[Qemu-devel] [PATCH 02/10] xen: backend driver core

[Gerd Hoffmann]

This patch adds infrastructure for xen backend drivers living in qemu,
so drivers don't need to implement common stuff on their own.  It's
mostly xenbus management stuff: some functions to access xentore,
setting up xenstore watches, callbacks on device discovery and state
changes, handle event channel, ...

Signed-off-by: Gerd Hoffmann <address@hidden>
---
 Makefile.target     |    2 +-
 hw/xen_backend.c    |  691 +++++++++++++++++++++++++++++++++++++++++++++++++++
 hw/xen_backend.h    |   86 +++++++
 hw/xen_common.h     |   34 +++
 hw/xen_machine_pv.c |    8 +-
 5 files changed, 819 insertions(+), 2 deletions(-)
 create mode 100644 hw/xen_backend.c
 create mode 100644 hw/xen_backend.h
 create mode 100644 hw/xen_common.h

diff --git a/Makefile.target b/Makefile.target
index 1cad75e..5d9dfc7 100644
--- a/Makefile.target
+++ b/Makefile.target
@@ -558,7 +558,7 @@ LIBS += $(CONFIG_BLUEZ_LIBS)
 endif
 
 # xen backend driver support
-XEN_OBJS := xen_machine_pv.o
+XEN_OBJS := xen_machine_pv.o xen_backend.o
 ifeq ($(CONFIG_XEN), yes)
   OBJS += $(XEN_OBJS)
   LIBS += $(XEN_LIBS)
diff --git a/hw/xen_backend.c b/hw/xen_backend.c
new file mode 100644
index 0000000..b13c3f9
--- /dev/null
+++ b/hw/xen_backend.c
@@ -0,0 +1,691 @@
+/*
+ *  xen backend driver infrastructure
+ *  (c) 2008 Gerd Hoffmann <address@hidden>
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation; under version 2 of the License.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License along
+ *  with this program; if not, write to the Free Software Foundation, Inc.,
+ *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+/*
+ * TODO: add some xenbus / xenstore concepts overview here.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdarg.h>
+#include <string.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <inttypes.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+#include <sys/signal.h>
+
+#include <xs.h>
+#include <xenctrl.h>
+#include <xen/grant_table.h>
+
+#include "hw.h"
+#include "qemu-char.h"
+#include "xen_backend.h"
+
+/* ------------------------------------------------------------- */
+
+/* public */
+int xen_xc;
+struct xs_handle *xenstore = NULL;
+
+/* private */
+static TAILQ_HEAD(XenDeviceHead, XenDevice) xendevs = 
TAILQ_HEAD_INITIALIZER(xendevs);
+static int debug = 0;
+
+/* ------------------------------------------------------------- */
+
+int xenstore_write_str(const char *base, const char *node, const char *val)
+{
+    char abspath[XEN_BUFSIZE];
+
+    snprintf(abspath, sizeof(abspath), "%s/%s", base, node);
+    if (!xs_write(xenstore, 0, abspath, val, strlen(val)))
+       return -1;
+    return 0;
+}
+
+char *xenstore_read_str(const char *base, const char *node)
+{
+    char abspath[XEN_BUFSIZE];
+    unsigned int len;
+
+    snprintf(abspath, sizeof(abspath), "%s/%s", base, node);
+    return xs_read(xenstore, 0, abspath, &len);
+}
+
+int xenstore_write_int(const char *base, const char *node, int ival)
+{
+    char val[32];
+
+    snprintf(val, sizeof(val), "%d", ival);
+    return xenstore_write_str(base, node, val);
+}
+
+int xenstore_read_int(const char *base, const char *node, int *ival)
+{
+    char *val;
+    int rc = -1;
+
+    val = xenstore_read_str(base, node);
+    if (val && 1 == sscanf(val, "%d", ival))
+       rc = 0;
+    qemu_free(val);
+    return rc;
+}
+
+int xenstore_write_be_str(struct XenDevice *xendev, const char *node, const 
char *val)
+{
+    return xenstore_write_str(xendev->be, node, val);
+}
+
+int xenstore_write_be_int(struct XenDevice *xendev, const char *node, int ival)
+{
+    return xenstore_write_int(xendev->be, node, ival);
+}
+
