[04/18] xen: create xenstore areas for XenDevice-s

Commit Message

Paul Durrant Nov. 21, 2018, 3:11 p.m. UTC

This patch adds a new source module, xen-bus-helper.c, which builds on
basic libxenstore primitives to provide functions to create (setting
permissions appropriately) and destroy xenstore areas, and functions to
'printf' and 'scanf' nodes therein. The main xen-bus code then uses
these primitives [1] to initialize and destroy the frontend and backend
areas for a XenDevice during realize and unrealize respectively.

The 'xen-qdisk' implementation is extended with a 'get_name' method that
returns the VBD number. This number is reqired to 'name' the xenstore
areas.

NOTE: An exit handler is also added to make sure the xenstore areas are
      cleaned up if QEMU terminates without devices being unrealized.

[1] The 'scanf' functions are actually not yet needed, but they will be
    needed by code delivered in subsequent patches.

Signed-off-by: Paul Durrant <[e-mail]>
---
Cc: Stefano Stabellini <[e-mail]>
Cc: Anthony Perard <[e-mail]>
Cc: Kevin Wolf <[e-mail]>
Cc: Max Reitz <[e-mail]>
---
 hw/block/xen-qdisk.c            |  11 ++
 hw/xen/Makefile.objs            |   2 +-
 hw/xen/trace-events             |   6 +-
 hw/xen/xen-bus-helper.c         | 124 +++++++++++++++++
 hw/xen/xen-bus.c                | 288 +++++++++++++++++++++++++++++++++++++++-
 include/hw/xen/xen-bus-helper.h |  26 ++++
 include/hw/xen/xen-bus.h        |  12 ++
 7 files changed, 464 insertions(+), 5 deletions(-)
 create mode 100644 hw/xen/xen-bus-helper.c
 create mode 100644 include/hw/xen/xen-bus-helper.h

Comments

Anthony PERARD Nov. 29, 2018, 6:48 p.m. UTC | #1

On Wed, Nov 21, 2018 at 03:11:57PM +0000, Paul Durrant wrote:
> This patch adds a new source module, xen-bus-helper.c, which builds on
> basic libxenstore primitives to provide functions to create (setting
> permissions appropriately) and destroy xenstore areas, and functions to
> 'printf' and 'scanf' nodes therein. The main xen-bus code then uses
> these primitives [1] to initialize and destroy the frontend and backend
> areas for a XenDevice during realize and unrealize respectively.
> 
> The 'xen-qdisk' implementation is extended with a 'get_name' method that
> returns the VBD number. This number is reqired to 'name' the xenstore

                                         ^ required

> diff --git a/hw/xen/xen-bus-helper.c b/hw/xen/xen-bus-helper.c
> new file mode 100644
> index 0000000000..d9ee2ed6a0
> --- /dev/null
> +++ b/hw/xen/xen-bus-helper.c
[...]
> +void xs_node_destroy(struct xs_handle *xsh, const char *node)
> +{
> +    xs_rm(xsh, XBT_NULL, node);

We should check for error, and grab errno.

> +}
> +
> +void xs_node_vprintf(struct xs_handle *xsh, const char *node,
> +                     const char *key, const char *fmt, va_list ap)
> +{
> +    char *path, *value;
> +
> +    path = (strlen(node) != 0) ? g_strdup_printf("%s/%s", node, key) :
> +        g_strdup(key);

A comment would be helpful to findout how to use that function,
especialy the fact that with node="", we write to $key instead of
$node/$key.

> +    value = g_strdup_vprintf(fmt, ap);

Looks like g_vasprintf() would be better, since it returns the lenght as
well.

> +
> +    xs_write(xsh, XBT_NULL, path, value, strlen(value));

You should check for failures, and grab errno.

> +    g_free(value);
> +    g_free(path);
> +}
> +
> +int xs_node_vscanf(struct xs_handle *xsh, const char *node, const char *key,
> +                   const char *fmt, va_list ap)
> +{
> +    char *path, *value;
> +    int rc;
> +
> +    path = (strlen(node) != 0) ? g_strdup_printf("%s/%s", node, key) :
> +        g_strdup(key);
> +
> +    value = xs_read(xsh, XBT_NULL, path, NULL);

The xenstore.h isn't clear about failure of this function, it is
supposed to return a malloced value. Do we actually need to check if value
is NULL?

> +
> +    rc = value ? vsscanf(value, fmt, ap) : EOF;
> +
> +    free(value);
> +    g_free(path);
> +
> +    return rc;
> +}
> +
> diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
> index dede2d914a..663aa8e117 100644
> --- a/hw/xen/xen-bus.c
> +++ b/hw/xen/xen-bus.c
[...]

> +static void xen_device_backend_destroy(XenDevice *xendev)
> +{
> +    XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
> +
> +    if (!xendev->backend_path) {
> +        return;
> +    }
> +
> +    g_assert(xenbus->xsh);
> +
> +    xs_node_destroy(xenbus->xsh, xendev->backend_path);
> +    g_free(xendev->backend_path);

It would be nice to also set backend_path to NULL.

> diff --git a/include/hw/xen/xen-bus-helper.h b/include/hw/xen/xen-bus-helper.h
> new file mode 100644
> index 0000000000..53570650db
> --- /dev/null
> +++ b/include/hw/xen/xen-bus-helper.h
> @@ -0,0 +1,26 @@
> +/*
> + * Copyright (c) Citrix Systems Inc.
> + * All rights reserved.
> + */
> +
> +#ifndef HW_XEN_BUS_HELPER_H
> +#define HW_XEN_BUS_HELPER_H

That should probably include xen_common.h, to have `enum xenbus_state`,
`struct xs_handle`, ..

