }
+                    return ret;
+                }
+                cur_offset += iov[i].iov_len;
+                nsio++;
+            } else if (iov[i].iov_len > 0) {
+                /*
+                 * This case is where one element of the io vector is > 4MB.
+                 */
+                cur.iov_base = iov[i].iov_base;
+                cur.iov_len = IIO_IO_BUF_SIZE;
+                cur_write_len = 0;
+                while (1) {
+                    nsio++;
+                    errno = 0;
+                    ret = iio_writev(qnio_ctx, rfd, &cur, 1, cur_offset, ctx,
+                                     flags);
+                    if (ret == -1) {
+                        trace_vxhs_qnio_iio_writev_err(i, cur.iov_len, errno);
+                        /*
+                         * Need to adjust the AIOCB segment count to prevent
+                         * blocking of AIOCB completion within the
+                         * QEMU block driver.
+                         */
+                        if (segcount > 0 && (segcount - nsio) > 0) {
+                            vxhs_dec_acb_segment_count(ctx, segcount - nsio);
+                        }
+                        return ret;
+                    }
+
+                    cur_write_len += cur.iov_len;
+                    if (cur_write_len == iov[i].iov_len) {
+                        break;
+                    }
+                    cur_offset += cur.iov_len;
+                    cur.iov_base += cur.iov_len;
+                    if ((iov[i].iov_len - cur_write_len) >
+                                                IIO_IO_BUF_SIZE) {
+                        cur.iov_len = IIO_IO_BUF_SIZE;
+                    } else {
+                        cur.iov_len = (iov[i].iov_len - cur_write_len);
+                    }
+                }
+            }
+        }
+    }
+    return ret;
+}
+
+/*
+ * Iterate over the i/o vector and send read request
+ * to QNIO one by one.
+ */
+static int32_t
+vxhs_qnio_iio_readv(void *qnio_ctx, uint32_t rfd, QEMUIOVector *qiov,
+               uint64_t offset, void *ctx, uint32_t flags)
+{
+    uint64_t read_offset = offset;
+    void *buffer = NULL;
+    size_t size;
+    int aligned, segcount;
+    int i, ret = 0;
+    int iovcnt = qiov->niov;
+    struct iovec *iov = qiov->iov;
+
+    aligned = vxhs_is_iovector_read_aligned(qiov, BDRV_SECTOR_SIZE);
+    size = qiov->size;
+
+    if (!aligned) {
+        buffer = vxhs_convert_iovector_to_buffer(qiov);
+        if (buffer == NULL) {
+            return -ENOMEM;
+        }
+
+        errno = 0;
+        ret = iio_read(qnio_ctx, rfd, buffer, size, read_offset, ctx, flags);
+        if (ret != 0) {
+            trace_vxhs_qnio_iio_readv(ctx, ret, errno);
+            qemu_vfree(buffer);
+            return ret;
+        }
+        vxhs_set_acb_buffer(ctx, buffer);
+        return ret;
+    }
+
+    /*
+     * Since read IO request is going to split based on
+     * number of IOvectors hence increment the segment
+     * count depending on the number of IOVectors before
+     * submitting the read request to QNIO.
+     * This is needed to protect the QEMU block driver
+     * IO completion while read request for the same IO
+     * is being submitted to QNIO.
+     */
+    segcount = iovcnt - 1;
+    if (segcount > 0) {
+        vxhs_inc_acb_segment_count(ctx, segcount);
+    }
+
+    for (i = 0; i < iovcnt; i++) {
+        errno = 0;
+        ret = iio_read(qnio_ctx, rfd, iov[i].iov_base, iov[i].iov_len,
+                       read_offset, ctx, flags);
+        if (ret != 0) {
+            trace_vxhs_qnio_iio_readv(ctx, ret, errno);
+            /*
+             * Need to adjust the AIOCB segment count to prevent
+             * blocking of AIOCB completion within QEMU block driver.
