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

[Thomas Huth <thuth@redhat.com>]

On 16/01/2020 13.28, David Hildenbrand wrote:
> On 16.01.20 13:26, Thomas Huth wrote:
>> On 16/01/2020 13.23, David Hildenbrand wrote:
>>> On 16.01.20 13:20, Thomas Huth wrote:
>>>> 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.
>>>>
>>>> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1756946
>>>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>>>> ---
>>>>  hw/s390x/s390-virtio-ccw.c         |  4 ++++
>>>>  include/hw/s390x/s390-virtio-ccw.h |  4 ++++
>>>>  target/s390x/kvm.c                 | 11 ++++++++---
>>>>  3 files changed, 16 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
>>>> index e7eadd14e8..6f43136396 100644
>>>> --- a/hw/s390x/s390-virtio-ccw.c
>>>> +++ b/hw/s390x/s390-virtio-ccw.c
>>>> @@ -456,6 +456,7 @@ 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;
>>>> @@ -662,6 +663,9 @@ 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);
>>>> +
>>>> +    s390mc->kvm_ais_allowed = false;
>>>>      ccw_machine_5_0_class_options(mc);
>>>>      compat_props_add(mc->compat_props, hw_compat_4_2, hw_compat_4_2_len);
>>>>  }
>>>> diff --git a/include/hw/s390x/s390-virtio-ccw.h b/include/hw/s390x/s390-virtio-ccw.h
>>>> index 8aa27199c9..f142d379c6 100644
>>>> --- a/include/hw/s390x/s390-virtio-ccw.h
>>>> +++ b/include/hw/s390x/s390-virtio-ccw.h
>>>> @@ -21,6 +21,9 @@
>>>>  #define S390_MACHINE_CLASS(klass) \
>>>>      OBJECT_CLASS_CHECK(S390CcwMachineClass, (klass), TYPE_S390_CCW_MACHINE)
>>>>  
>>>> +#define S390_CCW_MACHINE_OBJ_GET_CLASS(obj) \
>>>> +    OBJECT_GET_CLASS(S390CcwMachineClass, obj, TYPE_S390_CCW_MACHINE)
>>>> +
>>>>  typedef struct S390CcwMachineState {
>>>>      /*< private >*/
>>>>      MachineState parent_obj;
>>>> @@ -40,6 +43,7 @@ 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 */
>>>> diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c
>>>> index 15260aeb9a..4c1c8c0208 100644
>>>> --- a/target/s390x/kvm.c
>>>> +++ b/target/s390x/kvm.c
>>>> @@ -329,6 +329,8 @@ static void ccw_machine_class_foreach(ObjectClass *oc, void *opaque)
>>>>  
>>>>  int kvm_arch_init(MachineState *ms, KVMState *s)
>>>>  {
>>>> +    S390CcwMachineClass *smc = S390_CCW_MACHINE_OBJ_GET_CLASS(ms);
>>>> +
>>>>      object_class_foreach(ccw_machine_class_foreach, TYPE_S390_CCW_MACHINE,
>>>>                           false, NULL);
>>>>  
>>>> @@ -365,10 +367,13 @@ 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 enable this for newer
>>>> +     * machine types and if KVM_CAP_S390_AIS_MIGRATION is available.
>>>>       */
>>>> -    /* kvm_vm_enable_cap(s, KVM_CAP_S390_AIS, 0); */
>>>> +    if (smc->kvm_ais_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;
>>>>
>>>
>>> We have ri_allowed(), cpu_model_allowed(), hpage_1m_allowed().
>>>
>>> Care to create a similar wrapper?
>>
>> Honestly, why do we need these wrappers at all? They look cumbersome to
>> me. I'd rather remove them in case they are not urgently needed (so far
>> I don't see the point... could someone enlighten me why we have them?).
> 
> I assume to minimize the number of places you have to lookup the
> machine/machine class.

I don't think that any of these functions is performance critical, since
they are only used during the initialization phase...
But looking more closely, cpu_model_allowed() and hpage_1m_allowed() are
used in functions where the current machine state / class is not
directly available, so the wrappers indeed make sense there.
We could remove the ri_allowed() wrapper, though, since this is also
only used in kvm_arch_init() where the machine state is easily available.

 Thomas