From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Borntraeger Subject: Re: [PATCH 04/12] KVM: s390: implement GISA IPM related primitives Date: Fri, 19 Jan 2018 11:17:41 +0100 Message-ID: <3b8b07ac-e809-b0c3-0afc-a07ba6b1b17e@de.ibm.com> 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> <20180119101132.GA4519@osiris> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Content-Language: en-US Sender: kvm-owner@vger.kernel.org List-Archive: List-Post: To: David Hildenbrand , Heiko Carstens Cc: mimu@linux.vnet.ibm.com, Cornelia Huck , KVM , linux-s390 , Janosch Frank List-ID: On 01/19/2018 11:16 AM, David Hildenbrand wrote: > On 19.01.2018 11:11, Heiko Carstens wrote: >> 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. >> > > That's unfortunate... and still looks hacky to me :) But if it works ... I had looked at the same place, but the alignment thing makes this really hard and I did not find a better solution.