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Fri, 11 Sep 2020 13:41:35 +0000 (UTC) Subject: Re: [PATCH v5 6/8] s390/sclp: add extended-length sccb support for kvm guest To: Collin Walling , qemu-devel@nongnu.org, qemu-s390x@nongnu.org References: <20200910093655.255774-1-walling@linux.ibm.com> <20200910093655.255774-7-walling@linux.ibm.com> From: Thomas Huth Message-ID: <43af1e04-b9c9-2704-9cef-735a9b283be7@redhat.com> Date: Fri, 11 Sep 2020 15:41:34 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200910093655.255774-7-walling@linux.ibm.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=thuth@redhat.com X-Mimecast-Spam-Score: 0.003 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US Received-SPF: pass client-ip=207.211.31.81; envelope-from=thuth@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/11 00:33:53 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -45 X-Spam_score: -4.6 X-Spam_bar: ---- X-Spam_report: (-4.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-2.469, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: frankja@linux.ibm.com, mst@redhat.com, cohuck@redhat.com, david@redhat.com, pasic@linux.ibm.com, borntraeger@de.ibm.com, pbonzini@redhat.com, sumanthk@linux.ibm.com, mihajlov@linux.ibm.com, rth@twiddle.net Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 10/09/2020 11.36, Collin Walling wrote: > As more features and facilities are added to the Read SCP Info (RSCPI) > response, more space is required to store them. The space used to store > these new features intrudes on the space originally used to store CPU > entries. This means as more features and facilities are added to the > RSCPI response, less space can be used to store CPU entries. > > With the Extended-Length SCCB (ELS) facility, a KVM guest can execute > the RSCPI command and determine if the SCCB is large enough to store a > complete reponse. If it is not large enough, then the required length > will be set in the SCCB header. > > The caller of the SCLP command is responsible for creating a > large-enough SCCB to store a complete response. Proper checking should > be in place, and the caller should execute the command once-more with > the large-enough SCCB. > > This facility also enables an extended SCCB for the Read CPU Info > (RCPUI) command. > > When this facility is enabled, the boundary violation response cannot > be a result from the RSCPI, RSCPI Forced, or RCPUI commands. > > In order to tolerate kernels that do not yet have full support for this > feature, a "fixed" offset to the start of the CPU Entries within the > Read SCP Info struct is set to allow for the original 248 max entries > when this feature is disabled. > > Additionally, this is introduced as a CPU feature to protect the guest > from migrating to a machine that does not support storing an extended > SCCB. This could otherwise hinder the VM from being able to read all > available CPU entries after migration (such as during re-ipl). > > Signed-off-by: Collin Walling > --- [...] > /* Provide information about the configuration, CPUs and storage */ > static void read_SCP_info(SCLPDevice *sclp, SCCB *sccb) > { > @@ -89,10 +112,15 @@ static void read_SCP_info(SCLPDevice *sclp, SCCB *sccb) > int rnsize, rnmax; > IplParameterBlock *ipib = s390_ipl_get_iplb(); > int required_len = SCCB_REQ_LEN(ReadInfo, machine->possible_cpus->len); > - int offset_cpu = offsetof(ReadInfo, entries); > + int offset_cpu = s390_has_feat(S390_FEAT_EXTENDED_LENGTH_SCCB) ? > + offsetof(ReadInfo, entries) : > + SCLP_READ_SCP_INFO_FIXED_CPU_OFFSET; Sorry, but I'm having somewhat trouble to understand this... What's the difference between offsetof(ReadInfo, entries) and SCLP_READ_SCP_INFO_FIXED_CPU_OFFSET ? Aren't both terms resulting in the value 128 ? Thomas