[Qemu-devel] [PATCH] xen-bus: only set the xen device frontend state if it is missing

[Qemu-devel] [PATCH] xen-bus: only set the xen device frontend state if it is missing

[Paul Durrant]

From: Mark Syms <mark.syms@citrix.com>

Some toolstack implementations will set the frontend xenstore
keys to Initialising which will then trigger the in guest PV
drivers to begin initialising and some implementations will
then set their state to Closing. If this has occurred then
device realize must not overwrite the frontend keys as then
the handshake will stall.

Signed-off-by: Mark Syms <mark.syms@citrix.com>

Also avoid creating the frontend area if it already exists.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
---
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
---
 hw/xen/xen-bus.c | 47 +++++++++++++++++++++++++++++++----------------
 1 file changed, 31 insertions(+), 16 deletions(-)

diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
index 55c157393d..c2ad22a42d 100644
--- a/hw/xen/xen-bus.c
+++ b/hw/xen/xen-bus.c
@@ -857,6 +857,13 @@ static void xen_device_frontend_changed(void *opaque)
     }
 }
 
+static bool xen_device_frontend_exists(XenDevice *xendev)
+{
+    enum xenbus_state state;
+
+    return (xen_device_frontend_scanf(xendev, "state", "%u", &state) == 1);
+}
+
 static void xen_device_frontend_create(XenDevice *xendev, Error **errp)
 {
     XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
@@ -865,19 +872,25 @@ static void xen_device_frontend_create(XenDevice *xendev, Error **errp)
 
     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;
+    /*
+     * The frontend area may have already been created by a legacy
+     * toolstack.
+     */
+    if (!xen_device_frontend_exists(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);
+        g_assert(xenbus->xsh);
 
-    xs_node_create(xenbus->xsh, XBT_NULL, xendev->frontend_path, perms,
-                   ARRAY_SIZE(perms), &local_err);
-    if (local_err) {
-        error_propagate_prepend(errp, local_err,
-                                "failed to create frontend: ");
-        return;
+        xs_node_create(xenbus->xsh, XBT_NULL, xendev->frontend_path, perms,
+                       ARRAY_SIZE(perms), &local_err);
+        if (local_err) {
+            error_propagate_prepend(errp, local_err,
+                                    "failed to create frontend: ");
+            return;
+        }
     }
 
     xendev->frontend_state_watch =
@@ -1290,12 +1303,14 @@ static void xen_device_realize(DeviceState *dev, Error **errp)
     xen_device_backend_set_online(xendev, true);
     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);
+    if (!xen_device_frontend_exists(xendev)) {
+        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, true);
+        xen_device_frontend_set_state(xendev, XenbusStateInitialising, true);
+    }
 
     xendev->exit.notify = xen_device_exit;
     qemu_add_exit_notifier(&xendev->exit);
-- 
2.20.1.2.gb21ebb671

Re: [PATCH] xen-bus: only set the xen device frontend state if it is missing

[Anthony PERARD]

On Wed, Sep 18, 2019 at 12:57:44PM +0100, Paul Durrant wrote:
> From: Mark Syms <mark.syms@citrix.com>
> 
> Some toolstack implementations will set the frontend xenstore
> keys to Initialising which will then trigger the in guest PV
> drivers to begin initialising and some implementations will
> then set their state to Closing. If this has occurred then
> device realize must not overwrite the frontend keys as then
> the handshake will stall.
> 
> Signed-off-by: Mark Syms <mark.syms@citrix.com>
> 
> Also avoid creating the frontend area if it already exists.
> 
> Signed-off-by: Paul Durrant <paul.durrant@citrix.com>

Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>

Thanks,

-- 
Anthony PERARD