+char *xenstore_read_be_str(struct XenDevice *xendev, const char *node)
+{
+    return xenstore_read_str(xendev->be, node);
+}
+
+int xenstore_read_be_int(struct XenDevice *xendev, const char *node, int *ival)
+{
+    return xenstore_read_int(xendev->be, node, ival);
+}
+
+char *xenstore_read_fe_str(struct XenDevice *xendev, const char *node)
+{
+    return xenstore_read_str(xendev->fe, node);
+}
+
+int xenstore_read_fe_int(struct XenDevice *xendev, const char *node, int *ival)
+{
+    return xenstore_read_int(xendev->fe, node, ival);
+}
+
+/* ------------------------------------------------------------- */
+
+const char *xenbus_strstate(enum xenbus_state state)
+{
+       static const char *const name[] = {
+               [ XenbusStateUnknown      ] = "Unknown",
+               [ XenbusStateInitialising ] = "Initialising",
+               [ XenbusStateInitWait     ] = "InitWait",
+               [ XenbusStateInitialised  ] = "Initialised",
+               [ XenbusStateConnected    ] = "Connected",
+               [ XenbusStateClosing      ] = "Closing",
+               [ XenbusStateClosed       ] = "Closed",
+       };
+       return (state < ARRAY_SIZE(name)) ? name[state] : "INVALID";
+}
+
+int xen_be_set_state(struct XenDevice *xendev, enum xenbus_state state)
+{
+    int rc;
+
+    rc = xenstore_write_be_int(xendev, "state", state);
+    if (rc < 0)
+       return rc;
+    xen_be_printf(xendev, 1, "backend state: %s -> %s\n",
+                 xenbus_strstate(xendev->be_state), xenbus_strstate(state));
+    xendev->be_state = state;
+    return 0;
+}
+
+/* ------------------------------------------------------------- */
+
+struct XenDevice *xen_be_find_xendev(const char *type, int dom, int dev)
+{
+    struct XenDevice *xendev;
+
+    TAILQ_FOREACH(xendev, &xendevs, next) {
+       if (xendev->dom != dom)
+           continue;
+       if (xendev->dev != dev)
+           continue;
+       if (0 != strcmp(xendev->type, type))
+           continue;
+       return xendev;
+    }
+    return NULL;
+}
+
+/*
+ * get xen backend device, allocate a new one if it doesn't exist.
+ */
+static struct XenDevice *xen_be_get_xendev(const char *type, int dom, int dev,
+                                           struct XenDevOps *ops)
+{
+    struct XenDevice *xendev;
+    char *dom0;
+
+    xendev = xen_be_find_xendev(type, dom, dev);
+    if (xendev)
+       return xendev;
+
+    /* init new xendev */
+    xendev = qemu_mallocz(ops->size);
+    if (!xendev)
+        return NULL;
+    xendev->type  = type;
+    xendev->dom   = dom;
+    xendev->dev   = dev;
+    xendev->ops   = ops;
+
+    dom0 = xs_get_domain_path(xenstore, 0);
+    snprintf(xendev->be, sizeof(xendev->be), "%s/backend/%s/%d/%d",
+            dom0, xendev->type, xendev->dom, xendev->dev);
+    snprintf(xendev->name, sizeof(xendev->name), "%s-%d",
+            xendev->type, xendev->dev);
+    free(dom0);
+
+    xendev->debug      = debug;
+    xendev->local_port = -1;
+
+    xendev->evtchndev = xc_evtchn_open();
+    if (xendev->evtchndev < 0) {
+       fprintf(stderr, "can't open evtchn device\n");
+       qemu_free(xendev);
+       return NULL;
+    }
+    fcntl(xc_evtchn_fd(xendev->evtchndev), F_SETFD, FD_CLOEXEC);
+
+    if (ops->flags & DEVOPS_FLAG_NEED_GNTDEV) {
+       xendev->gnttabdev = xc_gnttab_open();
+       if (xendev->gnttabdev < 0) {
+           fprintf(stderr, "can't open gnttab device\n");
+           xc_evtchn_close(xendev->evtchndev);
+           qemu_free(xendev);
+           return NULL;
+       }
+    } else {
+       xendev->gnttabdev = -1;
+    }
+
+    TAILQ_INSERT_TAIL(&xendevs, xendev, next);
+
+    if (xendev->ops->alloc)
+       xendev->ops->alloc(xendev);
+
+    return xendev;
+}
+
+/*
+ * release xen backend device.
