qemu: Use host-model CPU on s390 by default

On s390 machines host-passthrough and host-model CPUs result in the same
guest ABI (with QEMU new enough to be able to tell us what "host" CPU is
expanded to, which was implemented around 2.9.0). So instead of using
host-passthrough CPU when there's no CPU specified in a domain XML we
can safely use host-model and benefit from CPU compatibility checks
during migration, snapshot restore and similar operations.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

src/qemu/qemu_domain.c

@@ -4435,6 +4435,7 @@ qemuDomainDefVcpusPostParse(virDomainDefPtr def)
 
 static int
 qemuDomainDefSetDefaultCPU(virDomainDefPtr def,
+                           virCapsPtr caps,
                            virQEMUCapsPtr qemuCaps)
 {
     const char *model;
@@ -4465,26 +4466,36 @@ qemuDomainDefSetDefaultCPU(virDomainDefPtr def,
         return -1;
     }
 
-    VIR_DEBUG("Setting default CPU model for domain '%s' to %s",
-              def->name, model);
-
     if (!def->cpu)
         def->cpu = g_new0(virCPUDef, 1);
 
-    /* We need to turn off all CPU checks when the domain is started because
-     * the default CPU (e.g., qemu64) may not be runnable on any host. QEMU
-     * will just disable the unavailable features and we will update the CPU
-     * definition accordingly and set check to FULL when starting the domain. */
     def->cpu->type = VIR_CPU_TYPE_GUEST;
-    def->cpu->check = VIR_CPU_CHECK_NONE;
 
     if (STREQ(model, "host")) {
-        def->cpu->mode = VIR_CPU_MODE_HOST_PASSTHROUGH;
+        if (ARCH_IS_S390(def->os.arch) &&
+            virQEMUCapsIsCPUModeSupported(qemuCaps, caps, def->virtType,
+                                          VIR_CPU_MODE_HOST_MODEL)) {
+            def->cpu->mode = VIR_CPU_MODE_HOST_MODEL;
+        } else {
+            def->cpu->mode = VIR_CPU_MODE_HOST_PASSTHROUGH;
+        }
+
+        VIR_DEBUG("Setting default CPU mode for domain '%s' to %s",
+                  def->name, virCPUModeTypeToString(def->cpu->mode));
     } else {
+        /* We need to turn off all CPU checks when the domain is started
+         * because the default CPU (e.g., qemu64) may not be runnable on any
+         * host. QEMU will just disable the unavailable features and we will
+         * update the CPU definition accordingly and set check to FULL when
+         * starting the domain. */
+        def->cpu->check = VIR_CPU_CHECK_NONE;
         def->cpu->mode = VIR_CPU_MODE_CUSTOM;
         def->cpu->match = VIR_CPU_MATCH_EXACT;
         def->cpu->fallback = VIR_CPU_FALLBACK_FORBID;
         def->cpu->model = g_strdup(model);
+
+        VIR_DEBUG("Setting default CPU model for domain '%s' to %s",
+                  def->name, model);
     }
 
     return 0;
@@ -4669,7 +4680,7 @@ qemuDomainDefPostParseBasic(virDomainDefPtr def,
 
 static int
 qemuDomainDefPostParse(virDomainDefPtr def,
-                       virCapsPtr caps G_GNUC_UNUSED,
+                       virCapsPtr caps,
                        unsigned int parseFlags,
                        void *opaque,
                        void *parseOpaque)
@@ -4701,7 +4712,7 @@ qemuDomainDefPostParse(virDomainDefPtr def,
     if (qemuCanonicalizeMachine(def, qemuCaps) < 0)
         return -1;
 
-    if (qemuDomainDefSetDefaultCPU(def, qemuCaps) < 0)
+    if (qemuDomainDefSetDefaultCPU(def, caps, qemuCaps) < 0)
         return -1;
 
     qemuDomainDefEnableDefaultFeatures(def, qemuCaps);

tests/qemuxml2argvdata/s390-default-cpu-kvm-ccw-virtio-4.2.s390x-latest.args

@@ -13,7 +13,9 @@ QEMU_AUDIO_DRV=none \
 -object secret,id=masterKey0,format=raw,\
 file=/tmp/lib/domain--1-test/master-key.aes \
 -machine s390-ccw-virtio-4.2,accel=kvm,usb=off,dump-guest-core=off \
--cpu host \
+-cpu z13.2-base,aen=on,aefsi=on,msa5=on,msa4=on,msa3=on,msa2=on,msa1=on,\
+sthyi=on,edat=on,ri=on,edat2=on,vx=on,ipter=on,ap=on,esop=on,apft=on,apqci=on,\
+cte=on,bpb=on,ppa15=on,zpci=on,sea_esop2=on,te=on,cmm=on \
 -m 256 \
 -overcommit mem-lock=off \
 -smp 1,sockets=1,cores=1,threads=1 \

tests/qemuxml2xmloutdata/s390-default-cpu-kvm-ccw-virtio-4.2.s390x-latest.xml

@@ -8,7 +8,7 @@
     <type arch='s390x' machine='s390-ccw-virtio-4.2'>hvm</type>
     <boot dev='hd'/>
   </os>
-  <cpu mode='host-passthrough' check='none'/>
+  <cpu mode='host-model' check='partial'/>
   <clock offset='utc'/>
   <on_poweroff>destroy</on_poweroff>
   <on_reboot>restart</on_reboot>