Re: [Qemu-devel] [libvirt] Question about the host-model CPU mode

[Christian Borntraeger]

On 10/20/2017 03:51 PM, David Hildenbrand wrote:
[...]
>> The problem goes much further.
>> A fresh guest with
>>
>>     <os>
>>      <type arch='s390x' machine='s390-ccw-virtio-2.9'>hvm</type>
>>    </os>
>>    <cpu mode='host-model'/>
>>
>> does not start. No migration from an older system is necessary.
>>
> 
> Yes, as stated in the documentation "copying host CPU definition from
> capabilities XML" this can not work. And it works just as documented.
> Not saying this is a nice thing :)
> 
> I think we should try to fix gs_allowed (if possible) and avoid
> something like that in the future. This would avoid other complexity
> involved when suddenly having X host models.

Maybe this one is really a proper fix. It will allow the guest to start
and on migration the cpu model will complain if the target cannot provide gs.
Similar things can happen if - for example - the host kernel lacks some 
features.


diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index 77169bb..97a08fa 100644
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -430,7 +430,6 @@ static void ccw_machine_class_init(ObjectClass *oc, void 
*data)
     s390mc->ri_allowed = true;
     s390mc->cpu_model_allowed = true;
     s390mc->css_migration_enabled = true;
-    s390mc->gs_allowed = true;
     mc->init = ccw_init;
     mc->reset = s390_machine_reset;
     mc->hot_add_cpu = s390_hot_add_cpu;
@@ -509,12 +508,6 @@ bool cpu_model_allowed(void)
     return get_machine_class()->cpu_model_allowed;
 }
 
-bool gs_allowed(void)
-{
-    /* for "none" machine this results in true */
-    return get_machine_class()->gs_allowed;
-}
-
 static char *machine_get_loadparm(Object *obj, Error **errp)
 {
     S390CcwMachineState *ms = S390_CCW_MACHINE(obj);
@@ -757,7 +750,6 @@ static void ccw_machine_2_9_class_options(MachineClass *mc)
 {
     S390CcwMachineClass *s390mc = S390_MACHINE_CLASS(mc);
 
-    s390mc->gs_allowed = false;
     ccw_machine_2_10_class_options(mc);
     SET_MACHINE_COMPAT(mc, CCW_COMPAT_2_9);
     s390mc->css_migration_enabled = false;
diff --git a/include/hw/s390x/s390-virtio-ccw.h 
b/include/hw/s390x/s390-virtio-ccw.h
index a9a90c2..1de53b0 100644
--- a/include/hw/s390x/s390-virtio-ccw.h
+++ b/include/hw/s390x/s390-virtio-ccw.h
@@ -40,7 +40,6 @@ typedef struct S390CcwMachineClass {
     bool ri_allowed;
     bool cpu_model_allowed;
     bool css_migration_enabled;
-    bool gs_allowed;
 } S390CcwMachineClass;
 
 /* runtime-instrumentation allowed by the machine */
diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c
index a0d5052..3f13fc2 100644
--- a/target/s390x/kvm.c
+++ b/target/s390x/kvm.c
@@ -362,7 +362,7 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
             cap_ri = 1;
         }
     }
-    if (gs_allowed()) {
+    if (cpu_model_allowed()) {
         if (kvm_vm_enable_cap(s, KVM_CAP_S390_GS, 0) == 0) {
             cap_gs = 1;
         }