linux-kernel - Re: [PATCH v5 10/15] KVM: s390: add functions to (un)register GISC with GISA

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Date:   Tue, 8 Jan 2019 14:41:35 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     Michael Mueller <mimu@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>,
        KVM Mailing List <kvm@...r.kernel.org>,
        Linux-S390 Mailing List <linux-s390@...r.kernel.org>,
        linux-kernel@...r.kernel.org,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v5 10/15] KVM: s390: add functions to (un)register GISC
 with GISA

On Tue, 8 Jan 2019 14:36:13 +0100
Halil Pasic <pasic@...ux.ibm.com> wrote:

> On Tue, 8 Jan 2019 11:34:44 +0100
> Cornelia Huck <cohuck@...hat.com> wrote:
> 
> > > >>      
> > > >>> +	spin_unlock(&kvm->arch.iam_ref_lock);
> > > >>> +
> > > >>> +	return gib->nisc;
> > > >>> +}
> > > >>> +EXPORT_SYMBOL_GPL(kvm_s390_gisc_register);
> > > >>> +
> > > >>> +int kvm_s390_gisc_unregister(struct kvm *kvm, u32 gisc)
> > > >>> +{
> > > >>> +	int rc = 0;
> > > >>> +
> > > >>> +	if (!kvm->arch.gib_in_use)
> > > >>> +		return -ENODEV;
> > > >>> +	if (gisc > MAX_ISC)
> > > >>> +		return -ERANGE;
> > > >>> +
> > > >>> +	spin_lock(&kvm->arch.iam_ref_lock);
> > > >>> +	if (kvm->arch.iam_ref_count[gisc] == 0) {
> > > >>> +		rc = -EINVAL;
> > > >>> +		goto out;
> > > >>> +	}
> > > >>> +	kvm->arch.iam_ref_count[gisc]--;
> > > >>> +	if (kvm->arch.iam_ref_count[gisc] == 0) {
> > > >>> +		kvm->arch.iam &= ~(0x80 >> gisc);
> > > >>> +		set_iam(kvm->arch.gisa, kvm->arch.iam);      
> > > > 
> > > > Any chance of this function failing here? If yes, would there be any
> > > > implications?      
> > > 
> > > It is the same here.    
> > 
> > I'm not sure that I follow: This is the reverse operation
> > (unregistering the gisc). Can we rely on get_ipm() to do any fixup
> > later? Is that a problem for the caller?  
> 
> IMHO gib alerts are all about not loosing initiative. I.e. avoiding
> substantially delayed delivery of interrupts due to vCPUs in wait
> state.
> 
> Thus we must avoid letting go before setting up stuff (gisa.iam under
> consideration of gisa ipm, gisa.next_alert, and gib.alert_list_origin)
> so that we can react on the next interrupt (if necessary).
> 
> On the other hand, getting more gisa alerts than necessary is not
> fatal -- better avoided if not too much hassle of course.

Yes, unless the caller does not expect any more alerts after
unregistering (I guess that's what you're saying?)

> 
> Bottom line is, while it may be critical that the IAM changes implied
> by register take place immediately, unregister is a different story.

It does feel a bit weird, though. But maybe I just have trouble
grasping the design :/