+ */
+static struct XenDevice *xen_be_del_xendev(int dom, int dev)
+{
+    struct XenDevice *xendev, *xnext;
+
+    /*
+     * This is pretty much like TAILQ_FOREACH(xendev, &xendevs, next) but
+     * we save the next pointer in xnext because we might free xendev.
+     */
+    xnext = xendevs.tqh_first;
+    while (xnext) {
+        xendev = xnext;
+        xnext = xendev->next.tqe_next;
+
+       if (xendev->dom != dom)
+           continue;
+       if (xendev->dev != dev && dev != -1)
+           continue;
+
+       if (xendev->ops->free)
+           xendev->ops->free(xendev);
+
+       if (xendev->fe) {
+           char token[XEN_BUFSIZE];
+           snprintf(token, sizeof(token), "fe:%p", xendev);
+           xs_unwatch(xenstore, xendev->fe, token);
+           qemu_free(xendev->fe);
+       }
+
+       if (xendev->evtchndev >= 0)
+           xc_evtchn_close(xendev->evtchndev);
+       if (xendev->gnttabdev >= 0)
+           xc_gnttab_close(xendev->gnttabdev);
+
+       TAILQ_REMOVE(&xendevs, xendev, next);
+       qemu_free(xendev);
+    }
+    return NULL;
+}
+
+/*
+ * Sync internal data structures on xenstore updates.
+ * Node specifies the changed field.  node = NULL means
+ * update all fields (used for initialization).
+ */
+static void xen_be_backend_changed(struct XenDevice *xendev, const char *node)
+{
+    if (node == NULL  ||  strcmp(node, "online") == 0) {
+       if (xenstore_read_be_int(xendev, "online", &xendev->online) == -1)
+           xendev->online = 0;
+    }
+
+    if (node) {
+       xen_be_printf(xendev, 2, "backend update: %s\n", node);
+       if (xendev->ops->backend_changed)
+           xendev->ops->backend_changed(xendev, node);
+    }
+}
+
+static void xen_be_frontend_changed(struct XenDevice *xendev, const char *node)
+{
+    int fe_state;
+
+    if (node == NULL  ||  strcmp(node, "state") == 0) {
+       if (xenstore_read_fe_int(xendev, "state", &fe_state) == -1)
+           fe_state = XenbusStateUnknown;
+       if (xendev->fe_state != fe_state)
+           xen_be_printf(xendev, 1, "frontend state: %s -> %s\n",
+                         xenbus_strstate(xendev->fe_state),
+                         xenbus_strstate(fe_state));
+       xendev->fe_state = fe_state;
+    }
+    if (node == NULL  ||  strcmp(node, "protocol") == 0) {
+       qemu_free(xendev->protocol);
+       xendev->protocol = xenstore_read_fe_str(xendev, "protocol");
+       if (xendev->protocol)
+           xen_be_printf(xendev, 1, "frontend protocol: %s\n", 
xendev->protocol);
+    }
+
+    if (node) {
+       xen_be_printf(xendev, 2, "frontend update: %s\n", node);
+       if (xendev->ops->frontend_changed)
+           xendev->ops->frontend_changed(xendev, node);
+    }
+}
+
+/* ------------------------------------------------------------- */
+/* Check for possible state transitions and perform them.        */
+
+/*
+ * Initial xendev setup.  Read frontend path, register watch for it.
