accel: Unbreak accelerator fallback

When the user specifies a list of accelerators, we pick the first one
that initializes successfully.  Recent commit 1a3ec8c1564 broke that.
Reproducer:

    $ qemu-system-x86_64 --machine accel=xen:tcg
    xencall: error: Could not obtain handle on privileged command interface: No such file or directory
    xen be core: xen be core: can't open xen interface
    can't open xen interface
    qemu-system-x86_64: failed to initialize Xen: Operation not permitted
    qemu-system-x86_64: /home/armbru/work/qemu/qom/object.c:436: object_set_accelerator_compat_props: Assertion `!object_compat_props[0]' failed.

Root cause: we register accelerator compat properties even when the
accelerator fails.  The failed assertion is
object_set_accelerator_compat_props() telling us off.  Fix by calling
it only for the accelerator that succeeded.

Fixes: 1a3ec8c1564f51628cce10d435a2e22559ea29fd
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20190401090827.20793-6-armbru@redhat.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/accel/accel.c b/accel/accel.c
index 8deb475b5d..454fef9d92 100644
--- a/accel/accel.c
+++ b/accel/accel.c
@@ -65,8 +65,9 @@ static int accel_init_machine(AccelClass *acc, MachineState *ms)
         ms->accelerator = NULL;
         *(acc->allowed) = false;
         object_unref(OBJECT(accel));
+    } else {
+        object_set_accelerator_compat_props(acc->compat_props);
     }
-    object_set_accelerator_compat_props(acc->compat_props);
     return ret;
 }