[RFC PATCH] tools/libxl: force running dm when device_model_args set

[Alex Bennée <alex.bennee@xxxxxxxxxx>]

When device_model_args is set we almost certainly want to spawn the
device model. This is most useful when debugging with QEMU and you
want to specify a new device type for testing without teaching libxl
about the device itself.

Signed-off-by: Alex Bennée <alex.bennee@xxxxxxxxxx>

---
This is very much an RFC, it's useful to me as a developer but I can
see an argument for being more cautious for production.
---
 tools/libs/light/libxl_dm.c      | 6 ++++++
 tools/libs/light/libxl_types.idl | 2 ++
 tools/xl/xl_parse.c              | 2 ++
 3 files changed, 10 insertions(+)

diff --git a/tools/libs/light/libxl_dm.c b/tools/libs/light/libxl_dm.c
index 620f381560..5774510aa0 100644
--- a/tools/libs/light/libxl_dm.c
+++ b/tools/libs/light/libxl_dm.c
@@ -3969,7 +3969,13 @@ int libxl__need_xenpv_qemu(libxl__gc *gc, 
libxl_domain_config *d_config)
         }
     }
 
+    /* finally if we've slurped extra args we almost certainly do */
+    if (d_config->need_ext_dm) {
+        ret = 1;
+    }
+
 out:
+    LOGD(INFO, domid, "need_qemu: %s", ret == 1 ? "yes":"no");
     return ret;
 }
 
diff --git a/tools/libs/light/libxl_types.idl b/tools/libs/light/libxl_types.idl
index 2be194eb19..e7705bebe9 100644
--- a/tools/libs/light/libxl_types.idl
+++ b/tools/libs/light/libxl_types.idl
@@ -1113,6 +1113,8 @@ libxl_domain_config = Struct("domain_config", [
     ("on_watchdog", libxl_action_on_shutdown),
     ("on_crash", libxl_action_on_shutdown),
     ("on_soft_reset", libxl_action_on_shutdown),
+    # true when we need to launch dm to serve the guest
+    ("need_ext_dm", bool),
     ], dir=DIR_IN)
 
 libxl_diskinfo = Struct("diskinfo", [
diff --git a/tools/xl/xl_parse.c b/tools/xl/xl_parse.c
index 52e20134a9..be0ea3e3dc 100644
--- a/tools/xl/xl_parse.c
+++ b/tools/xl/xl_parse.c
@@ -3083,6 +3083,8 @@ skip_usbdev:
     if (e && e != ESRCH) {                                                \
         fprintf(stderr,"xl: Unable to parse device_model_args"#type".\n");\
         exit(-ERROR_FAIL);                                                \
+    } else if (e) {                                                     \
+        d_config->need_ext_dm = true;                                   \
     }
 
     /* parse extra args for qemu, common to both pv, hvm */
-- 
2.39.2