> +const char *xs_strstate(enum xenbus_state state);
> +
> +void xs_node_create(struct xs_handle *xsh, const char *node,
> +                    struct xs_permissions perms[],
> +                    unsigned int nr_perms, Error **errp);
> +void xs_node_destroy(struct xs_handle *xsh, const char *node);
> +
> +void xs_node_vprintf(struct xs_handle *xsh, const char *node,
> +                     const char *key, const char *fmt, va_list ap);
> +void xs_node_printf(struct xs_handle *xsh, const char *node, const char *key,
> +                    const char *fmt, ...);

This prototype needs GCC_FMT_ATTR(), that's the printf format
__attribute__.

> +
> +int xs_node_vscanf(struct xs_handle *xsh, const char *node, const char *key,
> +                   const char *fmt, va_list ap);
> +int xs_node_scanf(struct xs_handle *xsh, const char *node, const char *key,
> +                  const char *fmt, ...);

Maybe here as well.

Paul Durrant Dec. 5, 2018, 12:05 p.m. UTC | #2

> -----Original Message-----
> From: Anthony PERARD [mailto:[e-mail]]
> Sent: 29 November 2018 18:49
> To: Paul Durrant <[e-mail]>
> Cc: [e-mail]; [e-mail]; xen-
> [e-mail]; Stefano Stabellini <[e-mail]>;
> Kevin Wolf <[e-mail]>; Max Reitz <[e-mail]>
> Subject: Re: [PATCH 04/18] xen: create xenstore areas for XenDevice-s
> 
> On Wed, Nov 21, 2018 at 03:11:57PM +0000, Paul Durrant wrote:
> > This patch adds a new source module, xen-bus-helper.c, which builds on
> > basic libxenstore primitives to provide functions to create (setting
> > permissions appropriately) and destroy xenstore areas, and functions to
> > 'printf' and 'scanf' nodes therein. The main xen-bus code then uses
> > these primitives [1] to initialize and destroy the frontend and backend
> > areas for a XenDevice during realize and unrealize respectively.
> >
> > The 'xen-qdisk' implementation is extended with a 'get_name' method that
> > returns the VBD number. This number is reqired to 'name' the xenstore
> 
>                                          ^ required
> 

Ok.

> > diff --git a/hw/xen/xen-bus-helper.c b/hw/xen/xen-bus-helper.c
> > new file mode 100644
> > index 0000000000..d9ee2ed6a0
> > --- /dev/null
> > +++ b/hw/xen/xen-bus-helper.c
> [...]
> > +void xs_node_destroy(struct xs_handle *xsh, const char *node)
> > +{
> > +    xs_rm(xsh, XBT_NULL, node);
> 
> We should check for error, and grab errno.
> 

I'll make all of them fill in an Error *

> > +}
> > +
> > +void xs_node_vprintf(struct xs_handle *xsh, const char *node,
> > +                     const char *key, const char *fmt, va_list ap)
> > +{
> > +    char *path, *value;
> > +
> > +    path = (strlen(node) != 0) ? g_strdup_printf("%s/%s", node, key) :
> > +        g_strdup(key);
> 
> A comment would be helpful to findout how to use that function,
> especialy the fact that with node="", we write to $key instead of
> $node/$key.

Ok, I'll add comments into the header.

> 
> > +    value = g_strdup_vprintf(fmt, ap);
> 
> Looks like g_vasprintf() would be better, since it returns the lenght as
> well.
> 

Yes.

> > +
> > +    xs_write(xsh, XBT_NULL, path, value, strlen(value));
> 
> You should check for failures, and grab errno.
> 
> > +    g_free(value);
> > +    g_free(path);
> > +}
> > +
> > +int xs_node_vscanf(struct xs_handle *xsh, const char *node, const char
> *key,
> > +                   const char *fmt, va_list ap)
> > +{
> > +    char *path, *value;
> > +    int rc;
> > +
> > +    path = (strlen(node) != 0) ? g_strdup_printf("%s/%s", node, key) :
> > +        g_strdup(key);
> > +
> > +    value = xs_read(xsh, XBT_NULL, path, NULL);
> 
> The xenstore.h isn't clear about failure of this function, it is
> supposed to return a malloced value. Do we actually need to check if value
> is NULL?

The comment above xs_read() in xs.c is:

/* Get the value of a single file, nul terminated.                          
 * Returns a malloced value: call free() on it after use.                   
 * len indicates length in bytes, not including the nul.                    
 */

and I think we should check it for NULL before passing it to vsscanf().

> 
> > +
> > +    rc = value ? vsscanf(value, fmt, ap) : EOF;
> > +
> > +    free(value);
> > +    g_free(path);
> > +
> > +    return rc;
> > +}
> > +
> > diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
> > index dede2d914a..663aa8e117 100644
> > --- a/hw/xen/xen-bus.c
> > +++ b/hw/xen/xen-bus.c
> [...]
> 
> > +static void xen_device_backend_destroy(XenDevice *xendev)
> > +{
> > +    XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
> > +
> > +    if (!xendev->backend_path) {
> > +        return;
> > +    }
> > +
> > +    g_assert(xenbus->xsh);
> > +
> > +    xs_node_destroy(xenbus->xsh, xendev->backend_path);
> > +    g_free(xendev->backend_path);
> 
> It would be nice to also set backend_path to NULL.
> 

Yes, it should be for idempotency.

