From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:57034) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1e7NJS-0006kd-7P for qemu-devel@nongnu.org; Wed, 25 Oct 2017 11:09:47 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1e7NJM-0001ak-6c for qemu-devel@nongnu.org; Wed, 25 Oct 2017 11:09:46 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:59050 helo=mx0a-001b2d01.pphosted.com) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1e7NJL-0001WH-U0 for qemu-devel@nongnu.org; Wed, 25 Oct 2017 11:09:40 -0400 Received: from pps.filterd (m0098414.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id v9PF3Dfo144432 for ; Wed, 25 Oct 2017 11:09:35 -0400 Received: from e06smtp15.uk.ibm.com (e06smtp15.uk.ibm.com [195.75.94.111]) by mx0b-001b2d01.pphosted.com with ESMTP id 2dtu63pxnr-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Wed, 25 Oct 2017 11:09:34 -0400 Received: from localhost by e06smtp15.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 25 Oct 2017 16:09:32 +0100 References: <20171020145437.18549-1-borntraeger@de.ibm.com> <1ac27ce2-091d-e1b7-fc94-ebdf696a35e9@redhat.com> From: Boris Fiuczynski Date: Wed, 25 Oct 2017 17:09:28 +0200 MIME-Version: 1.0 In-Reply-To: <1ac27ce2-091d-e1b7-fc94-ebdf696a35e9@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Message-Id: <9f4037d8-93a3-be0f-8cb4-4424158e236b@linux.vnet.ibm.com> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH/QEMU] s390x/kvm: use cpu_model_available for guarded storage on compat machines List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: David Hildenbrand , Christian Borntraeger , Marc Hartmayer , Cornelia Huck Cc: libvir-list@redhat.com, qemu-devel@nongnu.org, Jiri Denemark , "Jason J . Herne" , Viktor Mihajlovski , Halil Pasic On 10/25/2017 12:23 PM, David Hildenbrand wrote: > On 25.10.2017 12:18, Christian Borntraeger wrote: >> Ping, I plan to submit belows patch for 2.11. We can then still look i= nto >> a libvirt<->qemu interface for limiting host-model depending on machin= e versions >> (or not). >=20 > I think this would be sufficient for now. >=20 > Having different host models, depending on the the machine type sounds > wrong. But maybe we'll need it in the future. >=20 David, I disagree if your proposal is to generally tolerate new cpu=20 features in old machine types. This *might* work for gs but how do you=20 guaranty that guests do not behave differently/wrong when suddenly new=20 cpu features are made available to guest when (re-)starting them. That is my feedback for the qemu side of this mater. Regarding the libvirt side of this: When looking at https://libvirt.org/formatdomain.html#elementsCPU I=20 found the following sentence: Since the CPU definition is copied just before starting a domain,=20 exactly the same XML can be used on different hosts while still=20 providing the best guest CPU each host supports. My interpretation of "the best guest CPU each host supports" is that=20 besides limiting factors like hardware, kernel and qemu capabilities the=20 requested machine type for the guest is a limiting factor as well. Nevertheless if my interpretation is found to be incorrect than we=20 should think about another new cpu mode that includes the machine type=20 into the "best guest CPU" detection. My assumption is that we must not require the users to know which cpu=20 model they manually need to define to match a specific machine type AND we want to guarantee that guests run without risking any side=20 effects by tolerating any additional cpu features. --=20 Mit freundlichen Gr=C3=BC=C3=9Fen/Kind regards Boris Fiuczynski IBM Deutschland Research & Development GmbH Vorsitzender des Aufsichtsrats: Martina K=C3=B6deritz Gesch=C3=A4ftsf=C3=BChrung: Dirk Wittkopp Sitz der Gesellschaft: B=C3=B6blingen Registergericht: Amtsgericht Stuttgart, HRB 243294