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

[David Hildenbrand <david@redhat.com>]

On 21.01.20 17:33, 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.
> 
> 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);
> +    }

I just remembered that the availability of CPU features should in
general not depend on the selected machine. We only have compat handling
for pre-cpu-model-support machines (e.g., vx).

The issue is, that the probed host cpu model is otherwise not guaranteed
to run om a selected machine and you get misleading errors.

Can someone remind me why we need kvm_ais_allowed() at all and cannot
simply rely on cpu model checks to properly handle this (like all other
features)?

-- 
Thanks,

David / dhildenb