> > diff --git a/include/hw/xen/xen-bus-helper.h b/include/hw/xen/xen-bus-
> helper.h
> > new file mode 100644
> > index 0000000000..53570650db
> > --- /dev/null
> > +++ b/include/hw/xen/xen-bus-helper.h
> > @@ -0,0 +1,26 @@
> > +/*
> > + * Copyright (c) Citrix Systems Inc.
> > + * All rights reserved.
> > + */
> > +
> > +#ifndef HW_XEN_BUS_HELPER_H
> > +#define HW_XEN_BUS_HELPER_H
> 
> That should probably include xen_common.h, to have `enum xenbus_state`,
> `struct xs_handle`, ..

Ok.

> 
> > +const char *xs_strstate(enum xenbus_state state);
> > +
> > +void xs_node_create(struct xs_handle *xsh, const char *node,
> > +                    struct xs_permissions perms[],
> > +                    unsigned int nr_perms, Error **errp);
> > +void xs_node_destroy(struct xs_handle *xsh, const char *node);
> > +
> > +void xs_node_vprintf(struct xs_handle *xsh, const char *node,
> > +                     const char *key, const char *fmt, va_list ap);
> > +void xs_node_printf(struct xs_handle *xsh, const char *node, const char
> *key,
> > +                    const char *fmt, ...);
> 
> This prototype needs GCC_FMT_ATTR(), that's the printf format
> __attribute__.
> 
> > +
> > +int xs_node_vscanf(struct xs_handle *xsh, const char *node, const char
> *key,
> > +                   const char *fmt, va_list ap);
> > +int xs_node_scanf(struct xs_handle *xsh, const char *node, const char
> *key,
> > +                  const char *fmt, ...);
> 
> Maybe here as well.

Will do.

 Paul

> 
> 
> --
> Anthony PERARD

Paul Durrant Dec. 5, 2018, 12:43 p.m. UTC | #3

> -----Original Message-----
> From: Qemu-devel [mailto:qemu-devel-
> bounces+paul.durrant=[e-mail]] On Behalf Of Paul Durrant
> Sent: 05 December 2018 12:05
> To: Anthony Perard <[e-mail]>
> Cc: Kevin Wolf <[e-mail]>; Stefano Stabellini
> <[e-mail]>; [e-mail]; [e-mail];
> Max Reitz <[e-mail]>; [e-mail]
> Subject: Re: [Qemu-devel] [PATCH 04/18] xen: create xenstore areas for
> XenDevice-s
> 
> > -----Original Message-----
> > From: Anthony PERARD [mailto:[e-mail]]
> > Sent: 29 November 2018 18:49
> > To: Paul Durrant <[e-mail]>
> > Cc: [e-mail]; [e-mail]; xen-
> > [e-mail]; Stefano Stabellini <[e-mail]>;
> > Kevin Wolf <[e-mail]>; Max Reitz <[e-mail]>
> > Subject: Re: [PATCH 04/18] xen: create xenstore areas for XenDevice-s
> >
> > On Wed, Nov 21, 2018 at 03:11:57PM +0000, Paul Durrant wrote:
> > > This patch adds a new source module, xen-bus-helper.c, which builds on
> > > basic libxenstore primitives to provide functions to create (setting
> > > permissions appropriately) and destroy xenstore areas, and functions
> to
> > > 'printf' and 'scanf' nodes therein. The main xen-bus code then uses
> > > these primitives [1] to initialize and destroy the frontend and
> backend
> > > areas for a XenDevice during realize and unrealize respectively.
> > >
> > > The 'xen-qdisk' implementation is extended with a 'get_name' method
> that
> > > returns the VBD number. This number is reqired to 'name' the xenstore
> >
> >                                          ^ required
> >
> 
> Ok.
> 
> > > diff --git a/hw/xen/xen-bus-helper.c b/hw/xen/xen-bus-helper.c
> > > new file mode 100644
> > > index 0000000000..d9ee2ed6a0
> > > --- /dev/null
> > > +++ b/hw/xen/xen-bus-helper.c
> > [...]
> > > +void xs_node_destroy(struct xs_handle *xsh, const char *node)
> > > +{
> > > +    xs_rm(xsh, XBT_NULL, node);
> >
> > We should check for error, and grab errno.
> >
> 
> I'll make all of them fill in an Error *
> 
> > > +}
> > > +
> > > +void xs_node_vprintf(struct xs_handle *xsh, const char *node,
> > > +                     const char *key, const char *fmt, va_list ap)
> > > +{
> > > +    char *path, *value;
> > > +
> > > +    path = (strlen(node) != 0) ? g_strdup_printf("%s/%s", node, key)
> :
> > > +        g_strdup(key);
> >
> > A comment would be helpful to findout how to use that function,
> > especialy the fact that with node="", we write to $key instead of
> > $node/$key.
> 
> Ok, I'll add comments into the header.
> 
> >
> > > +    value = g_strdup_vprintf(fmt, ap);
> >
> > Looks like g_vasprintf() would be better, since it returns the lenght as
> > well.
> >
> 
> Yes.

I tried this and it appears not to exist in the version of glib in my environment so I guess I'll stick with g_strdup_printf().

