[xen stable-4.15] libxl/PCI: Fix PV hotplug & stubdom coldplug

[patchbot@xxxxxxx]

commit 7ca8706ad53d5967fd15fa94c2dd5ada8d42d812
Author:     Jason Andryuk <jandryuk@xxxxxxxxx>
AuthorDate: Tue Jan 25 13:45:09 2022 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Jan 25 13:45:09 2022 +0100

    libxl/PCI: Fix PV hotplug & stubdom coldplug
    
    commit 0fdb48ffe7a1 "libxl: Make sure devices added by pci-attach are
    reflected in the config" broken PCI hotplug (xl pci-attach) for PV
    domains when it moved libxl__create_pci_backend() later in the function.
    
    This also broke HVM + stubdom PCI passthrough coldplug.  For that, the
    PCI devices are hotplugged to a running PV stubdom, and then the QEMU
    QMP device_add commands are made to QEMU inside the stubdom.
    
    A running PV domain calls libxl__wait_for_backend().  With the current
    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:
    1) The domain is PV
    2) The domain is running
    3) The backend is already present
    
    This is because:
    
    1) xen-pcifront is only used for PV.  It does not load for HVM domains
       where QEMU is used.
    
    2) If the domain is not running (starting), then the frontend state will
       be Initialising.  xen-pciback waits for the frontend to transition to
       at Initialised before attempting to connect.  So a wait for a
       non-running domain is not applicable as the backend will not
       transition to Connected.
    
    3) For presence, 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 or changing to
       some other state like closing.  If we are creating the backend, then
       we don't have to worry about the state since it is being created.
    
    Fixes: 0fdb48ffe7a1 ("libxl: Make sure devices added by pci-attach are
    reflected in the config")
    
    Signed-off-by: Jason Andryuk <jandryuk@xxxxxxxxx>
    Reviewed-by: Paul Durrant <paul@xxxxxxx>
    Reviewed-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
    master commit: 73ee2795aaef2cb086ac078bffe1c6b33c0ea91b
    master date: 2022-01-13 14:33:16 +0100
---
 tools/libs/light/libxl_pci.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/tools/libs/light/libxl_pci.c b/tools/libs/light/libxl_pci.c
index 1a1c263080..92bf86b2be 100644
--- a/tools/libs/light/libxl_pci.c
+++ b/tools/libs/light/libxl_pci.c
@@ -157,9 +157,11 @@ 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)
-            return ERROR_FAIL;
+    /* Wait is only needed if the backend already exists (num_devs != NULL) */
+    if (num_devs && !starting && domtype == LIBXL_DOMAIN_TYPE_PV) {
+        rc = libxl__wait_for_backend(gc, be_path,
+                                     GCSPRINTF("%d", XenbusStateConnected));
+        if (rc) return rc;
     }
 
     back = flexarray_make(gc, 16, 1);
--
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