+             */
+            if (segcount > 0 && (segcount - i) > 0) {
+                vxhs_dec_acb_segment_count(ctx, segcount - i);
+            }
+            return ret;
+        }
+        read_offset += iov[i].iov_len;
+    }
+
+    return ret;
+}
+
+static int vxhs_restart_aio(VXHSAIOCB *acb)
+{
+    BDRVVXHSState *s = NULL;
+    int iio_flags = 0;
+    int res = 0;
+
+    s = acb->common.bs->opaque;
+
+    if (acb->direction == VDISK_AIO_WRITE) {
+        vxhs_inc_vdisk_iocount(s, 1);
+        vxhs_inc_acb_segment_count(acb, 1);
+        iio_flags = (IIO_FLAG_DONE | IIO_FLAG_ASYNC);
+        res = vxhs_qnio_iio_writev(s->qnio_ctx,
+                s->vdisk_hostinfo[s->vdisk_cur_host_idx].vdisk_rfd,
+                acb->qiov, acb->io_offset, (void *)acb, iio_flags);
+    }
+
+    if (acb->direction == VDISK_AIO_READ) {
+        vxhs_inc_vdisk_iocount(s, 1);
+        vxhs_inc_acb_segment_count(acb, 1);
+        iio_flags = (IIO_FLAG_DONE | IIO_FLAG_ASYNC);
+        res = vxhs_qnio_iio_readv(s->qnio_ctx,
+                s->vdisk_hostinfo[s->vdisk_cur_host_idx].vdisk_rfd,
+                acb->qiov, acb->io_offset, (void *)acb, iio_flags);
+    }
+
+    if (res != 0) {
+        vxhs_dec_vdisk_iocount(s, 1);
+        vxhs_dec_acb_segment_count(acb, 1);
+        trace_vxhs_restart_aio(acb->direction, res, errno);
+    }
+
+    return res;
+}
+
+static QemuOptsList runtime_opts = {
+    .name = "vxhs",
+    .head = QTAILQ_HEAD_INITIALIZER(runtime_opts.head),
+    .desc = {
+        {
+            .name = VXHS_OPT_FILENAME,
+            .type = QEMU_OPT_STRING,
+            .help = "URI to the Veritas HyperScale image",
+        },
+        {
+            .name = VXHS_OPT_VDISK_ID,
+            .type = QEMU_OPT_STRING,
+            .help = "UUID of the VxHS vdisk",
+        },
+        { /* end of list */ }
+    },
+};
+
+static QemuOptsList runtime_tcp_opts = {
+    .name = "vxhs_tcp",
+    .head = QTAILQ_HEAD_INITIALIZER(runtime_tcp_opts.head),
+    .desc = {
+        {
+            .name = VXHS_OPT_HOST,
+            .type = QEMU_OPT_STRING,
+            .help = "host address (ipv4 addresses)",
+        },
+        {
+            .name = VXHS_OPT_PORT,
+            .type = QEMU_OPT_NUMBER,
+            .help = "port number on which VxHSD is listening (default 9999)",
+            .def_value_str = "9999"
+        },
+        { /* end of list */ }
+    },
+};
+
+/*
+ * Parse the incoming URI and populate *options with the host information.
+ * URI syntax has the limitation of supporting only one host info.
+ * To pass multiple host information, use the JSON syntax.