+ * Should succeed once xend finished setting up the backend device.
+ *
+ * Also sets initial state (-> Initializing) when done.  Which
+ * only affects the xendev->be_state variable as xenbus should
+ * already be put into that state by xend.
+ */
+static int xen_be_try_setup(struct XenDevice *xendev)
+{
+    char token[XEN_BUFSIZE];
+    int be_state;
+
+    if (xenstore_read_be_int(xendev, "state", &be_state) == -1) {
+       xen_be_printf(xendev, 0, "reading backend state failed\n");
+       return -1;
+    }
+
+    if (be_state != XenbusStateInitialising) {
+       xen_be_printf(xendev, 0, "initial backend state is wrong (%s)\n",
+                     xenbus_strstate(be_state));
+       return -1;
+    }
+
+    xendev->fe = xenstore_read_be_str(xendev, "frontend");
+    if (xendev->fe == NULL) {
+       xen_be_printf(xendev, 0, "reading frontend path failed\n");
+       return -1;
+    }
+
+    /* setup frontend watch */
+    snprintf(token, sizeof(token), "fe:%p", xendev);
+    if (!xs_watch(xenstore, xendev->fe, token)) {
+       xen_be_printf(xendev, 0, "watching frontend path (%s) failed\n",
+                     xendev->fe);
+       return -1;
+    }
+    xen_be_set_state(xendev, XenbusStateInitialising);
+
+    xen_be_backend_changed(xendev, NULL);
+    xen_be_frontend_changed(xendev, NULL);
+    return 0;
+}
+
+/*
+ * Try initialize xendev.  Prepare everything the backend can do
+ * without synchronizing with the frontend.  Fakes hotplug-status.  No
+ * hotplug involved here because this is about userspace drivers, thus
+ * there are kernel backend devices which could invoke hotplug.
+ *
+ * Goes to InitWait on success.
+ */
+static int xen_be_try_init(struct XenDevice *xendev)
+{
+    int rc = 0;
+
+    if (!xendev->online) {
+       xen_be_printf(xendev, 1, "not online\n");
+       return -1;
+    }
+
+    if (xendev->ops->init)
+       rc = xendev->ops->init(xendev);
+    if (0 != rc) {
+       xen_be_printf(xendev, 1, "init() failed\n");
+       return rc;
+    }
+
+    xenstore_write_be_str(xendev, "hotplug-status", "connected");
+    xen_be_set_state(xendev, XenbusStateInitWait);
+    return 0;
+}
+
+/*
+ * Try to connect xendev.  Depends on the frontend being ready
+ * for it (shared ring and evtchn info in xenstore, state being
+ * Initialised or Connected).
+ *
+ * Goes to Connected on success.
+ */
+static int xen_be_try_connect(struct XenDevice *xendev)
+{
+    int rc = 0;
+
+    if (xendev->fe_state != XenbusStateInitialised  &&
+       xendev->fe_state != XenbusStateConnected) {
+       if (xendev->ops->flags & DEVOPS_FLAG_IGNORE_STATE) {
+           xen_be_printf(xendev, 2, "frontend not ready, ignoring\n");
+       } else {
+           xen_be_printf(xendev, 2, "frontend not ready (yet)\n");
+           return -1;
+       }
+    }
+
+    if (xendev->ops->connect)
+       rc = xendev->ops->connect(xendev);
+    if (0 != rc) {
+       xen_be_printf(xendev, 0, "connect() failed\n");
+       return rc;
+    }
+
+    xen_be_set_state(xendev, XenbusStateConnected);
+    return 0;
+}
+
+/*
+ * Teardown connection.
