[PATCH v4] target/s390x/kvm: Enable adapter interruption suppression again

[Thomas Huth <thuth@redhat.com>]

The AIS feature has been disabled late in the v2.10 development cycle since
there were some issues with migration (see commit 3f2d07b3b01ea61126b -
"s390x/ais: for 2.10 stable: disable ais facility"). We originally wanted
to enable it again for newer machine types, but apparently we forgot to do
this so far. Let's do it for the new s390-ccw-virtio-5.0 machine now.

While at it, also add a more verbose comment why we need the *_allowed()
wrappers in s390-virtio-ccw.c.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1756946
Reviewed-by: David Hildenbrand <david@redhat.com>
Tested-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 v4: Use kvm_kernel_irqchip_allowed() for avoiding problems when running
     with -machine s390-ccw-virtio,kernel_irqchip=off

 hw/s390x/s390-virtio-ccw.c         | 20 +++++++++++++++++---
 include/hw/s390x/s390-virtio-ccw.h |  3 +++
 target/s390x/kvm.c                 |  9 ++++++---
 3 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index e0e28139a2..76254e8447 100644
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -452,6 +452,7 @@ static void ccw_machine_class_init(ObjectClass *oc, void *data)
     s390mc->cpu_model_allowed = true;
     s390mc->css_migration_enabled = true;
     s390mc->hpage_1m_allowed = true;
+    s390mc->kvm_ais_allowed = true;
     mc->init = ccw_init;
     mc->reset = s390_machine_reset;
     mc->hot_add_cpu = s390_hot_add_cpu;
@@ -505,6 +506,14 @@ static inline void machine_set_dea_key_wrap(Object *obj, bool value,
 
 static S390CcwMachineClass *current_mc;
 
+/*
+ * Get the class of the s390-ccw-virtio machine that is currently in use.
+ * Note: libvirt is using the "none" machine to probe for the features of the
+ * host CPU, so in case this is called with the "none" machine, the function
+ * returns the TYPE_S390_CCW_MACHINE base class. In this base class, all the
+ * various "*_allowed" variables are enabled, so that the *_allowed() wrappers
+ * below return the correct default value for the "none" machine.
+ */
 static S390CcwMachineClass *get_machine_class(void)
 {
     if (unlikely(!current_mc)) {
@@ -521,22 +530,24 @@ static S390CcwMachineClass *get_machine_class(void)
 
 bool ri_allowed(void)
 {
-    /* for "none" machine this results in true */
     return get_machine_class()->ri_allowed;
 }
 
 bool cpu_model_allowed(void)
 {
-    /* for "none" machine this results in true */
     return get_machine_class()->cpu_model_allowed;
 }
 
 bool hpage_1m_allowed(void)
 {
-    /* for "none" machine this results in true */
     return get_machine_class()->hpage_1m_allowed;
 }
 
+bool kvm_ais_allowed(void)
+{
+    return get_machine_class()->kvm_ais_allowed;
+}
+
 static char *machine_get_loadparm(Object *obj, Error **errp)
 {
     S390CcwMachineState *ms = S390_CCW_MACHINE(obj);
@@ -658,8 +669,11 @@ static void ccw_machine_4_2_instance_options(MachineState *machine)
 
 static void ccw_machine_4_2_class_options(MachineClass *mc)
 {
+    S390CcwMachineClass *s390mc = S390_MACHINE_CLASS(mc);
+
     ccw_machine_5_0_class_options(mc);
     compat_props_add(mc->compat_props, hw_compat_4_2, hw_compat_4_2_len);
+    s390mc->kvm_ais_allowed = false;
 }
 DEFINE_CCW_MACHINE(4_2, "4.2", false);
 
diff --git a/include/hw/s390x/s390-virtio-ccw.h b/include/hw/s390x/s390-virtio-ccw.h
index 8aa27199c9..e3ba3b88b1 100644
--- a/include/hw/s390x/s390-virtio-ccw.h
+++ b/include/hw/s390x/s390-virtio-ccw.h
@@ -40,6 +40,7 @@ typedef struct S390CcwMachineClass {
     bool cpu_model_allowed;
     bool css_migration_enabled;
     bool hpage_1m_allowed;
+    bool kvm_ais_allowed;
 } S390CcwMachineClass;
 
 /* runtime-instrumentation allowed by the machine */
@@ -48,6 +49,8 @@ bool ri_allowed(void);
 bool cpu_model_allowed(void);
 /* 1M huge page mappings allowed by the machine */
 bool hpage_1m_allowed(void);
+/* adapter-interrupt suppression allowed by the machine? */
+bool kvm_ais_allowed(void);
 
 /**
  * Returns true if (vmstate based) migration of the channel subsystem
diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c
index 15260aeb9a..1602a2c33d 100644
--- a/target/s390x/kvm.c
+++ b/target/s390x/kvm.c
@@ -365,10 +365,13 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
     /*
      * The migration interface for ais was introduced with kernel 4.13
      * but the capability itself had been active since 4.12. As migration
-     * support is considered necessary let's disable ais in the 2.10
-     * machine.
+     * support is considered necessary, we only try to enable this for
+     * newer machine types if KVM_CAP_S390_AIS_MIGRATION is available.
      */
-    /* kvm_vm_enable_cap(s, KVM_CAP_S390_AIS, 0); */
+    if (kvm_ais_allowed() && kvm_kernel_irqchip_allowed() &&
+        kvm_check_extension(s, KVM_CAP_S390_AIS_MIGRATION)) {
+        kvm_vm_enable_cap(s, KVM_CAP_S390_AIS, 0);
+    }
 
     kvm_set_max_memslot_size(KVM_SLOT_MAX_BYTES);
     return 0;
-- 
2.18.1