[PATCH v2] libxl: Fix stubdom PCI passthrough

[Jason Andryuk <jandryuk@gmail.com>]

commit 0fdb48ffe7a1 "libxl: Make sure devices added by pci-attach are
reflected in the config" broken stubdom PCI passthrough when it moved
libxl__create_pci_backend later in the function.  xl pci-attach for a
running PV domain may also have been broken, but that was not verified.

The stubdomain is running (!starting) and PV, so it calls
libxl__wait_for_backend.  With the new placement of
libxl__create_pci_backend, the path does not exist and the call
immediately fails.
libxl: error: libxl_device.c:1388:libxl__wait_for_backend: Backend /local/domain/0/backend/pci/43/0 does not exist
libxl: error: libxl_pci.c:1764:device_pci_add_done: Domain 42:libxl__device_pci_add failed for PCI device 0:2:0.0 (rc -3)
libxl: error: libxl_create.c:1857:domcreate_attach_devices: Domain 42:unable to add pci devices

The wait is only relevant when the backend is already present.  num_devs
is already used to determine if the backend needs to be created.  Re-use
num_devs to determine if the backend wait is necessary.  The wait is
necessary to avoid racing with another PCI attachment reconfiguring the
front/back. If we are creating the backend, then we don't have to worry
about a racing reconfigure.

Fixes: 0fdb48ffe7a1 ("libxl: Make sure devices added by pci-attach are
reflected in the config")

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
---
v2:
Add Fixes
Expand num_devs use in commit message
---
 tools/libs/light/libxl_pci.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/tools/libs/light/libxl_pci.c b/tools/libs/light/libxl_pci.c
index 1a1c263080..19daf1d4ee 100644
--- a/tools/libs/light/libxl_pci.c
+++ b/tools/libs/light/libxl_pci.c
@@ -157,8 +157,10 @@ static int libxl__device_pci_add_xenstore(libxl__gc *gc,
     if (domtype == LIBXL_DOMAIN_TYPE_INVALID)
         return ERROR_FAIL;
 
-    if (!starting && domtype == LIBXL_DOMAIN_TYPE_PV) {
-        if (libxl__wait_for_backend(gc, be_path, GCSPRINTF("%d", XenbusStateConnected)) < 0)
+    /* wait is only needed if the backend already exists (num_devs != NULL) */
+    if (num_devs && !starting && domtype == LIBXL_DOMAIN_TYPE_PV) {
+        if (libxl__wait_for_backend(gc, be_path,
+                                    GCSPRINTF("%d", XenbusStateConnected)) < 0)
             return ERROR_FAIL;
     }
 
-- 
2.30.2