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Tue, 28 Jan 2020 14:38:36 +0000 (GMT) Received: from b03ledav006.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 284D2C6062; Tue, 28 Jan 2020 14:38:35 +0000 (GMT) Received: from [9.56.58.87] (unknown [9.56.58.87]) by b03ledav006.gho.boulder.ibm.com (Postfix) with ESMTP; Tue, 28 Jan 2020 14:38:35 +0000 (GMT) Subject: Re: [PATCH v6 2/2] s390: diagnose 318 info reset and migration support To: Cornelia Huck References: <1579904044-20790-1-git-send-email-walling@linux.ibm.com> <1579904044-20790-3-git-send-email-walling@linux.ibm.com> <20200127124756.3627f754.cohuck@redhat.com> <1cbd5354-d9ca-a10e-0053-a61a00223f53@linux.ibm.com> <20200127183504.2de2654f.cohuck@redhat.com> <20200128122418.7533f4bb.cohuck@redhat.com> From: Collin Walling Organization: IBM Message-ID: Date: Tue, 28 Jan 2020 09:38:34 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20200128122418.7533f4bb.cohuck@redhat.com> 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=2.50.10434:6.0.138, 18.0.572 definitions=2020-01-28_03:2020-01-28, 2020-01-28 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 phishscore=0 mlxlogscore=999 spamscore=0 clxscore=1015 malwarescore=0 priorityscore=1501 mlxscore=0 adultscore=0 bulkscore=0 suspectscore=0 impostorscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-1911200001 definitions=main-2001280117 X-detected-operating-system: by eggs.gnu.org: GNU/Linux 3.x [generic] X-Received-From: 148.163.156.1 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: borntraeger@de.ibm.com, qemu-s390x@nongnu.org, rth@twiddle.net, qemu-devel@nongnu.org, david@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 1/28/20 6:24 AM, Cornelia Huck wrote: > On Mon, 27 Jan 2020 18:05:36 -0500 > Collin Walling wrote: > >> On 1/27/20 12:35 PM, Cornelia Huck wrote: >>> On Mon, 27 Jan 2020 11:39:02 -0500 >>> Collin Walling wrote: >>> >>>> On 1/27/20 6:47 AM, Cornelia Huck wrote: >>>>> On Fri, 24 Jan 2020 17:14:04 -0500 >>>>> Collin Walling wrote: >>>>> >> >> [...] >> >>>>>> >>>>>> The availability of this instruction is determined by byte 134, bit 0 >>>>>> of the Read Info block. This coincidentally expands into the space used >>>>> >>>>> "SCLP Read Info" >>>>> >>>>>> for CPU entries by taking away one byte, which means VMs running with >>>>>> the diag318 capability will not be able to retrieve information regarding >>>>>> the 248th CPU. This will not effect performance, and VMs can still be >>>>>> ran with 248 CPUs. >>>>> >>>>> Are there other ways in which that might affect guests? I assume Linux >>>>> can deal with it? Is it ok architecture-wise? >>>>> >>>>> In any case, should go into the patch description :) >>>>> >>>> >>>> Same as above. I'll try to provide more information regarding what happens >>>> here in my next reply. >>> >>> I think you can lift some stuff from the cover letter. >>> >> >> Here's what I found out: >> >> Each CPU entry holds info regarding the CPU's address / ID as well as an >> indication of the availability of certain CPU features. With these patches, >> we lose a CPU entry for one CPU (essentially what would be the CPU at the >> tail-end of the list). This CPU exists, but is essentially in limbo... the >> machine cannot access any information regarding it. > > s/machine/guest/ ? > Correct. >> >> So, a VM can run with the original N max CPUs, but in reality we can only >> utilize n-1. > > s/we/the guest/ ? > Correct again. > With those changes, it makes sense to put your explanations into the > patch description (for later reference). > >> >>>> >>>>>> >> >> [...] >> >> > > -- Respectfully, - Collin Walling