+ */
+static int vxhs_parse_uri(const char *filename, QDict *options)
+{
+    URI *uri = NULL;
+    char *hoststr, *portstr;
+    char *port;
+    int ret = 0;
+
+    trace_vxhs_parse_uri_filename(filename);
+
+    uri = uri_parse(filename);
+    if (!uri || !uri->server || !uri->path) {
+        uri_free(uri);
+        return -EINVAL;
+    }
+
+    hoststr = g_strdup(VXHS_OPT_SERVER"0.host");
+    qdict_put(options, hoststr, qstring_from_str(uri->server));
+    g_free(hoststr);
+
+    portstr = g_strdup(VXHS_OPT_SERVER"0.port");
+    if (uri->port) {
+        port = g_strdup_printf("%d", uri->port);
+        qdict_put(options, portstr, qstring_from_str(port));
+        g_free(port);
+    }
+    g_free(portstr);
+
+    if (strstr(uri->path, "vxhs") == NULL) {
+        qdict_put(options, "vdisk_id", qstring_from_str(uri->path));
+    }
+
+    trace_vxhs_parse_uri_hostinfo(1, uri->server, uri->port);
+    uri_free(uri);
+
+    return ret;
+}
+
+static void vxhs_parse_filename(const char *filename, QDict *options,
+                               Error **errp)
+{
+    if (qdict_haskey(options, "vdisk_id")
+        || qdict_haskey(options, "server"))
+    {
+        error_setg(errp, "vdisk_id/server and a file name may not be specified "
+                         "at the same time");
+        return;
+    }
+
+    if (strstr(filename, "://")) {
+        int ret = vxhs_parse_uri(filename, options);
+        if (ret < 0) {
+            error_setg(errp, "Invalid URI. URI should be of the form "
+                       "  vxhs://<host_ip>:<port>/{<vdisk_id>}");
+        }
+    }
+}
+
+static int vxhs_qemu_init(QDict *options, BDRVVXHSState *s,
+                              int *cfd, int *rfd, Error **errp)
+{
+    QDict *backing_options = NULL;
+    QemuOpts *opts, *tcp_opts;
+    const char *vxhs_filename;
+    char *of_vsa_addr = NULL;
+    Error *local_err = NULL;
+    const char *vdisk_id_opt;
+    char *file_name = NULL;
+    size_t num_servers = 0;
+    char *str = NULL;
+    int ret = 0;
+    int i;
+
+    opts = qemu_opts_create(&runtime_opts, NULL, 0, &error_abort);
+    qemu_opts_absorb_qdict(opts, options, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        ret = -EINVAL;
+        goto out;
+    }
+
+    vxhs_filename = qemu_opt_get(opts, VXHS_OPT_FILENAME);
+    if (vxhs_filename) {
+        trace_vxhs_qemu_init_filename(vxhs_filename);
+    }
+
+    vdisk_id_opt = qemu_opt_get(opts, VXHS_OPT_VDISK_ID);
+    if (!vdisk_id_opt) {
+        error_setg(&local_err, QERR_MISSING_PARAMETER, VXHS_OPT_VDISK_ID);
+        ret = -EINVAL;
+        goto out;
+    }
+    s->vdisk_guid = g_strdup(vdisk_id_opt);
+    trace_vxhs_qemu_init_vdisk(vdisk_id_opt);
+
+    num_servers = qdict_array_entries(options, VXHS_OPT_SERVER);
+    if (num_servers < 1) {
+        error_setg(&local_err, QERR_MISSING_PARAMETER, "server");
+        ret = -EINVAL;
+        goto out;
+    } else if (num_servers > VXHS_MAX_HOSTS) {
+        error_setg(&local_err, QERR_INVALID_PARAMETER, "server");
+        error_append_hint(errp, "Maximum %d servers allowed.\n",
+                          VXHS_MAX_HOSTS);
+        ret = -EINVAL;
+        goto out;
+    }
+    trace_vxhs_qemu_init_numservers(num_servers);
+
+    for (i = 0; i < num_servers; i++) {
+        str = g_strdup_printf(VXHS_OPT_SERVER"%d.", i);
+        qdict_extract_subqdict(options, &backing_options, str);
+
+        /* Create opts info from runtime_tcp_opts list */
+        tcp_opts = qemu_opts_create(&runtime_tcp_opts, NULL, 0, &error_abort);
+        qemu_opts_absorb_qdict(tcp_opts, backing_options, &local_err);
+        if (local_err) {
+            qdict_del(backing_options, str);
+            qemu_opts_del(tcp_opts);
+            g_free(str);
+            ret = -EINVAL;
+            goto out;
+        }
+
+        s->vdisk_hostinfo[i].hostip = g_strdup(qemu_opt_get(tcp_opts,
+                                                            VXHS_OPT_HOST));
+        s->vdisk_hostinfo[i].port = g_ascii_strtoll(qemu_opt_get(tcp_opts,
+                                                                 VXHS_OPT_PORT),
+                                                    NULL, 0);
+
+        s->vdisk_hostinfo[i].qnio_cfd = -1;
+        s->vdisk_hostinfo[i].vdisk_rfd = -1;
+        trace_vxhs_qemu_init(s->vdisk_hostinfo[i].hostip,
+                             s->vdisk_hostinfo[i].port);
+
+        qdict_del(backing_options, str);
+        qemu_opts_del(tcp_opts);
+        g_free(str);
+    }
+
+    s->vdisk_nhosts = i;
+    s->vdisk_cur_host_idx = 0;
+    file_name = g_strdup_printf("%s%s", vdisk_prefix, s->vdisk_guid);
+    of_vsa_addr = g_strdup_printf("of://%s:%d",
+                                s->vdisk_hostinfo[s->vdisk_cur_host_idx].hostip,
+                                s->vdisk_hostinfo[s->vdisk_cur_host_idx].port);
+
+    /*
+     * .bdrv_open() and .bdrv_create() run under the QEMU global mutex.
