libvirt: Remove unnecessary TODO

The 'guest.os_mach_type' property is set to the value returned by
'libvirt.utils.get_machine_type'. This will attempt to pull a machine
type from image metadata and fallback to a sensible default. As such, we
never need to worry about 'guest.os_mach_type' being unset on x86 hosts.

Change-Id: Ibc6d3dcaaa704599b7963bf97bdd901788f5af45
Signed-off-by: Stephen Finucane <[email protected]>

nova/virt/libvirt/driver.py

@@ -6811,38 +6811,28 @@ def _guest_needs_pcie(self, guest):
         """
         caps = self._host.get_capabilities()
 
-        # TODO(kchamart) In the third 'if' conditional below, for 'x86'
-        # arch, we're assuming: when 'os_mach_type' is 'None', you'll
-        # have "pc" machine type.  That assumption, although it is
-        # correct for the "forseeable future", it will be invalid when
-        # libvirt / QEMU changes the default machine types.
-        #
-        # From libvirt 4.7.0 onwards (September 2018), it will ensure
-        # that *if* 'pc' is available, it will be used as the default --
-        # to not break existing applications.  (Refer:
-        # https://libvirt.org/git/?p=libvirt.git;a=commit;h=26cfb1a3
-        # --"qemu: ensure default machine types don't change if QEMU
-        # changes").
-        #
-        # But even if libvirt (>=v4.7.0) handled the default case,
-        # relying on such assumptions is not robust.  Instead we should
-        # get the default machine type for a given architecture reliably
-        # -- by Nova setting it explicitly (we already do it for Arm /
-        # AArch64 & s390x).  A part of this bug is being tracked here:
-        # https://bugs.launchpad.net/nova/+bug/1780138).
-
         # Add PCIe root port controllers for PCI Express machines
         # but only if their amount is configured
 
         if not CONF.libvirt.num_pcie_ports:
             return False
-        if (caps.host.cpu.arch == fields.Architecture.AARCH64 and
-                guest.os_mach_type.startswith('virt')):
+
+        # Only certain architectures and machine types can handle PCIe ports;
+        # the latter will be handled by libvirt.utils.get_machine_type
+
+        if (
+            caps.host.cpu.arch == fields.Architecture.AARCH64 and
+            guest.os_mach_type.startswith('virt')
+        ):
             return True
-        if (caps.host.cpu.arch == fields.Architecture.X86_64 and
-                guest.os_mach_type is not None and
-                'q35' in guest.os_mach_type):
+
+        if (
+            caps.host.cpu.arch == fields.Architecture.X86_64 and
+            guest.os_mach_type is not None and
+            'q35' in guest.os_mach_type
+        ):
             return True
+
         return False
 
     def _get_guest_config(self, instance, network_info, image_meta,