vm: improve error message about missing IOMMU

Handle this case specifically, as way too many users ignore the message
during installation and complain it doesn't work later.

Name the problem explicitly, instead of pointing at libvirt error log.

Fixes QubesOS/qubes-issues#4689

qubes/app.py

@@ -314,6 +314,16 @@ class QubesHost:
             raise NotImplementedError('This function requires Xen hypervisor')
         return int(self._physinfo['free_memory'])
 
+    def is_iommu_supported(self):
+        """Check if IOMMU is supported on this platform"""
+        if self._physinfo is None:
+            try:
+                self._physinfo = self.app.vmm.xc.physinfo()
+            except AttributeError:
+                raise NotImplementedError(
+                    'This function requires Xen hypervisor')
+        return 'hvm_directio' in self._physinfo['virt_caps']
+
     def get_vm_stats(self, previous_time=None, previous=None, only_vm=None):
         """Measure cpu usage for all domains at once.
 

qubes/vm/qubesvm.py

@@ -1115,6 +1115,19 @@ class QubesVM(qubes.vm.mix.net.NetVMMixin, qubes.vm.BaseVM):
                 self.libvirt_domain.createWithFlags(
                     libvirt.VIR_DOMAIN_START_PAUSED)
 
+            except libvirt.libvirtError as exc:
+                # missing IOMMU?
+                if self.virt_mode == 'hvm' and \
+                        list(self.devices['pci'].persistent()) and \
+                        not self.app.host.is_iommu_supported():
+                    exc = qubes.exc.QubesException(
+                        'Failed to start an HVM qube with PCI devices assigned '
+                        '- hardware does not support IOMMU/VT-d/AMD-Vi')
+                self.log.error('Start failed: %s', str(exc))
+                yield from self.fire_event_async('domain-start-failed',
+                                                 reason=str(exc))
+                yield from self.storage.stop()
+                raise exc
             except Exception as exc:
                 self.log.error('Start failed: %s', str(exc))
                 # let anyone receiving domain-pre-start know that startup failed