If a Q35 domain has huge number of vCPUS (over 255, currently), then
it needs IOMMU with Extended Interrupt Mode enabled (see check in
qemuValidateDomainVCpuTopology()).

Well, we already add some devices and to other tricks when
parsing new domain XML. Might as well add IOMMU device if above
condition is met.

Resolves: https://issues.redhat.com/browse/RHEL-65844
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
 src/qemu/qemu_domain.c                        | 19 +++++++++++++++++++
 .../intel-iommu-eim-autoadd.xml               |  1 -
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
index 39f3e38351..f07f0505b1 100644
--- a/src/qemu/qemu_domain.c
+++ b/src/qemu/qemu_domain.c
@@ -4262,6 +4262,7 @@ qemuDomainDefAddDefaultDevices(virQEMUDriver *driver,
     bool addDefaultUSBMouse = false;
     bool addPanicDevice = false;
     bool addITCOWatchdog = false;
+    bool addIOMMU = false;
 
     /* add implicit input devices */
     if (qemuDomainDefAddImplicitInputDevice(def, qemuCaps) < 0)
@@ -4284,6 +4285,10 @@ qemuDomainDefAddDefaultDevices(virQEMUDriver *driver,
             addImplicitSATA = true;
             addITCOWatchdog = true;
 
+            if (virDomainDefGetVcpusMax(def) > QEMU_MAX_VCPUS_WITHOUT_EIM) {
+                addIOMMU = true;
+            }
+
             /* Prefer adding a USB3 controller if supported, fall back
              * to USB2 if there is no USB3 available, and if that's
              * unavailable don't add anything.
@@ -4530,6 +4535,20 @@ qemuDomainDefAddDefaultDevices(virQEMUDriver *driver,
         }
     }
 
+    if (addIOMMU && !def->iommu &&
+        virQEMUCapsGet(qemuCaps, QEMU_CAPS_DEVICE_INTEL_IOMMU) &&
+        virQEMUCapsGet(qemuCaps, QEMU_CAPS_INTEL_IOMMU_EIM)) {
+        g_autoptr(virDomainIOMMUDef) iommu = NULL;
+
+        iommu = virDomainIOMMUDefNew();
+        iommu->model = VIR_DOMAIN_IOMMU_MODEL_INTEL;
+        /* eim requires intremap. */
+        iommu->intremap = VIR_TRISTATE_SWITCH_ON;
+        iommu->eim = VIR_TRISTATE_SWITCH_ON;
+
+        def->iommu = g_steal_pointer(&iommu);
+    }
+
     if (qemuDomainDefAddDefaultAudioBackend(driver, def) < 0)
         return -1;
 
diff --git a/tests/qemuxmlconfdata/intel-iommu-eim-autoadd.xml b/tests/qemuxmlconfdata/intel-iommu-eim-autoadd.xml
index 7c294fe2f9..fa3aaf0d44 100644
--- a/tests/qemuxmlconfdata/intel-iommu-eim-autoadd.xml
+++ b/tests/qemuxmlconfdata/intel-iommu-eim-autoadd.xml
@@ -30,6 +30,5 @@
     <audio id='1' type='none'/>
     <watchdog model='itco' action='reset'/>
     <memballoon model='none'/>
-    <iommu model='intel'/>
   </devices>
 </domain>
-- 
2.45.2