qemu: Use GICv2 for aarch64/virt TCG guests

There are currently some limitations in the emulated GICv3
that make it unsuitable as a default. Use GICv2 instead.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1450433

Signed-off-by: Andrea Bolognani <abologna@redhat.com>

src/qemu/qemu_domain.c

@@ -2563,6 +2563,24 @@ qemuDomainDefEnableDefaultFeatures(virDomainDefPtr def,
         for (version = VIR_GIC_VERSION_LAST - 1;
              version > VIR_GIC_VERSION_NONE;
              version--) {
+
+            /* We want to use the highest available GIC version for guests;
+             * however, the emulated GICv3 is currently lacking a MSI controller,
+             * making it unsuitable for the pure PCIe topology we aim for.
+             *
+             * For that reason, we skip this step entirely for TCG guests,
+             * and rely on the code below to pick the default version, GICv2,
+             * which supports all the features we need.
+             *
+             * We'll want to revisit this once MSI support for GICv3 has been
+             * implemented in QEMU.
+             *
+             * See https://bugzilla.redhat.com/show_bug.cgi?id=1414081 */
+            if (version == VIR_GIC_VERSION_3 &&
+                def->virtType == VIR_DOMAIN_VIRT_QEMU) {
+                continue;
+            }
+
             if (virQEMUCapsSupportsGICVersion(qemuCaps,
                                               def->virtType,
                                               version)) {
@@ -2580,8 +2598,11 @@ qemuDomainDefEnableDefaultFeatures(virDomainDefPtr def,
 
     /* Use the default GIC version if no version was specified */
     if (def->features[VIR_DOMAIN_FEATURE_GIC] == VIR_TRISTATE_SWITCH_ON &&
-        def->gic_version == VIR_GIC_VERSION_NONE)
+        def->gic_version == VIR_GIC_VERSION_NONE) {
+        VIR_DEBUG("Using GIC version %s (default)",
+                  virGICVersionTypeToString(VIR_GIC_VERSION_DEFAULT));
         def->gic_version = VIR_GIC_VERSION_DEFAULT;
+    }
 }
 
 

tests/qemuxml2argvdata/qemuxml2argv-aarch64-gic-none-tcg.args

@@ -7,7 +7,7 @@ QEMU_AUDIO_DRV=none \
 /usr/bin/qemu-system-aarch64 \
 -name guest \
 -S \
--machine virt,accel=tcg,gic-version=3 \
+-machine virt,accel=tcg \
 -cpu cortex-a57 \
 -m 1024 \
 -smp 1,sockets=1,cores=1,threads=1 \

tests/qemuxml2xmloutdata/qemuxml2xmlout-aarch64-gic-none-tcg.xml

@@ -9,7 +9,7 @@
     <boot dev='hd'/>
   </os>
   <features>
-    <gic version='3'/>
+    <gic version='2'/>
   </features>
   <cpu mode='custom' match='exact' check='none'>
     <model fallback='allow'>cortex-a57</model>