xen-bus: reduce scope of backend watch

Currently a single watch on /local/domain/X/backend is registered by each
QEMU process running in service domain X (where X is usually 0). The purpose
of this watch is to ensure that QEMU is notified when the Xen toolstack
creates a new device backend area.
Such a backend area is specific to a single frontend area created for a
specific guest domain and, since each QEMU process is also created to service
a specfic guest domain, it is unnecessary and inefficient to notify all QEMU
processes.
Only the QEMU process associated with the same guest domain need
receive the notification. This patch re-factors the watch registration code
such that notifications are targetted appropriately.

Reported-by: Jerome Leseinne <jerome.leseinne@gmail.com>
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20201001081500.1026-1-paul@xen.org>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/hw/xen/xen-backend.c b/hw/xen/xen-backend.c
index 10199fb58d..5b0fb76eae 100644
--- a/hw/xen/xen-backend.c
+++ b/hw/xen/xen-backend.c
@@ -41,6 +41,12 @@ static void xen_backend_table_add(XenBackendImpl *impl)
     g_hash_table_insert(xen_backend_table_get(), (void *)impl->type, impl);
 }
 
+static const char **xen_backend_table_keys(unsigned int *count)
+{
+    return (const char **)g_hash_table_get_keys_as_array(
+        xen_backend_table_get(), count);
+}
+
 static const XenBackendImpl *xen_backend_table_lookup(const char *type)
 {
     return g_hash_table_lookup(xen_backend_table_get(), type);
@@ -70,6 +76,11 @@ void xen_backend_register(const XenBackendInfo *info)
     xen_backend_table_add(impl);
 }
 
+const char **xen_backend_get_types(unsigned int *count)
+{
+    return xen_backend_table_keys(count);
+}
+
 static QLIST_HEAD(, XenBackendInstance) backend_list;
 
 static void xen_backend_list_add(XenBackendInstance *backend)