+     */
+    if (global_qnio_ctx == NULL) {
+        global_qnio_ctx = vxhs_setup_qnio();
+        if (global_qnio_ctx == NULL) {
+            error_setg(&local_err, "Failed vxhs_setup_qnio");
+            ret = -EINVAL;
+            goto out;
+        }
+    }
+
+    ret = vxhs_qnio_iio_open(cfd, of_vsa_addr, rfd, file_name);
+    if (!ret) {
+        error_setg(&local_err, "Failed qnio_iio_open");
+        ret = -EIO;
+    }
+
+out:
+    g_free(file_name);
+    g_free(of_vsa_addr);
+    qemu_opts_del(opts);
+
+    if (ret < 0) {
+        for (i = 0; i < num_servers; i++) {
+            g_free(s->vdisk_hostinfo[i].hostip);
+        }
+        g_free(s->vdisk_guid);
+        s->vdisk_guid = NULL;
+        errno = -ret;
+    }
+    error_propagate(errp, local_err);
+
+    return ret;
+}
+
+static int vxhs_open(BlockDriverState *bs, QDict *options,
+              int bdrv_flags, Error **errp)
+{
+    BDRVVXHSState *s = bs->opaque;
+    AioContext *aio_context;
+    int qemu_qnio_cfd = -1;
+    int device_opened = 0;
+    int qemu_rfd = -1;
+    int ret = 0;
+    int i;
+
+    ret = vxhs_qemu_init(options, s, &qemu_qnio_cfd, &qemu_rfd, errp);
+    if (ret < 0) {
+        trace_vxhs_open_fail(ret);
+        return ret;
+    }
+
+    device_opened = 1;
+    s->qnio_ctx = global_qnio_ctx;
+    s->vdisk_hostinfo[0].qnio_cfd = qemu_qnio_cfd;
+    s->vdisk_hostinfo[0].vdisk_rfd = qemu_rfd;
+    s->vdisk_size = 0;
+    QSIMPLEQ_INIT(&s->vdisk_aio_retryq);
+
+    /*
+     * Create a pipe for communicating between two threads in different
+     * context. Set handler for read event, which gets triggered when
+     * IO completion is done by non-QEMU context.
+     */
+    ret = qemu_pipe(s->fds);
+    if (ret < 0) {
+        trace_vxhs_open_epipe('.');
+        ret = -errno;
+        goto errout;
+    }
+    fcntl(s->fds[VDISK_FD_READ], F_SETFL, O_NONBLOCK);
+
+    aio_context = bdrv_get_aio_context(bs);
+    aio_set_fd_handler(aio_context, s->fds[VDISK_FD_READ],
+                       false, vxhs_aio_event_reader, NULL, s);
+
+    /*
+     * Initialize the spin-locks.
+     */
+    qemu_spin_init(&s->vdisk_lock);
+    qemu_spin_init(&s->vdisk_acb_lock);
+
+    return 0;
+
+errout:
+    /*
+     * Close remote vDisk device if it was opened earlier
+     */
+    if (device_opened) {
+        for (i = 0; i < s->vdisk_nhosts; i++) {
+            vxhs_qnio_iio_close(s, i);
+        }
+    }
+    trace_vxhs_open_fail(ret);
+    return ret;
+}
+
+static const AIOCBInfo vxhs_aiocb_info = {
+    .aiocb_size = sizeof(VXHSAIOCB)
+};
+
+/*
+ * This allocates QEMU-VXHS callback for each IO
+ * and is passed to QNIO. When QNIO completes the work,
+ * it will be passed back through the callback.
