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

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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)
-       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!

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

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