  Paul

> 
> > > +
> > > +    xs_write(xsh, XBT_NULL, path, value, strlen(value));
> >
> > You should check for failures, and grab errno.
> >
> > > +    g_free(value);
> > > +    g_free(path);
> > > +}
> > > +
> > > +int xs_node_vscanf(struct xs_handle *xsh, const char *node, const
> char
> > *key,
> > > +                   const char *fmt, va_list ap)
> > > +{
> > > +    char *path, *value;
> > > +    int rc;
> > > +
> > > +    path = (strlen(node) != 0) ? g_strdup_printf("%s/%s", node, key)
> :
> > > +        g_strdup(key);
> > > +
> > > +    value = xs_read(xsh, XBT_NULL, path, NULL);
> >
> > The xenstore.h isn't clear about failure of this function, it is
> > supposed to return a malloced value. Do we actually need to check if
> value
> > is NULL?
> 
> The comment above xs_read() in xs.c is:
> 
> /* Get the value of a single file, nul terminated.
>  * Returns a malloced value: call free() on it after use.
>  * len indicates length in bytes, not including the nul.
>  */
> 
> and I think we should check it for NULL before passing it to vsscanf().
> 
> >
> > > +
> > > +    rc = value ? vsscanf(value, fmt, ap) : EOF;
> > > +
> > > +    free(value);
> > > +    g_free(path);
> > > +
> > > +    return rc;
> > > +}
> > > +
> > > diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
> > > index dede2d914a..663aa8e117 100644
> > > --- a/hw/xen/xen-bus.c
> > > +++ b/hw/xen/xen-bus.c
> > [...]
> >
> > > +static void xen_device_backend_destroy(XenDevice *xendev)
> > > +{
> > > +    XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
> > > +
> > > +    if (!xendev->backend_path) {
> > > +        return;
> > > +    }
> > > +
> > > +    g_assert(xenbus->xsh);
> > > +
> > > +    xs_node_destroy(xenbus->xsh, xendev->backend_path);
> > > +    g_free(xendev->backend_path);
> >
> > It would be nice to also set backend_path to NULL.
> >
> 
> Yes, it should be for idempotency.
> 
> > > diff --git a/include/hw/xen/xen-bus-helper.h b/include/hw/xen/xen-bus-
> > helper.h
> > > new file mode 100644
> > > index 0000000000..53570650db
> > > --- /dev/null
> > > +++ b/include/hw/xen/xen-bus-helper.h
> > > @@ -0,0 +1,26 @@
> > > +/*
> > > + * Copyright (c) Citrix Systems Inc.
> > > + * All rights reserved.
> > > + */
> > > +
> > > +#ifndef HW_XEN_BUS_HELPER_H
> > > +#define HW_XEN_BUS_HELPER_H
> >
> > That should probably include xen_common.h, to have `enum xenbus_state`,
> > `struct xs_handle`, ..
> 
> Ok.
> 
> >
> > > +const char *xs_strstate(enum xenbus_state state);
> > > +
> > > +void xs_node_create(struct xs_handle *xsh, const char *node,
> > > +                    struct xs_permissions perms[],
> > > +                    unsigned int nr_perms, Error **errp);
> > > +void xs_node_destroy(struct xs_handle *xsh, const char *node);
> > > +
> > > +void xs_node_vprintf(struct xs_handle *xsh, const char *node,
> > > +                     const char *key, const char *fmt, va_list ap);
> > > +void xs_node_printf(struct xs_handle *xsh, const char *node, const
> char
> > *key,
> > > +                    const char *fmt, ...);
> >
> > This prototype needs GCC_FMT_ATTR(), that's the printf format
> > __attribute__.
> >
> > > +
> > > +int xs_node_vscanf(struct xs_handle *xsh, const char *node, const
> char
> > *key,
> > > +                   const char *fmt, va_list ap);
> > > +int xs_node_scanf(struct xs_handle *xsh, const char *node, const char
> > *key,
> > > +                  const char *fmt, ...);
> >
> > Maybe here as well.
> 
> Will do.
> 
>  Paul
> 
> >
> >
> > --
> > Anthony PERARD

Anthony PERARD Dec. 5, 2018, 1:58 p.m. UTC | #4

On Wed, Dec 05, 2018 at 12:43:57PM +0000, Paul Durrant wrote:
> > > > +    value = g_strdup_vprintf(fmt, ap);
> > >
> > > Looks like g_vasprintf() would be better, since it returns the lenght as
> > > well.
> > >
> > 
> > Yes.
> 
> I tried this and it appears not to exist in the version of glib in my environment so I guess I'll stick with g_strdup_printf().

It's probably because you need to include "glib/gprintf.h", I've
suggested it because I've seen the function use elsewhere in QEMU. But
g_strdup_printf is fine too.

https://developer.gnome.org/glib/stable/glib-String-Utility-Functions.html#g-vasprintf

Paul Durrant Dec. 5, 2018, 2:24 p.m. UTC | #5

> -----Original Message-----
> From: Anthony PERARD [mailto:[e-mail]]
> Sent: 05 December 2018 13:59
> To: Paul Durrant <[e-mail]>
> Cc: Kevin Wolf <[e-mail]>; Stefano Stabellini
> <[e-mail]>; [e-mail]; [e-mail];
> Max Reitz <[e-mail]>; [e-mail]
> Subject: Re: [PATCH 04/18] xen: create xenstore areas for XenDevice-s
> 
> On Wed, Dec 05, 2018 at 12:43:57PM +0000, Paul Durrant wrote:
> > > > > +    value = g_strdup_vprintf(fmt, ap);
> > > >
> > > > Looks like g_vasprintf() would be better, since it returns the
> lenght as
> > > > well.
> > > >
> > >
> > > Yes.
> >
> > I tried this and it appears not to exist in the version of glib in my
> environment so I guess I'll stick with g_strdup_printf().
> 
> It's probably because you need to include "glib/gprintf.h", I've
> suggested it because I've seen the function use elsewhere in QEMU. But
> g_strdup_printf is fine too.
> 
> https://developer.gnome.org/glib/stable/glib-String-Utility-
> Functions.html#g-vasprintf

Ah, that's what I needed. Thanks,

  Paul

> 
> --
> Anthony PERARD

Anthony PERARD Dec. 5, 2018, 4:28 p.m. UTC | #6

On Wed, Dec 05, 2018 at 12:05:23PM +0000, Paul Durrant wrote:
> > > +    value = xs_read(xsh, XBT_NULL, path, NULL);
> > 
> > The xenstore.h isn't clear about failure of this function, it is
> > supposed to return a malloced value. Do we actually need to check if value
> > is NULL?
> 
> The comment above xs_read() in xs.c is:
> 
> /* Get the value of a single file, nul terminated.                          
>  * Returns a malloced value: call free() on it after use.                   
>  * len indicates length in bytes, not including the nul.                    
>  */
> 
> and I think we should check it for NULL before passing it to vsscanf().