+ */
+static BlockAIOCB *vxhs_aio_rw(BlockDriverState *bs,
+                                int64_t sector_num, QEMUIOVector *qiov,
+                                int nb_sectors,
+                                BlockCompletionFunc *cb,
+                                void *opaque, int iodir)
+{
+    VXHSAIOCB *acb = NULL;
+    BDRVVXHSState *s = bs->opaque;
+    size_t size;
+    uint64_t offset;
+    int iio_flags = 0;
+    int ret = 0;
+    void *qnio_ctx = s->qnio_ctx;
+    uint32_t rfd = s->vdisk_hostinfo[s->vdisk_cur_host_idx].vdisk_rfd;
+
+    offset = sector_num * BDRV_SECTOR_SIZE;
+    size = nb_sectors * BDRV_SECTOR_SIZE;
+
+    acb = qemu_aio_get(&vxhs_aiocb_info, bs, cb, opaque);
+    /*
+     * Setup or initialize VXHSAIOCB.
+     * Every single field should be initialized since
+     * acb will be picked up from the slab without
+     * initializing with zero.
+     */
+    acb->io_offset = offset;
+    acb->size = size;
+    acb->ret = 0;
+    acb->flags = 0;
+    acb->aio_done = VXHS_IO_INPROGRESS;
+    acb->segments = 0;
+    acb->buffer = 0;
+    acb->qiov = qiov;
+    acb->direction = iodir;
+
+    qemu_spin_lock(&s->vdisk_lock);
+    if (OF_VDISK_FAILED(s)) {
+        trace_vxhs_aio_rw(s->vdisk_guid, iodir, size, offset);
+        qemu_spin_unlock(&s->vdisk_lock);
+        goto errout;
+    }
+    if (OF_VDISK_IOFAILOVER_IN_PROGRESS(s)) {
+        QSIMPLEQ_INSERT_TAIL(&s->vdisk_aio_retryq, acb, retry_entry);
+        s->vdisk_aio_retry_qd++;
+        OF_AIOCB_FLAGS_SET_QUEUED(acb);
+        qemu_spin_unlock(&s->vdisk_lock);
+        trace_vxhs_aio_rw_retry(s->vdisk_guid, acb, 1);
+        goto out;
+    }
+    s->vdisk_aio_count++;
+    qemu_spin_unlock(&s->vdisk_lock);
+
+    iio_flags = (IIO_FLAG_DONE | IIO_FLAG_ASYNC);
+
+    switch (iodir) {
+    case VDISK_AIO_WRITE:
+            vxhs_inc_acb_segment_count(acb, 1);
+            ret = vxhs_qnio_iio_writev(qnio_ctx, rfd, qiov,
+                                       offset, (void *)acb, iio_flags);
+            break;
+    case VDISK_AIO_READ:
+            vxhs_inc_acb_segment_count(acb, 1);
+            ret = vxhs_qnio_iio_readv(qnio_ctx, rfd, qiov,
+                                       offset, (void *)acb, iio_flags);
+            break;
+    default:
+            trace_vxhs_aio_rw_invalid(iodir);
+            goto errout;
+    }
+
+    if (ret != 0) {
+        trace_vxhs_aio_rw_ioerr(
+                  s->vdisk_guid, iodir, size, offset,
+                  acb, acb->segments, ret, errno);
+        /*
+         * Don't retry I/Os against vDisk having no
+         * redundancy or stateful storage on compute
+         *
+         * TODO: Revisit this code path to see if any
+         *       particular error needs to be handled.
+         *       At this moment failing the I/O.
