From mboxrd@z Thu Jan 1 00:00:00 1970 From: Halil Pasic Subject: Re: [PATCH v5 10/15] KVM: s390: add functions to (un)register GISC with GISA Date: Tue, 8 Jan 2019 15:23:57 +0100 Message-ID: <20190108152357.39fea477@oc2783563651> References: <20181219191756.57973-1-mimu@linux.ibm.com> <20181219191756.57973-11-mimu@linux.ibm.com> <20190104141836.0ca98a77.cohuck@redhat.com> <20190108113444.56e76f13.cohuck@redhat.com> <20190108143613.2ca1a9d3@oc2783563651> <20190108144135.3fe5c23b.cohuck@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20190108144135.3fe5c23b.cohuck@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Archive: List-Post: To: Cornelia Huck Cc: Michael Mueller , Pierre Morel , KVM Mailing List , Linux-S390 Mailing List , linux-kernel@vger.kernel.org, Martin Schwidefsky , Heiko Carstens , Christian Borntraeger , Janosch Frank , David Hildenbrand List-ID: On Tue, 8 Jan 2019 14:41:35 +0100 Cornelia Huck wrote: > On Tue, 8 Jan 2019 14:36:13 +0100 > Halil Pasic wrote: > > > On Tue, 8 Jan 2019 11:34:44 +0100 > > Cornelia Huck 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?) Just to make sure, the caller, which I guess would probably be some sort of a driver, does not process the alerts. It is kvm that does. But you are right, the properties of unregister need do be considered in the cleanup code (e.g. resets). Sure, we must avoid the process_gib_alert_list() (which is common, i.e. ain't tied to any VM) poking dead GISAs. Frankly, I've my hands full with the normal operation scenario, and I did not pay much attention to the cleanup yet. > > > > > 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 :/ > You are on the only one. I have extreme difficulties following some of the discussion here -- despite of the fact that I have quite some confidence in my understanding of the GISA/GIB architecture. Regards, Halil