+ *
+ * Goes to Closed when done.
+ */
+static void xen_be_disconnect(struct XenDevice *xendev, enum xenbus_state 
state)
+{
+    if (xendev->be_state != XenbusStateClosing &&
+        xendev->be_state != XenbusStateClosed  &&
+        xendev->ops->disconnect)
+       xendev->ops->disconnect(xendev);
+    if (xendev->be_state != state)
+        xen_be_set_state(xendev, state);
+}
+
+/*
+ * Try to reset xendev, for reconnection by another frontend instance.
+ */
+static int xen_be_try_reset(struct XenDevice *xendev)
+{
+    if (xendev->fe_state != XenbusStateInitialising)
+        return -1;
+
+    xen_be_printf(xendev, 1, "device reset (for re-connect)\n");
+    xen_be_set_state(xendev, XenbusStateInitialising);
+    return 0;
+}
+
+/*
+ * state change dispatcher function
+ */
+void xen_be_check_state(struct XenDevice *xendev)
+{
+    int rc = 0;
+
+    /* frontend may request shutdown from almost anywhere */
+    if (xendev->fe_state == XenbusStateClosing ||
+       xendev->fe_state == XenbusStateClosed) {
+       xen_be_disconnect(xendev, xendev->fe_state);
+       return;
+    }
+
+    /* check for possible backend state transitions */
+    for (;;) {
+       switch (xendev->be_state) {
+       case XenbusStateUnknown:
+           rc = xen_be_try_setup(xendev);
+           break;
+       case XenbusStateInitialising:
+           rc = xen_be_try_init(xendev);
+           break;
+       case XenbusStateInitWait:
+           rc = xen_be_try_connect(xendev);
+           break;
+        case XenbusStateClosed:
+            rc = xen_be_try_reset(xendev);
+            break;
+       default:
+           rc = -1;
+       }
+       if (0 != rc)
+           break;
+    }
+}
+
+/* ------------------------------------------------------------- */
+
+static int xenstore_scan(const char *type, int dom, struct XenDevOps *ops)
+{
+    struct XenDevice *xendev;
+    char path[XEN_BUFSIZE], token[XEN_BUFSIZE];
+    char **dev = NULL, *dom0;
+    unsigned int cdev, j;
+
+    /* setup watch */
+    dom0 = xs_get_domain_path(xenstore, 0);
+    snprintf(token, sizeof(token), "be:%p:%d:%p", type, dom, ops);
+    snprintf(path, sizeof(path), "%s/backend/%s/%d", dom0, type, dom);
+    free(dom0);
+    if (!xs_watch(xenstore, path, token)) {
+       fprintf(stderr, "xen be: watching backend path (%s) failed\n", path);
+       return -1;
+    }
+
+    /* look for backends */
+    dev = xs_directory(xenstore, 0, path, &cdev);
+    if (!dev)
+       return 0;
+    for (j = 0; j < cdev; j++) {
+       xendev = xen_be_get_xendev(type, dom, atoi(dev[j]), ops);
+       if (xendev == NULL)
+           continue;
+       xen_be_check_state(xendev);
+    }
+    qemu_free(dev);
+    return 0;
+}
+
+static void xenstore_update_be(char *watch, char *type, int dom,
+                              struct XenDevOps *ops)
+{
+    struct XenDevice *xendev;
+    char path[XEN_BUFSIZE], *dom0;
+    unsigned int len, dev;
+
+    dom0 = xs_get_domain_path(xenstore, 0);
+    len = snprintf(path, sizeof(path), "%s/backend/%s/%d", dom0, type, dom);
+    free(dom0);
+    if (0 != strncmp(path, watch, len))
+       return;
+    if (2 != sscanf(watch+len, "/%u/%255s", &dev, path)) {
+       strcpy(path, "");
+       if (1 != sscanf(watch+len, "/%u", &dev))
+           dev = -1;
+    }
+    if (dev == -1)
+       return;
+
+    if (0) {
+       /* FIXME: detect devices being deleted from xenstore ... */
+       xen_be_del_xendev(dom, dev);
+    }
+