+         */
+        qemu_spin_lock(&s->vdisk_lock);
+        if (s->vdisk_nhosts == 1) {
+            trace_vxhs_aio_rw_iofail(s->vdisk_guid);
+            s->vdisk_aio_count--;
+            vxhs_dec_acb_segment_count(acb, 1);
+            qemu_spin_unlock(&s->vdisk_lock);
+            goto errout;
+        }
+        if (OF_VDISK_FAILED(s)) {
+            trace_vxhs_aio_rw_devfail(
+                      s->vdisk_guid, iodir, size, offset);
+            s->vdisk_aio_count--;
+            vxhs_dec_acb_segment_count(acb, 1);
+            qemu_spin_unlock(&s->vdisk_lock);
+            goto errout;
+        }
+        if (OF_VDISK_IOFAILOVER_IN_PROGRESS(s)) {
+            /*
+             * Queue all incoming io requests after failover starts.
+             * Number of requests that can arrive is limited by io queue depth
+             * so an app blasting independent ios will not exhaust memory.
+             */
+            QSIMPLEQ_INSERT_TAIL(&s->vdisk_aio_retryq, acb, retry_entry);
+            s->vdisk_aio_retry_qd++;
+            OF_AIOCB_FLAGS_SET_QUEUED(acb);
+            s->vdisk_aio_count--;
+            vxhs_dec_acb_segment_count(acb, 1);
+            qemu_spin_unlock(&s->vdisk_lock);
+            trace_vxhs_aio_rw_retry(s->vdisk_guid, acb, 2);
+            goto out;
+        }
+        OF_VDISK_SET_IOFAILOVER_IN_PROGRESS(s);
+        QSIMPLEQ_INSERT_TAIL(&s->vdisk_aio_retryq, acb, retry_entry);
+        s->vdisk_aio_retry_qd++;
+        OF_AIOCB_FLAGS_SET_QUEUED(acb);
+        vxhs_dec_acb_segment_count(acb, 1);
+        trace_vxhs_aio_rw_retry(s->vdisk_guid, acb, 3);
+        /*
+         * Start I/O failover if there is no active
+         * AIO within vxhs block driver.
+         */
+        if (--s->vdisk_aio_count == 0) {
+            qemu_spin_unlock(&s->vdisk_lock);
+            /*
+             * Start IO failover
+             */
+            vxhs_failover_io(s);
+            goto out;
+        }
+        qemu_spin_unlock(&s->vdisk_lock);
+    }
+
+out:
+    return &acb->common;
+
+errout:
+    qemu_aio_unref(acb);
+    return NULL;
+}
+
+static BlockAIOCB *vxhs_aio_readv(BlockDriverState *bs,
+                                   int64_t sector_num, QEMUIOVector *qiov,
+                                   int nb_sectors,
+                                   BlockCompletionFunc *cb, void *opaque)
+{
+    return vxhs_aio_rw(bs, sector_num, qiov, nb_sectors,
+                         cb, opaque, VDISK_AIO_READ);
+}
+
+static BlockAIOCB *vxhs_aio_writev(BlockDriverState *bs,
+                                    int64_t sector_num, QEMUIOVector *qiov,
+                                    int nb_sectors,
+                                    BlockCompletionFunc *cb, void *opaque)
+{
+    return vxhs_aio_rw(bs, sector_num, qiov, nb_sectors,
+                         cb, opaque, VDISK_AIO_WRITE);
+}
+
+static void vxhs_close(BlockDriverState *bs)
+{
+    BDRVVXHSState *s = bs->opaque;
+    int i;
+
+    trace_vxhs_close(s->vdisk_guid);
+    close(s->fds[VDISK_FD_READ]);
+    close(s->fds[VDISK_FD_WRITE]);
+
+    /*
+     * Clearing all the event handlers for oflame registered to QEMU
+     */
+    aio_set_fd_handler(bdrv_get_aio_context(bs), s->fds[VDISK_FD_READ],
+                       false, NULL, NULL, NULL);
+    g_free(s->vdisk_guid);
+    s->vdisk_guid = NULL;
+
+    for (i = 0; i < VXHS_MAX_HOSTS; i++) {
+        vxhs_qnio_iio_close(s, i);
+        /*
+         * Free the dynamically allocated hostip string
+         */
+        g_free(s->vdisk_hostinfo[i].hostip);
+        s->vdisk_hostinfo[i].hostip = NULL;
+        s->vdisk_hostinfo[i].port = 0;
+    }
+}
+
+/*
+ * This is called by QEMU when a flush gets triggered from within
+ * a guest at the block layer, either for IDE or SCSI disks.
