Re: [PATCH v6 12/13] KVM: s390: add gib_alert_irq_handler()

Halil Pasic
2019-01-29T16:45:17Z

On Tue, 29 Jan 2019 16:29:40 +0100
Michael Mueller <mimu@linux.ibm.com> wrote:

>
>
> On 29.01.19 14:26, Halil Pasic wrote:
> > On Thu, 24 Jan 2019 13:59:38 +0100
> > Michael Mueller <mimu@linux.ibm.com> wrote:
> >
> >> The patch implements a handler for GIB alert interruptions
> >> on the host. Its task is to alert guests that interrupts are
> >> pending for them.
> >>
> >> A GIB alert interrupt statistic counter is added as well:
> >>
> >> $ cat /proc/interrupts
> >> CPU0 CPU1
> >> ...
> >> GAL: 23 37 [I/O] GIB Alert
> >> ...
> >>
> >> Signed-off-by: Michael Mueller <mimu@linux.ibm.com>
> > [..]
> >> +/**
> >> + * gisa_get_ipm_or_restore_iam - return IPM or restore GISA IAM
> >> + *
> >> + * @gi: gisa interrupt struct to work on
> >> + *
> >> + * Atomically restores the interruption alert mask if none of the
> >> + * relevant ISCs are pending and return the IPM.
> >
> > The word 'relevant' probably reflects some previous state. It does not
> > bother me too much.
>
> "relevant" refers to the ISCs handled by the GAL mechanism, i.e those
> registered in the kvm->arch.gisa_int.alert.mask.

Sorry it was me who overlooked the & with the mask.

> >
> > [..]
> >
> >>
> >> +static void __airqs_kick_single_vcpu(struct kvm *kvm, u8 deliverable_mask)
> >> +{
> >> + int vcpu_id, online_vcpus = atomic_read(&kvm->online_vcpus);
> >> + struct kvm_s390_gisa_interrupt *gi = &kvm->arch.gisa_int;
> >> + struct kvm_vcpu *vcpu;
> >> +
> >> + for_each_set_bit(vcpu_id, kvm->arch.idle_mask, online_vcpus) {
> >> + vcpu = kvm_get_vcpu(kvm, vcpu_id);
> >> + if (psw_ioint_disabled(vcpu))
> >> + continue;
> >> + deliverable_mask &= (u8)(vcpu->arch.sie_block->gcr[6] >> 24);
> >> + if (deliverable_mask) {
> >> + /* lately kicked but not yet running */
> >
> > How about /* was kicked but didn't run yet */?
>
> what is the difference here...

I read you comment like the vcpu is either not running yet or running.
However the vcpu could have went into sie processed the interrupt and
gone back to wait state: the bit in the kicked_mask would be clear
in this case, and we would do the right thing kick it again.

I'm not a grammar expert but that continuous does bother me. I may be
wrong.

> > [..]
> >
> >> +static void process_gib_alert_list(void)
> >> +{
> >> + struct kvm_s390_gisa_interrupt *gi;
> >> + struct kvm_s390_gisa *gisa;
> >> + struct kvm *kvm;
> >> + u32 final, origin = 0UL;
> >> +
> >> + do {
> >> + /*
> >> + * If the NONE_GISA_ADDR is still stored in the alert list
> >> + * origin, we will leave the outer loop. No further GISA has
> >> + * been added to the alert list by millicode while processing
> >> + * the current alert list.
> >> + */
> >> + final = (origin & NONE_GISA_ADDR);
> >> + /*
> >> + * Cut off the alert list and store the NONE_GISA_ADDR in the
> >> + * alert list origin to avoid further GAL interruptions.
> >> + * A new alert list can be build up by millicode in parallel
> >> + * for guests not in the yet cut-off alert list. When in the
> >> + * final loop, store the NULL_GISA_ADDR instead. This will re-
> >> + * enable GAL interruptions on the host again.
> >> + */
> >> + origin = xchg(&gib->alert_list_origin,
> >> + (!final) ? NONE_GISA_ADDR : NULL_GISA_ADDR);
> >> + /*
> >> + * Loop through the just cut-off alert list and start the
> >> + * gisa timers to kick idle vcpus to consume the pending
> >> + * interruptions asap.
> >> + */
> >> + while (origin & GISA_ADDR_MASK) {
> >> + gisa = (struct kvm_s390_gisa *)(u64)origin;
> >> + origin = gisa->next_alert;
> >> + gisa->next_alert = (u32)(u64)gisa;
> >> + kvm = container_of(gisa, struct sie_page2, gisa)->kvm;
> >> + gi = &kvm->arch.gisa_int;
> >> + if (hrtimer_active(&gi->timer))
> >> + hrtimer_cancel(&gi->timer);
> >> + hrtimer_start(&gi->timer, 0, HRTIMER_MODE_REL);
> >> + }
> >> + } while (!final);
> >> +
> >> +}
> >> +
> >> void kvm_s390_gisa_clear(struct kvm *kvm)
> >> {
> >> struct kvm_s390_gisa_interrupt *gi = &kvm->arch.gisa_int;
> >>
> >> if (!gi->origin)
> >> return;
> >> - memset(gi->origin, 0, sizeof(struct kvm_s390_gisa));
> >> - gi->origin->next_alert = (u32)(u64)gi->origin;
> >> + gisa_clear_ipm(gi->origin);
> >
> > This could be a separate patch. I would like little more explanation
> > to this.

