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Fri, 11 Sep 2020 14:52:25 +0000 (GMT) Subject: Re: [PATCH v5 6/8] s390/sclp: add extended-length sccb support for kvm guest To: Thomas Huth , qemu-devel@nongnu.org, qemu-s390x@nongnu.org References: <20200910093655.255774-1-walling@linux.ibm.com> <20200910093655.255774-7-walling@linux.ibm.com> <43af1e04-b9c9-2704-9cef-735a9b283be7@redhat.com> From: Collin Walling Message-ID: Date: Fri, 11 Sep 2020 10:52:24 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235, 18.0.687 definitions=2020-09-11_05:2020-09-10, 2020-09-11 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 bulkscore=0 malwarescore=0 priorityscore=1501 adultscore=0 phishscore=0 suspectscore=0 impostorscore=0 mlxlogscore=999 spamscore=0 mlxscore=0 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2009110114 Received-SPF: pass client-ip=148.163.158.5; envelope-from=walling@linux.ibm.com; helo=mx0a-001b2d01.pphosted.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/11 09:50:13 X-ACL-Warn: Detected OS = Linux 3.x [generic] X-Spam_score_int: -51 X-Spam_score: -5.2 X-Spam_bar: ----- X-Spam_report: (-5.2 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-2.469, RCVD_IN_DNSWL_LOW=-0.7, 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 9/11/20 9:54 AM, Thomas Huth wrote: > On 11/09/2020 15.41, Thomas Huth wrote: >> 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 ? > > Ah, well, the answer is clear after looking at patch 8/8 ... ReadInfo is > extended there, so offsetof(ReadInfo, entries) will result in a > different value. > Might have been better to move the above hunk into patch 8/8, but if you > want to keep it here, that's now ok for me, too. > > Reviewed-by: Thomas Huth > > I see your point. In retrospect, it might've been better to include it in patch 8/8 so it's more clear why these features are introduced within the same patch set. If there are any requests to change / fixup this patch in any other regard, then I'll consider moving the offset_cpu calculation to 8/8. Otherwise, I'll leave it here :) Thanks! -- Regards, Collin Stay safe and stay healthy