hw/s390x: Add a more verbose comment about get_machine_class() and the wrappers

While working on the "Enable adapter interruption suppression again"
recently, I had to discover that the meaning of get_machine_class()
and the related *_allowed() wrappers is not very obvious. Add a more
verbose comment here to clarify how these should be used.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200123170256.12386-1-thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

1 files changed, 13 insertions, 3 deletions

diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index e0e28139a2..e759eb5f83 100644
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -505,6 +505,19 @@ 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.
+ *
+ * Attention! Do *not* add additional new wrappers for CPU features (e.g. like
+ * the ri_allowed() wrapper) via this mechanism anymore. CPU features should
+ * be handled via the CPU models, i.e. checking with cpu_model_allowed() during
+ * CPU initialization and s390_has_feat() later should be sufficient.
+ */
 static S390CcwMachineClass *get_machine_class(void)
 {
     if (unlikely(!current_mc)) {
@@ -521,19 +534,16 @@ 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;
 }