From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 11F93FA372A for ; Thu, 17 Oct 2019 16:20:02 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id D3AEB2089C for ; Thu, 17 Oct 2019 16:20:01 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D3AEB2089C Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:52308 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iL8VM-0000Qu-H1 for qemu-devel@archiver.kernel.org; Thu, 17 Oct 2019 12:20:00 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:57927) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iL7k8-0006KI-QH for qemu-devel@nongnu.org; Thu, 17 Oct 2019 11:31:14 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iL7k4-0002zo-UK for qemu-devel@nongnu.org; Thu, 17 Oct 2019 11:31:12 -0400 Received: from mx1.redhat.com ([209.132.183.28]:51524) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iL7k4-0002zR-Ft for qemu-devel@nongnu.org; Thu, 17 Oct 2019 11:31:08 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id EDBF136961 for ; Thu, 17 Oct 2019 15:31:06 +0000 (UTC) Received: from [10.36.117.42] (ovpn-117-42.ams2.redhat.com [10.36.117.42]) by smtp.corp.redhat.com (Postfix) with ESMTP id E784A600C4; Thu, 17 Oct 2019 15:31:05 +0000 (UTC) Subject: Re: Default CPU models on s390x and ppc64 From: David Hildenbrand To: Jiri Denemark , qemu-devel@nongnu.org, David Hildenbrand , David Gibson References: <20191017151606.GA1880840@orkuz.int.mamuti.net> <82ea23ea-be23-c374-3f10-65d8f6e79432@redhat.com> Organization: Red Hat GmbH Message-ID: <3c2d2d20-682e-ab4f-ced4-c7f48473dd24@redhat.com> Date: Thu, 17 Oct 2019 17:31:04 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: <82ea23ea-be23-c374-3f10-65d8f6e79432@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Thu, 17 Oct 2019 15:31:06 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 17.10.19 17:18, David Hildenbrand wrote: > On 17.10.19 17:16, Jiri Denemark wrote: >> Hi David and David, >> >> I'm working on libvirt's support [1] for query-machines' >> default-cpu-type, which is supposed to return the type of the default >> CPU model that QEMU uses for each machine type. Rather than hard coding >> the default in libvirt (which we currently do on x86), we ask QEMU for >> the default CPU model and use it unless a user asks for a specific CPU >> model explicitly. >> >> We use query-cpu-definitions for translating the default CPU type to the >> actual CPU model we need to pass to -cpu by looking up the CPU model >> with matching typename. >> >> However, all this seems to work only with TCG on both s390x and ppc64. >> The issues I met with KVM on each architecture are described below. >> >> On ppc64 the default CPU type reported by query-machines is >> power*-powerpc64-cpu, while IIUC QEMU would effectively use -cpu host by >> default. In fact -cpu power8 is mostly just a fancy alias to -cpu host >> on a Power8 machine. But QEMU even rewrites typename of the current host >> CPU model to host-powerpc64-cpu. Which means on a Power8 host the power8 >> CPU model would have typename=host-powerpc64-cpu while the default CPU >> type would still use power8*-powerpc64-cpu. Thus we may just fail to >> find any CPU model corresponding to the default CPU model. >> >> And to make it even worse, the default CPU model changes with machine >> type. E.g., pseries-3.1 uses power8_v2.0-powerpc64-cpu while pseries-4.2 >> uses power9_v2.0-powerpc64-cpu. However, starting QEMU with pseries-4.2 >> machine type and the reported default -cpu power9 fails on any >> non-Power9 host. That said QEMU just lies about the default CPU model. >> >> So for all combinations of (pseries-3.1, pseries-4.2) machine types and >> (Power8, Power9) hosts, one will get the following mixed results: >> >> - pseries-3.1 on Power8: no default CPU model will be detected, QEMU >> starts fine with it's own default >> - pseries-4.2 on Power9: same as above >> - pseries-3.1 on Power9: -cpu power8 (not sure if this works, though) >> - pseries-4.2 on Power8: -cpu power9, QEMU doesn't start >> >> >> This situation on s390x is not so complicated, but not really better. >> The default CPU is said to be "qemu" for all machine types, which works >> fine for TCG domains, but it doesn't work on KVM because QEMU complains >> that some features requested in the CPU model are not available. In >> other words the "qemu" CPU model is not runnable on KVM. This a bit >> similar to what happens on x86_64, but QEMU just ignores missing >> features and starts happily there. > > The default model under KVM is "host", under TCG it's "qemu". We should > not use "qemu" under KVM, although it might work on some setups ... > > Where/how is this default model detected? > ... target/s390x/kvm.c:kvm_arch_init() MachineClass *mc = MACHINE_GET_CLASS(ms); mc->default_cpu_type = S390_CPU_TYPE_NAME("host"); I think the call order should make sure that "host" is set after "qemu" is set. I'll go ahead and verify that. -- Thanks, David / dhildenb