[PATCH v2 2/6] xen: backends: touch some XenStore nodes only if device...

[Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>]

was created by QEMU

Xen PV devices in QEMU can be created in two ways: either by QEMU
itself, if they were passed via command line, or by Xen toolstack. In
the latter case, QEMU scans XenStore entries and configures devices
accordingly.

In the second case we don't want QEMU to write/delete front-end
entries for two reasons: it might have no access to those entries if
it is running in un-privileged domain and it is just incorrect to
overwrite entries already provided by Xen toolstack, because toolstack
manages those nodes. For example, it might read backend- or frontend-
state to be sure that they are both disconnected and it is safe to
destroy a domain.

This patch checks presence of xendev->backend to check if Xen PV
device is acting as a backend (i.e. it was configured by Xen
toolstack) to decide if it should touch frontend entries in XenStore.
Also, when we need to remove XenStore entries during device teardown
only if they weren't created by Xen toolstack. If they were created by
toolstack, then it is toolstack's job to do proper clean-up.

Suggested-by: Paul Durrant <xadimgnik@xxxxxxxxx>
Suggested-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Co-Authored-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
---
 hw/block/xen-block.c  | 16 +++++++++-------
 hw/char/xen_console.c |  2 +-
 hw/net/xen_nic.c      | 18 ++++++++++--------
 hw/xen/xen-bus.c      | 14 +++++++++-----
 4 files changed, 29 insertions(+), 21 deletions(-)

diff --git a/hw/block/xen-block.c b/hw/block/xen-block.c
index c2ac9db4a2..dac519a6d3 100644
--- a/hw/block/xen-block.c
+++ b/hw/block/xen-block.c
@@ -390,13 +390,15 @@ static void xen_block_realize(XenDevice *xendev, Error 
**errp)
 
     xen_device_backend_printf(xendev, "info", "%u", blockdev->info);
 
-    xen_device_frontend_printf(xendev, "virtual-device", "%lu",
-                               vdev->number);
-    xen_device_frontend_printf(xendev, "device-type", "%s",
-                               blockdev->device_type);
-
-    xen_device_backend_printf(xendev, "sector-size", "%u",
-                              conf->logical_block_size);
+    if (!xendev->backend) {
+        xen_device_frontend_printf(xendev, "virtual-device", "%lu",
+                                   vdev->number);
+        xen_device_frontend_printf(xendev, "device-type", "%s",
+                                   blockdev->device_type);
+
+        xen_device_backend_printf(xendev, "sector-size", "%u",
+                                  conf->logical_block_size);
+    }
 
     xen_block_set_size(blockdev);
 
diff --git a/hw/char/xen_console.c b/hw/char/xen_console.c
index bef8a3a621..b52ddddabf 100644
--- a/hw/char/xen_console.c
+++ b/hw/char/xen_console.c
@@ -450,7 +450,7 @@ static void xen_console_realize(XenDevice *xendev, Error 
**errp)
 
     trace_xen_console_realize(con->dev, object_get_typename(OBJECT(cs)));
 
-    if (CHARDEV_IS_PTY(cs)) {
+    if (CHARDEV_IS_PTY(cs) && !xendev->backend) {
         /* Strip the leading 'pty:' */
         xen_device_frontend_printf(xendev, "tty", "%s", cs->filename + 4);
     }
diff --git a/hw/net/xen_nic.c b/hw/net/xen_nic.c
index afa10c96e8..27442bef38 100644
--- a/hw/net/xen_nic.c
+++ b/hw/net/xen_nic.c
@@ -315,14 +315,16 @@ static void xen_netdev_realize(XenDevice *xendev, Error 
**errp)
 
     qemu_macaddr_default_if_unset(&netdev->conf.macaddr);
 
-    xen_device_frontend_printf(xendev, "mac", "%02x:%02x:%02x:%02x:%02x:%02x",
-                               netdev->conf.macaddr.a[0],
-                               netdev->conf.macaddr.a[1],
-                               netdev->conf.macaddr.a[2],
-                               netdev->conf.macaddr.a[3],
-                               netdev->conf.macaddr.a[4],
-                               netdev->conf.macaddr.a[5]);
-
+    if (!xendev->backend) {
+        xen_device_frontend_printf(xendev, "mac",
+                                   "%02x:%02x:%02x:%02x:%02x:%02x",
+                                   netdev->conf.macaddr.a[0],
+                                   netdev->conf.macaddr.a[1],
+                                   netdev->conf.macaddr.a[2],
+                                   netdev->conf.macaddr.a[3],
+                                   netdev->conf.macaddr.a[4],
+                                   netdev->conf.macaddr.a[5]);
+    }
     netdev->nic = qemu_new_nic(&net_xen_info, &netdev->conf,
                                object_get_typename(OBJECT(xendev)),
                                DEVICE(xendev)->id, netdev);
diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
index dd0171ab98..d0f17aeb27 100644
--- a/hw/xen/xen-bus.c
+++ b/hw/xen/xen-bus.c
@@ -599,8 +599,10 @@ static void xen_device_backend_destroy(XenDevice *xendev)
 
     g_assert(xenbus->xsh);
 
-    xs_node_destroy(xenbus->xsh, XBT_NULL, xendev->backend_path,
-                    &local_err);
+    if (!xendev->backend) {
+        xs_node_destroy(xenbus->xsh, XBT_NULL, xendev->backend_path,
+                        &local_err);
+    }
     g_free(xendev->backend_path);
     xendev->backend_path = NULL;
 
@@ -764,8 +766,10 @@ static void xen_device_frontend_destroy(XenDevice *xendev)
 
     g_assert(xenbus->xsh);
 
-    xs_node_destroy(xenbus->xsh, XBT_NULL, xendev->frontend_path,
-                    &local_err);
+    if (!xendev->backend) {
+        xs_node_destroy(xenbus->xsh, XBT_NULL, xendev->frontend_path,
+                        &local_err);
+    }
     g_free(xendev->frontend_path);
     xendev->frontend_path = NULL;
 
@@ -1063,7 +1067,7 @@ static void xen_device_realize(DeviceState *dev, Error 
**errp)
     xen_device_backend_set_online(xendev, true);
     xen_device_backend_set_state(xendev, XenbusStateInitWait);
 
-    if (!xen_device_frontend_exists(xendev)) {
+    if (!xen_device_frontend_exists(xendev) && !xendev->backend) {
         xen_device_frontend_printf(xendev, "backend", "%s",
                                    xendev->backend_path);
         xen_device_frontend_printf(xendev, "backend-id", "%u",
-- 
2.42.0