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

Michael Mueller
2023-09-04T14:11:34Z

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);