qemu_capabilities: Disable CPU models on old s390 machine types

Starting a KVM domain on s390 with old machine type (such as
s390-ccw-virtio-2.5) and without any guest CPU model configured fails
with

    CPU models are not available: KVM doesn't support CPU models

QEMU error. This is cause by libvirt using host-model CPU as the default
CPU based on QEMU reporting "host" CPU model as being the default one
(see commit v5.9.0-402-g24d8202294: qemu: Use host-model CPU on s390 by
default). However, even though both QEMU and KVM support CPU models on
s390 and QEMU can give us the host-model CPU, we can't use it with old
machine types which only support -cpu host.

https://bugzilla.redhat.com/show_bug.cgi?id=1795651

Reported-by: Christian Ehrhardt <paelzer@gmail.com>
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

src/qemu/qemu_capabilities.c

@@ -2274,16 +2274,33 @@ virQEMUCapsIsVirtTypeSupported(virQEMUCapsPtr qemuCaps,
     return false;
 }
 
+const char *s390HostPassthroughOnlyMachines[] = {
+    "s390-ccw-virtio-2.4",
+    "s390-ccw-virtio-2.5",
+    "s390-ccw-virtio-2.6",
+    "s390-ccw-virtio-2.7",
+    NULL
+};
 
 bool
 virQEMUCapsIsCPUModeSupported(virQEMUCapsPtr qemuCaps,
                               virArch hostarch,
                               virDomainVirtType type,
                               virCPUMode mode,
-                              const char *machineType G_GNUC_UNUSED)
+                              const char *machineType)
 {
     qemuMonitorCPUDefsPtr cpus;
 
+    /* CPU models (except for "host") are not supported by QEMU for on s390
+     * KVM domains with old machine types regardless on QEMU version. */
+    if (ARCH_IS_S390(qemuCaps->arch) &&
+        type == VIR_DOMAIN_VIRT_KVM &&
+        mode != VIR_CPU_MODE_HOST_PASSTHROUGH &&
+        machineType &&
+        g_strv_contains(s390HostPassthroughOnlyMachines, machineType)) {
+        return false;
+    }
+
     switch (mode) {
     case VIR_CPU_MODE_HOST_PASSTHROUGH:
         return type == VIR_DOMAIN_VIRT_KVM &&

tests/qemuxml2argvdata/s390-default-cpu-kvm-ccw-virtio-2.7.s390x-latest.args

@@ -13,9 +13,7 @@ QEMU_AUDIO_DRV=none \
 -object secret,id=masterKey0,format=raw,\
 file=/tmp/lib/domain--1-test/master-key.aes \
 -machine s390-ccw-virtio-2.7,accel=kvm,usb=off,dump-guest-core=off \
--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 \
+-cpu host \
 -m 256 \
 -overcommit mem-lock=off \
 -smp 1,sockets=1,cores=1,threads=1 \

tests/qemuxml2xmloutdata/s390-default-cpu-kvm-ccw-virtio-2.7.s390x-latest.xml

@@ -8,7 +8,7 @@
     <type arch='s390x' machine='s390-ccw-virtio-2.7'>hvm</type>
     <boot dev='hd'/>
   </os>
-  <cpu mode='host-model' check='partial'/>
+  <cpu mode='host-passthrough' check='none'/>
   <clock offset='utc'/>
   <on_poweroff>destroy</on_poweroff>
   <on_reboot>restart</on_reboot>