CA-403633: Keep vPCI devices in the same order (#6176)

QEMU orders devices by the time of plugging. Parallelizing them
introduces randomness, which breaks the assumption that devices are
ordered in a deterministic way.

Serialize all PCI and VUSB plugs to restore behaviour.

This fix has been manually tested by @freddy77 against an affected VM
relying on the ordering

Author: psafont

ocaml/xenopsd/lib/xenops_server.ml

@@ -1630,12 +1630,11 @@ let rec atomics_of_operation = function
           ]
       ; [VM_create_device_model (id, false)]
         (* PCI and USB devices are hot-plugged into HVM guests via QEMU, so the
-           following operations occur after creating the device models *)
-      ; parallel_concat "Devices.plug (qemu)" ~id
-          [
-            List.map (fun pci -> PCI_plug (pci.Pci.id, true)) pcis_other
-          ; List.map (fun vusb -> VUSB_plug vusb.Vusb.id) vusbs
-          ]
+           following operations occur after creating the device models.
+           The order of PCI devices depends on the order they are plugged, they
+           must be kept serialized. *)
+      ; List.map (fun pci -> PCI_plug (pci.Pci.id, true)) pcis_other
+      ; List.map (fun vusb -> VUSB_plug vusb.Vusb.id) vusbs
         (* At this point the domain is considered survivable. *)
       ; [VM_set_domain_action_request (id, None)]
       ]
@@ -1698,10 +1697,10 @@ let rec atomics_of_operation = function
         )
       ; [VM_create_device_model (id, true)]
         (* PCI and USB devices are hot-plugged into HVM guests via QEMU, so
-           the following operations occur after creating the device models *)
-      ; parallel_map "PCIs.plug" ~id pcis_other (fun pci ->
-            [PCI_plug (pci.Pci.id, true)]
-        )
+           the following operations occur after creating the device models.
+           The order of PCI devices depends on the order they are plugged, they
+           must be kept serialized. *)
+      ; List.map (fun pci -> PCI_plug (pci.Pci.id, true)) pcis_other
       ]
       |> List.concat
   | VM_poweroff (id, timeout) ->