+    xendev = xen_be_get_xendev(type, dom, dev, ops);
+    if (NULL != xendev) {
+       xen_be_backend_changed(xendev, path);
+       xen_be_check_state(xendev);
+    }
+}
+
+static void xenstore_update_fe(char *watch, struct XenDevice *xendev)
+{
+    char *node;
+    unsigned int len;
+
+    len = strlen(xendev->fe);
+    if (0 != strncmp(xendev->fe, watch, len))
+       return;
+    if (watch[len] != '/')
+       return;
+    node = watch + len + 1;
+
+    xen_be_frontend_changed(xendev, node);
+    xen_be_check_state(xendev);
+}
+
+static void xenstore_update(void *unused)
+{
+    char **vec = NULL;
+    intptr_t type, ops, ptr;
+    unsigned int dom, count;
+
+    vec = xs_read_watch(xenstore, &count);
+    if (vec == NULL)
+       goto cleanup;
+
+    if (3 == sscanf(vec[XS_WATCH_TOKEN], "be:%" PRIxPTR ":%d:%" PRIxPTR,
+                   &type, &dom, &ops))
+       xenstore_update_be(vec[XS_WATCH_PATH], (void*)type, dom, (void*)ops);
+    if (1 == sscanf(vec[XS_WATCH_TOKEN], "fe:%" PRIxPTR, &ptr))
+       xenstore_update_fe(vec[XS_WATCH_PATH], (void*)ptr);
+
+cleanup:
+    qemu_free(vec);
+}
+
+static void xen_be_evtchn_event(void *opaque)
+{
+    struct XenDevice *xendev = opaque;
+    evtchn_port_t port;
+
+    port = xc_evtchn_pending(xendev->evtchndev);
+    if (port != xendev->local_port) {
+       xen_be_printf(xendev, 0, "xc_evtchn_pending returned %d (expected 
%d)\n",
+                     port, xendev->local_port);
+       return;
+    }
+    xc_evtchn_unmask(xendev->evtchndev, port);
+
+    if (xendev->ops->event)
+       xendev->ops->event(xendev);
+}
+
+/* -------------------------------------------------------------------- */
+
+int xen_be_init(void)
+{
+    xenstore = xs_daemon_open();
+    if (!xenstore) {
+       fprintf(stderr, "can't connect to xenstored\n");
+       return -1;
+    }
+
+    if (qemu_set_fd_handler(xs_fileno(xenstore), xenstore_update, NULL, NULL) 
< 0)
+       goto err;
+
+    xen_xc = xc_interface_open();
+    if (xen_xc == -1) {
+       fprintf(stderr, "can't open xen interface\n");
+       goto err;
+    }
+    return 0;
+
+err:
+    qemu_set_fd_handler(xs_fileno(xenstore), NULL, NULL, NULL);
+    xs_daemon_close(xenstore);
+    xenstore = NULL;
+
+    return -1;
+}
+
+int xen_be_register(const char *type, struct XenDevOps *ops)
+{
+    return xenstore_scan(type, xen_domid, ops);
+}
+
+int xen_be_bind_evtchn(struct XenDevice *xendev)
+{
+    if (xendev->local_port != -1)
+       return 0;
+    xendev->local_port = xc_evtchn_bind_interdomain
+       (xendev->evtchndev, xendev->dom, xendev->remote_port);
+    if (xendev->local_port == -1) {
+       xen_be_printf(xendev, 0, "xc_evtchn_bind_interdomain failed\n");
+       return -1;
+    }
+    xen_be_printf(xendev, 2, "bind evtchn port %d\n", xendev->local_port);
+    qemu_set_fd_handler(xc_evtchn_fd(xendev->evtchndev),
+                       xen_be_evtchn_event, NULL, xendev);
+    return 0;
+}
+
+void xen_be_unbind_evtchn(struct XenDevice *xendev)
+{
+    if (xendev->local_port == -1)
+       return;
+    qemu_set_fd_handler(xc_evtchn_fd(xendev->evtchndev), NULL, NULL, NULL);
+    xc_evtchn_unbind(xendev->evtchndev, xendev->local_port);
+    xen_be_printf(xendev, 2, "unbind evtchn port %d\n", xendev->local_port);
+    xendev->local_port = -1;
+}
+
+int xen_be_send_notify(struct XenDevice *xendev)
+{
+    return xc_evtchn_notify(xendev->evtchndev, xendev->local_port);
+}
+
+/*
+ * msg_level:
+ *  0 == errors.