I've sent a patch against xenstore.h to document that xs_read, and some
other functions, can return NULL.
<[e-mail]>

Patch

diff --git a/hw/block/xen-qdisk.c b/hw/block/xen-qdisk.c
index 72122073f7..0859643f7d 100644
--- a/hw/block/xen-qdisk.c
+++ b/hw/block/xen-qdisk.c
@@ -11,6 +11,14 @@ 
 #include "hw/xen/xen-qdisk.h"
 #include "trace.h"
 
+static char *xen_qdisk_get_name(XenDevice *xendev, Error **errp)
+{
+    XenQdiskDevice *qdiskdev = XEN_QDISK_DEVICE(xendev);
+    XenQdiskVdev *vdev = &qdiskdev->vdev;
+
+    return g_strdup_printf("%lu", vdev->number);
+}
+
 static void xen_qdisk_realize(XenDevice *xendev, Error **errp)
 {
     XenQdiskDevice *qdiskdev = XEN_QDISK_DEVICE(xendev);
@@ -234,6 +242,9 @@  static void xen_qdisk_class_init(ObjectClass *class, void *data)
     DeviceClass *dev_class = DEVICE_CLASS(class);
     XenDeviceClass *xendev_class = XEN_DEVICE_CLASS(class);
 
+    xendev_class->backend = "qdisk";
+    xendev_class->device = "vbd";
+    xendev_class->get_name = xen_qdisk_get_name;
     xendev_class->realize = xen_qdisk_realize;
     xendev_class->unrealize = xen_qdisk_unrealize;
 
diff --git a/hw/xen/Makefile.objs b/hw/xen/Makefile.objs
index d9d6d7b4f9..77c0868190 100644
--- a/hw/xen/Makefile.objs
+++ b/hw/xen/Makefile.objs
@@ -1,5 +1,5 @@ 
 # xen backend driver support
-common-obj-$(CONFIG_XEN) += xen-legacy-backend.o xen_devconfig.o xen_pvdev.o xen-common.o xen-bus.o
+common-obj-$(CONFIG_XEN) += xen-legacy-backend.o xen_devconfig.o xen_pvdev.o xen-common.o xen-bus.o xen-bus-helper.o
 
 obj-$(CONFIG_XEN_PCI_PASSTHROUGH) += xen-host-pci-device.o
 obj-$(CONFIG_XEN_PCI_PASSTHROUGH) += xen_pt.o xen_pt_config_init.o xen_pt_graphics.o xen_pt_msi.o
diff --git a/hw/xen/trace-events b/hw/xen/trace-events
index 0172cd4e26..fa8aea1da1 100644
--- a/hw/xen/trace-events
+++ b/hw/xen/trace-events
@@ -16,5 +16,7 @@  xen_domid_restrict(int err) "err: %u"
 # include/hw/xen/xen-bus.c
 xen_bus_realize(void) ""
 xen_bus_unrealize(void) ""
-xen_device_realize(const char *type) "type: %s"
-xen_device_unrealize(const char *type) "type: %s"
+xen_device_realize(const char *type, char *name) "type: %s name: %s"
+xen_device_unrealize(const char *type, char *name) "type: %s name: %s"
+xen_device_backend_state(const char *type, char *name, const char *state) "type: %s name: %s -> %s"
+xen_device_frontend_state(const char *type, char *name, const char *state) "type: %s name: %s -> %s"
diff --git a/hw/xen/xen-bus-helper.c b/hw/xen/xen-bus-helper.c
new file mode 100644
index 0000000000..d9ee2ed6a0
--- /dev/null
+++ b/hw/xen/xen-bus-helper.c
@@ -0,0 +1,124 @@ 
+/*
+ * Copyright (c) Citrix Systems Inc.
+ * All rights reserved.
+ */
+
+#include "qemu/osdep.h"
+#include "hw/hw.h"
+#include "hw/sysbus.h"
+#include "hw/xen/xen.h"
+#include "hw/xen/xen-bus.h"
+#include "hw/xen/xen-bus-helper.h"
+#include "qapi/error.h"
+
+struct xs_state {
+    enum xenbus_state statenum;
+    const char *statestr;
+};
+#define XS_STATE(state) { state, #state }
+
+static struct xs_state xs_state[] = {
+    XS_STATE(XenbusStateUnknown),
+    XS_STATE(XenbusStateInitialising),
+    XS_STATE(XenbusStateInitWait),
+    XS_STATE(XenbusStateInitialised),
+    XS_STATE(XenbusStateConnected),
+    XS_STATE(XenbusStateClosing),
+    XS_STATE(XenbusStateClosed),
+    XS_STATE(XenbusStateReconfiguring),
+    XS_STATE(XenbusStateReconfigured),
+};
+
+#undef XS_STATE
+
+const char *xs_strstate(enum xenbus_state state)
+{
+    unsigned int i;
+
+   for (i = 0; i < ARRAY_SIZE(xs_state); i++) {
+        if (xs_state[i].statenum == state) {
+            return xs_state[i].statestr;
+        }
+    }
+
+    return "INVALID";
+}
+
+void xs_node_create(struct xs_handle *xsh, const char *node,
+                    struct xs_permissions perms[],
+                    unsigned int nr_perms, Error **errp)
+{
+    if (!xs_write(xsh, XBT_NULL, node, "", 0)) {
+        error_setg_errno(errp, errno, "failed to create node '%s'", node);
+        return;
+    }
+
+    if (!xs_set_permissions(xsh, XBT_NULL, node,
+                            perms, nr_perms)) {
+        error_setg_errno(errp, errno, "failed to set node '%s' permissions",
+                         node);
+    }
+}
+
+void xs_node_destroy(struct xs_handle *xsh, const char *node)
+{
+    xs_rm(xsh, XBT_NULL, node);
+}
+
+void xs_node_vprintf(struct xs_handle *xsh, const char *node,
+                     const char *key, const char *fmt, va_list ap)
+{
+    char *path, *value;
+
+    path = (strlen(node) != 0) ? g_strdup_printf("%s/%s", node, key) :
+        g_strdup(key);
+
+    value = g_strdup_vprintf(fmt, ap);
+
+    xs_write(xsh, XBT_NULL, path, value, strlen(value));
+
+    g_free(value);
+    g_free(path);
+}
+
+void xs_node_printf(struct xs_handle *xsh, const char *node,
+                    const char *key, const char *fmt, ...)
+{
+    va_list ap;
+
+    va_start(ap, fmt);
+    xs_node_vprintf(xsh, node, key, fmt, ap);
+    va_end(ap);
+}
+
+int xs_node_vscanf(struct xs_handle *xsh, const char *node, const char *key,
+                   const char *fmt, va_list ap)
+{
+    char *path, *value;
+    int rc;
+
+    path = (strlen(node) != 0) ? g_strdup_printf("%s/%s", node, key) :
+        g_strdup(key);
+
+    value = xs_read(xsh, XBT_NULL, path, NULL);
+
+    rc = value ? vsscanf(value, fmt, ap) : EOF;
+
+    free(value);
+    g_free(path);
+
+    return rc;
+}
+
+int xs_node_scanf(struct xs_handle *xsh, const char *node, const char *key,
+                  const char *fmt, ...)
+{
+    va_list ap;
+    int rc;
+
+    va_start(ap, fmt);
+    rc = xs_node_vscanf(xsh, node, key, fmt, ap);
+    va_end(ap);
+
+    return rc;
+}
diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
index dede2d914a..663aa8e117 100644
--- a/hw/xen/xen-bus.c
+++ b/hw/xen/xen-bus.c
@@ -6,24 +6,102 @@ 
 #include "qemu/osdep.h"
 #include "hw/hw.h"
 #include "hw/sysbus.h"
+#include "hw/xen/xen.h"
 #include "hw/xen/xen-bus.h"
+#include "hw/xen/xen-bus-helper.h"
+#include "monitor/monitor.h"
 #include "qapi/error.h"
+#include "sysemu/sysemu.h"
 #include "trace.h"
 
