From mboxrd@z Thu Jan 1 00:00:00 1970 From: Heiko Carstens Subject: Re: [PATCH 04/12] KVM: s390: implement GISA IPM related primitives Date: Fri, 19 Jan 2018 11:11:32 +0100 Message-ID: <20180119101132.GA4519@osiris> References: <20180116200217.211897-1-borntraeger@de.ibm.com> <20180116200217.211897-5-borntraeger@de.ibm.com> <249b3566-b8f3-5825-1f2b-6e0662874ca0@redhat.com> <28160f0a-4056-626e-2b83-0e6f039f27fc@redhat.com> <2e96491f-c73e-2782-17b0-5d9764ecf5d7@linux.vnet.ibm.com> <46713077-a46c-c3e3-45d7-ad4dc4cbce2e@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <46713077-a46c-c3e3-45d7-ad4dc4cbce2e@redhat.com> Sender: kvm-owner@vger.kernel.org List-Archive: List-Post: To: David Hildenbrand Cc: mimu@linux.vnet.ibm.com, Christian Borntraeger , Cornelia Huck , KVM , linux-s390 , Janosch Frank List-ID: On Thu, Jan 18, 2018 at 09:45:03PM +0100, David Hildenbrand wrote: > Actually, the problem is there are no atomic byte-based operations on > s390x without Interlocked-Access-Facility 2. > > Even __sync_fetch_and_or(&gisa->ipm, value) falls back to a > Compare-And-Swap loop. And Compare-And-Swap also operates at least on 32bit. > > So I assume there isn't too much we can do about it. As storage > locations following the u8 are also written - but in an atomic matter, > it should in general not matter. > > But can we avoid starting the bitmap at the beginning of the gisa? > > What about something like this: > > +void kvm_s390_gisa_set_ipm_gisc(struct kvm_s390_gisa *gisa, u8 gisc) > +{ > + set_bit_inv(gisc, (unsigned long *) &gisa->ipm); > +} set_bit_inv() may use a csg instruction which requires an 8 byte alignment of the operand. What you propose would crash immediately. The code written by Michael is fine as-is.