+ */
+static int vxhs_co_flush(BlockDriverState *bs)
+{
+    BDRVVXHSState *s = bs->opaque;
+    int64_t size = 0;
+    int ret = 0;
+
+    /*
+     * VDISK_AIO_FLUSH ioctl is a no-op at present and will
+     * always return success. This could change in the future.
+     */
+    ret = vxhs_qnio_iio_ioctl(s->qnio_ctx,
+            s->vdisk_hostinfo[s->vdisk_cur_host_idx].vdisk_rfd,
+            VDISK_AIO_FLUSH, &size, NULL, IIO_FLAG_SYNC);
+
+    if (ret < 0) {
+        trace_vxhs_co_flush(s->vdisk_guid, ret, errno);
+        vxhs_close(bs);
+    }
+
+    return ret;
+}
+
+static unsigned long vxhs_get_vdisk_stat(BDRVVXHSState *s)
+{
+    int64_t vdisk_size = 0;
+    int ret = 0;
+
+    ret = vxhs_qnio_iio_ioctl(s->qnio_ctx,
+            s->vdisk_hostinfo[s->vdisk_cur_host_idx].vdisk_rfd,
+            VDISK_STAT, &vdisk_size, NULL, 0);
+
+    if (ret < 0) {
+        trace_vxhs_get_vdisk_stat_err(s->vdisk_guid, ret, errno);
+        return 0;
+    }
+
+    trace_vxhs_get_vdisk_stat(s->vdisk_guid, vdisk_size);
+    return vdisk_size;
+}
+
+/*
+ * Returns the size of vDisk in bytes. This is required
+ * by QEMU block upper block layer so that it is visible
+ * to guest.
+ */
+static int64_t vxhs_getlength(BlockDriverState *bs)
+{
+    BDRVVXHSState *s = bs->opaque;
+    int64_t vdisk_size = 0;
+
+    if (s->vdisk_size > 0) {
+        vdisk_size = s->vdisk_size;
+    } else {
+        /*
+         * Fetch the vDisk size using stat ioctl
+         */
+        vdisk_size = vxhs_get_vdisk_stat(s);
+        if (vdisk_size > 0) {
+            s->vdisk_size = vdisk_size;
+        }
+    }
+
+    if (vdisk_size > 0) {
+        return vdisk_size; /* return size in bytes */
+    }
+
+    return -EIO;
+}
+
+/*
+ * Returns actual blocks allocated for the vDisk.
+ * This is required by qemu-img utility.
+ */
+static int64_t vxhs_get_allocated_blocks(BlockDriverState *bs)
+{
+    BDRVVXHSState *s = bs->opaque;
+    int64_t vdisk_size = 0;
+
+    if (s->vdisk_size > 0) {
+        vdisk_size = s->vdisk_size;
+    } else {
+        /*
+         * TODO:
+         * Once HyperScale storage-virtualizer provides
+         * actual physical allocation of blocks then
+         * fetch that information and return back to the
+         * caller but for now just get the full size.