+static char *xen_device_get_backend_path(XenDevice *xendev)
+{
+    XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
+    XenDeviceClass *xendev_class = XEN_DEVICE_GET_CLASS(xendev);
+    const char *type = object_get_typename(OBJECT(xendev));
+    const char *backend = xendev_class->backend;
+
+    if (!backend) {
+        backend = type;
+    }
+
+    return g_strdup_printf("/local/domain/%u/backend/%s/%u/%s",
+                           xenbus->backend_id, backend, xendev->frontend_id,
+                           xendev->name);
+}
+
+static char *xen_device_get_frontend_path(XenDevice *xendev)
+{
+    XenDeviceClass *xendev_class = XEN_DEVICE_GET_CLASS(xendev);
+    const char *type = object_get_typename(OBJECT(xendev));
+    const char *device = xendev_class->device;
+
+    if (!device) {
+        device = type;
+    }
+
+    return g_strdup_printf("/local/domain/%u/device/%s/%s",
+                           xendev->frontend_id, device, xendev->name);
+}
+
+static void xen_bus_print_dev(Monitor *mon, DeviceState *dev, int indent)
+{
+    XenDevice *xendev = XEN_DEVICE(dev);
+
+    monitor_printf(mon, "%*sname = '%s' frontend_id = %u\n",
+                   indent, "", xendev->name, xendev->frontend_id);
+}
+
+static char *xen_bus_get_dev_path(DeviceState *dev)
+{
+    return xen_device_get_backend_path(XEN_DEVICE(dev));
+}
+
 static void xen_bus_unrealize(BusState *bus, Error **errp)
 {
+    XenBus *xenbus = XEN_BUS(bus);
+
     trace_xen_bus_unrealize();
+
+    if (!xenbus->xsh) {
+        return;
+    }
+
+    xs_close(xenbus->xsh);
 }
 
 static void xen_bus_realize(BusState *bus, Error **errp)
 {
+    XenBus *xenbus = XEN_BUS(bus);
+    unsigned int domid;
+
     trace_xen_bus_realize();
+
+    xenbus->xsh = xs_open(0);
+    if (!xenbus->xsh) {
+        error_setg_errno(errp, errno, "failed xs_open");
+        goto fail;
+    }
+
+    if (xs_node_scanf(xenbus->xsh, "", /* domain root node */
+                            "domid", "%u", &domid) == 1) {
+        xenbus->backend_id = domid;
+    } else {
+        xenbus->backend_id = 0; /* Assume lack of node means dom0 */
+    }
+
+    return;
+
+fail:
+    xen_bus_unrealize(bus, &error_abort);
 }
 
 static void xen_bus_class_init(ObjectClass *class, void *data)
 {
     BusClass *bus_class = BUS_CLASS(class);
 
