Re: [PATCH v4] KVM: s390: fix gisa destroy operation might lead to cpu stalls

[Michael Mueller <mimu@linux.ibm.com>]

On 04.09.23 09:05, Nico Boehr wrote:
> Quoting Michael Mueller (2023-09-01 12:58:23)
> [...]
>> diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
>> index 9bd0a873f3b1..96450e5c4b6f 100644
>> --- a/arch/s390/kvm/interrupt.c
>> +++ b/arch/s390/kvm/interrupt.c
> [...]
>>   static inline void gisa_set_ipm_gisc(struct kvm_s390_gisa *gisa, u32 gisc)
>>   {
>>          set_bit_inv(IPM_BIT_OFFSET + gisc, (unsigned long *) gisa);
>> @@ -3202,11 +3197,12 @@ void kvm_s390_gisa_destroy(struct kvm *kvm)
>>   
>>          if (!gi->origin)
>>                  return;
>> -       if (gi->alert.mask)
>> -               KVM_EVENT(3, "vm 0x%pK has unexpected iam 0x%02x",
>> -                         kvm, gi->alert.mask);
>> -       while (gisa_in_alert_list(gi->origin))
>> -               cpu_relax();
>> +       WARN(gi->alert.mask != 0x00,
>> +            "unexpected non zero alert.mask 0x%02x",
>> +            gi->alert.mask);
>> +       gi->alert.mask = 0x00;
>> +       if (gisa_set_iam(gi->origin, gi->alert.mask))
>> +               process_gib_alert_list();
> 
> I am not an expert for the GISA, so excuse my possibly stupid question:
> process_gib_alert_list() starts the timer. So can gisa_vcpu_kicker()
> already be running before we reach hrtimer_cancel() below? Is this fine?

You are right, It cannnot be running in that situation because 
gisa_vcpu_kicker() has returned with HRTIMER_NORESTART and no vcpus are 
defined anymore.

There is another case when the gisa specific timer is started not only 
in the process_gib_alert() case but also when a vcpu of the guest owning 
the gisa is put into wait state, see kvm_s390_handle_wait(). Thus yes, 
it could be running already in that situation. I remember having seen 
this situation when I write the gisa/gib code. But that's case in 
process_gib_alert_list()

It does not hurt to have the hrtimer_cancel() here but I don't want to 
add a change to this patch. Eventually a new one.

Thanks for asking!

> 
>>          hrtimer_cancel(&gi->timer);
>>          gi->origin = NULL;
>>          VM_EVENT(kvm, 3, "gisa 0x%pK destroyed", gisa);