nice

>
> I can break at out as I had before... ;)
>
> >
> >> VM_EVENT(kvm, 3, "gisa 0x%pK cleared", gi->origin);
> >> }
> >>
> >> @@ -2940,13 +3078,25 @@ void kvm_s390_gisa_init(struct kvm *kvm)
> >> gi->origin = &kvm->arch.sie_page2->gisa;
> >> gi->alert.mask = 0;
> >> spin_lock_init(&gi->alert.ref_lock);
> >> - kvm_s390_gisa_clear(kvm);
> >> + gi->expires = 50 * 1000; /* 50 usec */
> >
> > I blindly trust your choice here ;)
>
> You know I will increase it to 1 ms together with the change that I
> proposed. (gisa_get_ipm_or_restore_iam() in kvm_s390_handle_wait()).
>

Is probably OK with just one gsic registered. I will think about
it a bit more.

> >
> >> + hrtimer_init(&gi->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> >> + gi->timer.function = gisa_vcpu_kicker;
> >> + memset(gi->origin, 0, sizeof(struct kvm_s390_gisa));
> >> + gi->origin->next_alert = (u32)(u64)gi->origin;
> >> VM_EVENT(kvm, 3, "gisa 0x%pK initialized", gi->origin);
> >> }
> >>
> >> void kvm_s390_gisa_destroy(struct kvm *kvm)
> >> {
> >> - kvm->arch.gisa_int.origin = NULL;
> >> + struct kvm_s390_gisa_interrupt *gi = &kvm->arch.gisa_int;
> >> +
> >> + if (!gi->origin)
> >> + return;
> >> + hrtimer_cancel(&gi->timer);
> >
> > I'm not sure this cancel here is sufficient.
> >
> >> + WRITE_ONCE(gi->alert.mask, 0);
> >> + while (gisa_in_alert_list(gi->origin))
> >> + cpu_relax();
> >
> > If you end up waiting here, I guess, it's likely that a new
> > timer is going to get set up right after we do
> > gisa->next_alert = (u32)(u64)gisa;
> > in process_gib_alert_list().
>
> There will be no vcpus available anymore at this time, whence
> none will be kicked by the timer function. Thus canceling the
> timer will be sufficient after the loop.
>

Hm I'm not 100% convinced this is race free. I guess, I simply
don't understand enough of the tear-down. I don't want to delay
the series because of this. If the last interrupt arrived kind of
long ago we should be fine -- probably. Keep my ack ;)

> I have addressed the message as well, but will write it into
> the KVM trace.
>
> void kvm_s390_gisa_destroy(struct kvm *kvm)
> {
> - kvm->arch.gisa_int.origin = NULL;
> + struct kvm_s390_gisa_interrupt *gi = &kvm->arch.gisa_int;
> +
> + 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();
> + hrtimer_cancel(&gi->timer);
> + gi->origin = NULL;
> }
>
> >
> >> + gi->origin = NULL;
> >> }
> >>
> >> /**
> >> @@ -3037,11 +3187,23 @@ int kvm_s390_gisc_unregister(struct kvm *kvm, u32 gisc)
> >> }
> >> EXPORT_SYMBOL_GPL(kvm_s390_gisc_unregister);
> >>
> >
> >
> > Overall, there are couple of things I would prefer done differently,
> > but better something working today that something prefect in 6 months.
> > In that sense, provided my comment regarding destroy is addressed:
> >
> > Acked-by: Halil Pasic <pasic@linux.ibm.com>
> >
>
> Michael