+         */
+        vdisk_size = vxhs_get_vdisk_stat(s);
+        if (vdisk_size > 0) {
+            s->vdisk_size = vdisk_size;
+        }
+    }
+
+    if (vdisk_size > 0) {
+        return vdisk_size; /* return size in bytes */
+    }
+
+    return -EIO;
+}
+
+static void vxhs_detach_aio_context(BlockDriverState *bs)
+{
+    BDRVVXHSState *s = bs->opaque;
+
+    aio_set_fd_handler(bdrv_get_aio_context(bs), s->fds[VDISK_FD_READ],
+                       false, NULL, NULL, NULL);
+
+}
+
+static void vxhs_attach_aio_context(BlockDriverState *bs,
+                                   AioContext *new_context)
+{
+    BDRVVXHSState *s = bs->opaque;
+
+    aio_set_fd_handler(new_context, s->fds[VDISK_FD_READ],
+                       false, vxhs_aio_event_reader, NULL, s);
+}
+
+static BlockDriver bdrv_vxhs = {
+    .format_name                  = "vxhs",
+    .protocol_name                = "vxhs",
+    .instance_size                = sizeof(BDRVVXHSState),
+    .bdrv_file_open               = vxhs_open,
+    .bdrv_parse_filename          = vxhs_parse_filename,
+    .bdrv_close                   = vxhs_close,
+    .bdrv_getlength               = vxhs_getlength,
+    .bdrv_get_allocated_file_size = vxhs_get_allocated_blocks,
+    .bdrv_aio_readv               = vxhs_aio_readv,
+    .bdrv_aio_writev              = vxhs_aio_writev,
+    .bdrv_co_flush_to_disk        = vxhs_co_flush,
+    .bdrv_detach_aio_context      = vxhs_detach_aio_context,
+    .bdrv_attach_aio_context      = vxhs_attach_aio_context,
+};
+
+static void bdrv_vxhs_init(void)
+{
+    trace_vxhs_bdrv_init('.');
+    bdrv_register(&bdrv_vxhs);
+}
+
+block_init(bdrv_vxhs_init);
diff --git a/configure b/configure
index 8fa62ad..50fe935 100755
--- a/configure
+++ b/configure
@@ -320,6 +320,7 @@ numa=""
 tcmalloc="no"
 jemalloc="no"
 replication="yes"
+vxhs=""
 
 # parse CC options first
 for opt do
@@ -1159,6 +1160,11 @@ for opt do
   ;;
   --enable-replication) replication="yes"
   ;;
+  --disable-vxhs) vxhs="no"
+  ;;
+  --enable-vxhs) vxhs="yes"
+  ;;
+
   *)
       echo "ERROR: unknown option $opt"
       echo "Try '$0 --help' for more information"
@@ -1388,6 +1394,7 @@ disabled with --disable-FEATURE, default is enabled if available:
   tcmalloc        tcmalloc support
   jemalloc        jemalloc support
   replication     replication support
+  vxhs            Veritas HyperScale vDisk backend support
 
 NOTE: The object files are built at the place where configure is launched
 EOF
@@ -4513,6 +4520,33 @@ if do_cc -nostdlib -Wl,-r -Wl,--no-relax -o $TMPMO $TMPO; then
 fi
 
 ##########################################
+# Veritas HyperScale block driver VxHS
+# Check if libqnio is installed
+
+if test "$vxhs" != "no" ; then
+  cat > $TMPC <<EOF
+#include <stdint.h>
+#include <qnio/qnio_api.h>
+
+void *vxhs_callback;
+
+int main(void) {
+    iio_init(vxhs_callback);
+    return 0;
+}
+EOF
+  vxhs_libs="-lqnio"
+  if compile_prog "" "$vxhs_libs" ; then
+    vxhs=yes
+  else
+    if test "$vxhs" = "yes" ; then
+      feature_not_found "vxhs block device" "Install libqnio. See github"
+    fi
+    vxhs=no
+  fi
+fi
+
+##########################################
 # End of CC checks
 # After here, no more $cc or $ld runs
 
@@ -4877,6 +4911,7 @@ echo "tcmalloc support  $tcmalloc"
 echo "jemalloc support  $jemalloc"
 echo "avx2 optimization $avx2_opt"
 echo "replication support $replication"
+echo "VxHS block device $vxhs"
 
 if test "$sdl_too_old" = "yes"; then
 echo "-> Your SDL version is too old - please upgrade to have SDL support"
@@ -5465,6 +5500,12 @@ if test "$pthread_setname_np" = "yes" ; then
   echo "CONFIG_PTHREAD_SETNAME_NP=y" >> $config_host_mak
 fi
 
+if test "$vxhs" = "yes" ; then
+  echo "CONFIG_VXHS=y" >> $config_host_mak
+  echo "VXHS_CFLAGS=$vxhs_cflags" >> $config_host_mak
+  echo "VXHS_LIBS=$vxhs_libs" >> $config_host_mak
+fi
+
 if test "$tcg_interpreter" = "yes"; then
   QEMU_INCLUDES="-I\$(SRC_PATH)/tcg/tci $QEMU_INCLUDES"
 elif test "$ARCH" = "sparc64" ; then
-- 
2.5.5