+    bus_class->print_dev = xen_bus_print_dev;
+    bus_class->get_dev_path = xen_bus_get_dev_path;
     bus_class->realize = xen_bus_realize;
     bus_class->unrealize = xen_bus_unrealize;
 }
@@ -36,6 +114,138 @@  static const TypeInfo xen_bus_type_info = {
     .class_init = xen_bus_class_init,
 };
 
+static void xen_device_backend_printf(XenDevice *xendev, const char *key,
+                                      const char *fmt, ...)
+{
+    XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
+    va_list ap;
+
+    g_assert(xenbus->xsh);
+
+    va_start(ap, fmt);
+    xs_node_vprintf(xenbus->xsh, xendev->backend_path, key, fmt, ap);
+    va_end(ap);
+}
+
+static void xen_device_backend_set_state(XenDevice *xendev,
+                                         enum xenbus_state state)
+{
+    const char *type = object_get_typename(OBJECT(xendev));
+
+    if (xendev->backend_state == state) {
+        return;
+    }
+
+    trace_xen_device_backend_state(type, xendev->name,
+                                   xs_strstate(state));
+
+    xendev->backend_state = state;
+    xen_device_backend_printf(xendev, "state", "%u", state);
+}
+
+static void xen_device_backend_create(XenDevice *xendev, Error **errp)
+{
+    XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
+    struct xs_permissions perms[2];
+    Error *local_err = NULL;
+
+    xendev->backend_path = xen_device_get_backend_path(xendev);
+
+    perms[0].id = xenbus->backend_id;
+    perms[0].perms = XS_PERM_NONE;
+    perms[1].id = xendev->frontend_id;
+    perms[1].perms = XS_PERM_READ;
+
+    g_assert(xenbus->xsh);
+
+    xs_node_create(xenbus->xsh, xendev->backend_path, perms,
+                   ARRAY_SIZE(perms), &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        error_prepend(errp, "failed to create backend: ");
+    }
+}
+
+static void xen_device_backend_destroy(XenDevice *xendev)
+{
+    XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
+
+    if (!xendev->backend_path) {
+        return;
+    }
+
+    g_assert(xenbus->xsh);
+
+    xs_node_destroy(xenbus->xsh, xendev->backend_path);
+    g_free(xendev->backend_path);
+}
+
+static void xen_device_frontend_printf(XenDevice *xendev, const char *key,
+                                       const char *fmt, ...)
+{
+    XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
+    va_list ap;
+
+    g_assert(xenbus->xsh);
+
+    va_start(ap, fmt);
+    xs_node_vprintf(xenbus->xsh, xendev->frontend_path, key, fmt, ap);
+    va_end(ap);
+}
+
+static void xen_device_frontend_set_state(XenDevice *xendev,
+                                          enum xenbus_state state)
+{
+    const char *type = object_get_typename(OBJECT(xendev));
+
+    if (xendev->frontend_state == state) {
+        return;
+    }
+
+    trace_xen_device_frontend_state(type, xendev->name,
+                                    xs_strstate(state));
+
+    xendev->frontend_state = state;
+    xen_device_frontend_printf(xendev, "state", "%u", state);
+}
+
+static void xen_device_frontend_create(XenDevice *xendev, Error **errp)
+{
+    XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
+    struct xs_permissions perms[2];
+    Error *local_err = NULL;
+
+    xendev->frontend_path = xen_device_get_frontend_path(xendev);
+
+    perms[0].id = xendev->frontend_id;
+    perms[0].perms = XS_PERM_NONE;
+    perms[1].id = xenbus->backend_id;
+    perms[1].perms = XS_PERM_READ | XS_PERM_WRITE;
+
+    g_assert(xenbus->xsh);
+
+    xs_node_create(xenbus->xsh, xendev->frontend_path, perms,
+                   ARRAY_SIZE(perms), &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        error_prepend(errp, "failed to create frontend: ");
+    }
+}
+
+static void xen_device_frontend_destroy(XenDevice *xendev)
+{
+    XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
+
+    if (!xendev->frontend_path) {
+        return;
+    }
+
+    g_assert(xenbus->xsh);
+
+    xs_node_destroy(xenbus->xsh, xendev->frontend_path);
+    g_free(xendev->frontend_path);
+}
+
 static void xen_device_unrealize(DeviceState *dev, Error **errp)
 {
     XenDevice *xendev = XEN_DEVICE(dev);
@@ -43,7 +253,10 @@  static void xen_device_unrealize(DeviceState *dev, Error **errp)
     const char *type = object_get_typename(OBJECT(xendev));
     Error *local_err = NULL;
 
-    trace_xen_device_unrealize(type);
+    trace_xen_device_unrealize(type, xendev->name);
+
+    if (xendev->exit.notify)
+        qemu_remove_exit_notifier(&xendev->exit);
 
     if (xendev_class->unrealize) {
         xendev_class->unrealize(xendev, &local_err);
@@ -52,16 +265,62 @@  static void xen_device_unrealize(DeviceState *dev, Error **errp)
             return;
         }
     }
+
+    xen_device_frontend_destroy(xendev);
+    xen_device_backend_destroy(xendev);
+
+    g_free(xendev->name);
+}
+
+static void xen_device_exit(Notifier *n, void *data)
+{
+    XenDevice *xendev = container_of(n, XenDevice, exit);
+
+    xen_device_unrealize(DEVICE(xendev), &error_abort);
 }
 
 static void xen_device_realize(DeviceState *dev, Error **errp)
 {
     XenDevice *xendev = XEN_DEVICE(dev);
     XenDeviceClass *xendev_class = XEN_DEVICE_GET_CLASS(xendev);
+    XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
     const char *type = object_get_typename(OBJECT(xendev));
     Error *local_err = NULL;
 