+ *  1 == informative debug messages.
+ *  2 == noisy debug messages.
+ *  3 == will flood your log.
+ */
+void xen_be_printf(struct XenDevice *xendev, int msg_level, const char *fmt, 
...)
+{
+    va_list args;
+
+    if (msg_level > xendev->debug)
+       return;
+    fprintf(stderr, "xen be: %s: ", xendev->name);
+    va_start(args, fmt);
+    vfprintf(stderr, fmt, args);
+    va_end(args);
+}
diff --git a/hw/xen_backend.h b/hw/xen_backend.h
new file mode 100644
index 0000000..a1243f6
--- /dev/null
+++ b/hw/xen_backend.h
@@ -0,0 +1,86 @@
+#ifndef QEMU_HW_XEN_BACKEND_H
+#define QEMU_HW_XEN_BACKEND_H 1
+
+#include "xen_common.h"
+
+/* ------------------------------------------------------------- */
+
+#define XEN_BUFSIZE 1024
+
+struct XenDevice;
+
+/* driver uses grant tables  ->  open gntdev device (xendev->gnttabdev) */
+#define DEVOPS_FLAG_NEED_GNTDEV   1
+/* don't expect frontend doing correct state transitions (aka console quirk) */
+#define DEVOPS_FLAG_IGNORE_STATE  2
+
+struct XenDevOps {
+    size_t    size;
+    uint32_t  flags;
+    void      (*alloc)(struct XenDevice *xendev);
+    int       (*init)(struct XenDevice *xendev);
+    int       (*connect)(struct XenDevice *xendev);
+    void      (*event)(struct XenDevice *xendev);
+    void      (*disconnect)(struct XenDevice *xendev);
+    int       (*free)(struct XenDevice *xendev);
+    void      (*backend_changed)(struct XenDevice *xendev, const char *node);
+    void      (*frontend_changed)(struct XenDevice *xendev, const char *node);
+};
+
+struct XenDevice {
+    const char         *type;
+    int                dom;
+    int                dev;
+    char               name[64];
+    int                debug;
+
+    enum xenbus_state  be_state;
+    enum xenbus_state  fe_state;
+    int                online;
+    char               be[XEN_BUFSIZE];
+    char               *fe;
+    char               *protocol;
+    int                remote_port;
+    int                local_port;
+
+    int                evtchndev;
+    int                gnttabdev;
+
+    struct XenDevOps   *ops;
+    TAILQ_ENTRY(XenDevice) next;
+};
+
+/* ------------------------------------------------------------- */
+
+/* variables */
+extern int xen_xc;
+extern struct xs_handle *xenstore;
+
+/* xenstore helper functions */
+int xenstore_write_str(const char *base, const char *node, const char *val);
+int xenstore_write_int(const char *base, const char *node, int ival);
+char *xenstore_read_str(const char *base, const char *node);
+int xenstore_read_int(const char *base, const char *node, int *ival);
+
+int xenstore_write_be_str(struct XenDevice *xendev, const char *node, const 
char *val);
