From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [PATCH v3 1/2] s390/setup: diag318: remove bit check and refactor struct References: <20190402174636.15175-1-walling@linux.ibm.com> <20190402174636.15175-2-walling@linux.ibm.com> <92d69cf5-0510-cf22-9f72-ab3efe4ede9b@de.ibm.com> <13072d30-0ecb-e655-8f34-71c640a519df@redhat.com> From: Christian Borntraeger Date: Wed, 3 Apr 2019 14:10:38 +0200 MIME-Version: 1.0 In-Reply-To: <13072d30-0ecb-e655-8f34-71c640a519df@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Message-Id: Sender: linux-doc-owner@vger.kernel.org List-Archive: List-Post: To: David Hildenbrand , Collin Walling , pbonzini@redhat.com, cohuck@redhat.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org Cc: schwidefsky@de.ibm.com, heiko.carstens@de.ibm.com, frankja@linux.ibm.com List-ID: On 03.04.19 14:09, David Hildenbrand wrote: > On 03.04.19 13:28, Christian Borntraeger wrote: >> >> >> On 02.04.19 19:46, Collin Walling wrote: >>> Execution of DIAGNOSE 0x318 is fenced by checking an SCLP bit >>> for the availability of hardware support for the instruction. >>> >>> In order to support this instruction for a KVM/QEMU guest, we >>> would need to provide modifications to the SCLP Read SCP Info >>> data, which will in turn reduce the maximum number of CPUs that >>> may be provided to the guest. This issue introduces compatability >>> and legacy concerns. >>> >>> Let's circumvent this issue by removing the bit check and blindly >>> executing the instruction. An exception table rule is in place to >>> catch the case where hardware does not support this instruction. >> >> >> No, please keep the check. We have to extend the read scp field anyway >> for future extensions. > > Wasn't there already an SCLP-way of telling the guest that the read-scp > info response is bigger than 4k? Somehow rings a bell ... Yes, that would be a future extension (the Linux guest does not support this). Until then we probably have to go back to a smaller cpu number (e.g. 240 for machine type 4.1 and newer).