-    trace_xen_device_realize(type);
+    if (xendev->frontend_id == DOMID_INVALID) {
+        xendev->frontend_id = xen_domid;
+    }
+
+    if (xendev->frontend_id >= DOMID_FIRST_RESERVED) {
+        error_setg(errp, "invalid frontend-id");
+        return;
+    }
+
+    if (!xendev_class->get_name) {
+        error_setg(errp, "get_name method not implemented");
+        return;
+    }
+
+    xendev->name = xendev_class->get_name(xendev, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        error_prepend(errp, "failed to get device name: ");
+        return;
+    }
+
+    trace_xen_device_realize(type, xendev->name);
+
+    xen_device_backend_create(xendev, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        goto unrealize;
+    }
+
+    xen_device_frontend_create(xendev, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        goto unrealize;
+    }
 
     if (xendev_class->realize) {
         xendev_class->realize(xendev, &local_err);
@@ -71,18 +330,43 @@  static void xen_device_realize(DeviceState *dev, Error **errp)
         }
     }
 
+    xen_device_backend_printf(xendev, "frontend", "%s",
+                              xendev->frontend_path);
+    xen_device_backend_printf(xendev, "frontend-id", "%u",
+                              xendev->frontend_id);
+    xen_device_backend_printf(xendev, "online", "%u", 1);
+    xen_device_backend_printf(xendev, "hotplug-status", "connected");
+
+    xen_device_backend_set_state(xendev, XenbusStateInitWait);
+
+    xen_device_frontend_printf(xendev, "backend", "%s",
+                               xendev->backend_path);
+    xen_device_frontend_printf(xendev, "backend-id", "%u",
+                               xenbus->backend_id);
+
+    xen_device_frontend_set_state(xendev, XenbusStateInitialising);
+
+    xendev->exit.notify = xen_device_exit;
+    qemu_add_exit_notifier(&xendev->exit);
     return;
 
 unrealize:
     xen_device_unrealize(dev, &error_abort);
 }
 
+static Property xen_device_props[] = {
+    DEFINE_PROP_UINT16("frontend-id", XenDevice, frontend_id,
+                       DOMID_INVALID),
+    DEFINE_PROP_END_OF_LIST()
+};
+
 static void xen_device_class_init(ObjectClass *class, void *data)
 {
     DeviceClass *dev_class = DEVICE_CLASS(class);
 
     dev_class->realize = xen_device_realize;
     dev_class->unrealize = xen_device_unrealize;
+    dev_class->props = xen_device_props;
     dev_class->bus_type = TYPE_XEN_BUS;
 }
 
diff --git a/include/hw/xen/xen-bus-helper.h b/include/hw/xen/xen-bus-helper.h
new file mode 100644
index 0000000000..53570650db
--- /dev/null
+++ b/include/hw/xen/xen-bus-helper.h
@@ -0,0 +1,26 @@ 
+/*
+ * Copyright (c) Citrix Systems Inc.
+ * All rights reserved.
+ */
+
+#ifndef HW_XEN_BUS_HELPER_H
+#define HW_XEN_BUS_HELPER_H
+
+const char *xs_strstate(enum xenbus_state state);
+
+void xs_node_create(struct xs_handle *xsh, const char *node,
+                    struct xs_permissions perms[],
+                    unsigned int nr_perms, Error **errp);
+void xs_node_destroy(struct xs_handle *xsh, const char *node);
+
+void xs_node_vprintf(struct xs_handle *xsh, const char *node,
+                     const char *key, const char *fmt, va_list ap);
+void xs_node_printf(struct xs_handle *xsh, const char *node, const char *key,
+                    const char *fmt, ...);
+
+int xs_node_vscanf(struct xs_handle *xsh, const char *node, const char *key,
+                   const char *fmt, va_list ap);
+int xs_node_scanf(struct xs_handle *xsh, const char *node, const char *key,
+                  const char *fmt, ...);
+
+#endif /* HW_XEN_BUS_HELPER_H */
diff --git a/include/hw/xen/xen-bus.h b/include/hw/xen/xen-bus.h
index 41772dc73a..434d1dbf58 100644
--- a/include/hw/xen/xen-bus.h
+++ b/include/hw/xen/xen-bus.h
@@ -6,12 +6,19 @@ 
 #ifndef HW_XEN_BUS_H
 #define HW_XEN_BUS_H
 
+#include "hw/xen/xen_common.h"
 #include "hw/sysbus.h"
 
 typedef struct XenDevice {
     DeviceState qdev;
+    domid_t frontend_id;
+    char *name;
+    char *backend_path, *frontend_path;
+    enum xenbus_state backend_state, frontend_state;
+    Notifier exit;
 } XenDevice;
 
+typedef char *(*XenDeviceGetName)(XenDevice *xendev, Error **errp);
 typedef void (*XenDeviceRealize)(XenDevice *xendev, Error **errp);
 typedef void (*XenDeviceUnrealize)(XenDevice *xendev, Error **errp);
 
@@ -19,6 +26,9 @@  typedef struct XenDeviceClass {
     /*< private >*/
     DeviceClass parent_class;
     /*< public >*/
+    const char *backend;
+    const char *device;
+    XenDeviceGetName get_name;
     XenDeviceRealize realize;
     XenDeviceUnrealize unrealize;
 } XenDeviceClass;
@@ -33,6 +43,8 @@  typedef struct XenDeviceClass {
 
 typedef struct XenBus {
     BusState qbus;
+    domid_t backend_id;
+    struct xs_handle *xsh;
 } XenBus;
 
 typedef struct XenBusClass {