+int xenstore_write_be_int(struct XenDevice *xendev, const char *node, int 
ival);
+char *xenstore_read_be_str(struct XenDevice *xendev, const char *node);
+int xenstore_read_be_int(struct XenDevice *xendev, const char *node, int 
*ival);
+char *xenstore_read_fe_str(struct XenDevice *xendev, const char *node);
+int xenstore_read_fe_int(struct XenDevice *xendev, const char *node, int 
*ival);
+
+const char *xenbus_strstate(enum xenbus_state state);
+struct XenDevice *xen_be_find_xendev(const char *type, int dom, int dev);
+void xen_be_check_state(struct XenDevice *xendev);
+
+/* xen backend driver bits */
+int xen_be_init(void);
+int xen_be_register(const char *type, struct XenDevOps *ops);
+int xen_be_set_state(struct XenDevice *xendev, enum xenbus_state state);
+int xen_be_bind_evtchn(struct XenDevice *xendev);
+void xen_be_unbind_evtchn(struct XenDevice *xendev);
+int xen_be_send_notify(struct XenDevice *xendev);
+void xen_be_printf(struct XenDevice *xendev, int msg_level, const char *fmt, 
...)
+    __attribute__ ((format(printf, 3, 4)));
+
+#endif /* QEMU_HW_XEN_BACKEND_H */
diff --git a/hw/xen_common.h b/hw/xen_common.h
new file mode 100644
index 0000000..7562567
--- /dev/null
+++ b/hw/xen_common.h
@@ -0,0 +1,34 @@
+#ifndef QEMU_HW_XEN_COMMON_H
+#define QEMU_HW_XEN_COMMON_H 1
+
+#include <stddef.h>
+#include <inttypes.h>
+
+#include <xenctrl.h>
+#include <xs.h>
+#include <xen/io/xenbus.h>
+
+#include "hw.h"
+#include "xen.h"
+#include "sys-queue.h"   /* BSD list implementation */
+
+/*
+ * tweaks needed to build with different xen versions
+ *  0x00030205 -> 3.1.0
+ *  0x00030207 -> 3.2.0
+ *  0x00030208 -> unstable
+ */
+#include <xen/xen-compat.h>
+#if __XEN_LATEST_INTERFACE_VERSION__ < 0x00030205
+# define evtchn_port_or_error_t int
+#endif
+#if __XEN_LATEST_INTERFACE_VERSION__ < 0x00030207
+# define xc_map_foreign_pages xc_map_foreign_batch
+#endif
+#if __XEN_LATEST_INTERFACE_VERSION__ < 0x00030208
+# define xen_mb()  mb()
+# define xen_rmb() rmb()
+# define xen_wmb() wmb()
+#endif
+
+#endif /* QEMU_HW_XEN_COMMON_H */
diff --git a/hw/xen_machine_pv.c b/hw/xen_machine_pv.c
index 081df0b..6b2f8f6 100644
--- a/hw/xen_machine_pv.c
+++ b/hw/xen_machine_pv.c
@@ -26,7 +26,7 @@
 #include "pc.h"
 #include "sysemu.h"
 #include "boards.h"
-#include "xen.h"
+#include "xen_backend.h"
 
 uint32_t xen_domid;
 enum xen_mode xen_mode = XEN_EMULATE;
@@ -50,6 +50,12 @@ static void xen_init_pv(ram_addr_t ram_size, int 
vga_ram_size,
     }
     env = cpu_init(cpu_model);
     env->halted = 1;
+
+    /* Initialize backend core & drivers */
+    if (-1 == xen_be_init()) {
+        fprintf(stderr, "%s: xen backend core setup failed\n", __FUNCTION__);
+        exit(1);
+    }
 }
 
 QEMUMachine xenpv_machine = {
-- 
1.6.2.2