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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham 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 3A580C433DB for ; Thu, 18 Mar 2021 11:35:03 +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 8705C64E99 for ; Thu, 18 Mar 2021 11:35:02 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8705C64E99 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.de Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:51588 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lMqvd-0004RZ-Hf for qemu-devel@archiver.kernel.org; Thu, 18 Mar 2021 07:35:01 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:44958) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lMqtL-0003CR-AV; Thu, 18 Mar 2021 07:32:39 -0400 Received: from mx2.suse.de ([195.135.220.15]:38016) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lMqtG-0003JG-Ie; Thu, 18 Mar 2021 07:32:39 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id BB65AAC1E; Thu, 18 Mar 2021 11:32:31 +0000 (UTC) Subject: Re: arm_cpu_post_init (Was: Re: arm: "max" CPU class hierarchy changes possible?) From: Claudio Fontana To: Andrew Jones , Peter Maydell References: <11e9d3bb-c94c-4ad7-35b0-b698376c5e00@suse.de> <2e6a5d98-e022-0b39-5f30-92eb74491d3b@redhat.com> <2277fdf5-ec92-476a-8fe5-0d4eee23dfef@suse.de> <20210311191046.ykcelkwq7orajyu7@kamzik.brq.redhat.com> <5467e45c-cc8e-6422-0c56-398405a7c331@suse.de> Message-ID: Date: Thu, 18 Mar 2021 12:32:30 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: <5467e45c-cc8e-6422-0c56-398405a7c331@suse.de> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=195.135.220.15; envelope-from=cfontana@suse.de; helo=mx2.suse.de X-Spam_score_int: -41 X-Spam_score: -4.2 X-Spam_bar: ---- X-Spam_report: (-4.2 / 5.0 requ) BAYES_00=-1.9, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_MED=-2.3, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=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: Eduardo Habkost , Richard Henderson , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , qemu-devel , qemu-arm , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 3/18/21 12:06 PM, Claudio Fontana wrote: > On 3/11/21 8:10 PM, Andrew Jones wrote: >> On Thu, Mar 11, 2021 at 06:33:15PM +0000, Peter Maydell wrote: >>> On Thu, 11 Mar 2021 at 17:16, Claudio Fontana wrote: >>>> Maybe Peter you could clarify similarly what the intended meaning of "max" is on ARM? >>> >>> "max" is "best we can do, whatever that is". (On KVM this is "same as >>> the host".) >>> "host" is "whatever the host is (KVM only)". >>> >>>> KVM: (aarch64-only): aarch64_max_initfn(): >>>> >>>> The following comment in the code seems wrong to me: >>>> >>>> /* -cpu max: if KVM is enabled, like -cpu host (best possible with this host); */ >>>> >>>> This is not exactly true: >>>> >>>> "-cpu max" calls kvm_arm_set_cpu_features_from_host(), (which checks "dtb_compatible", and if not set gets the features from the host, if set ...?) >>>> After that, calls aarch64_add_sve_properties() and then adds also "svw-max-vq". This code is common with TCG. > > > As part of this research I noticed that arm_cpu_post_init() is quite confusing, seems really inconsistent to me. > > Apparently the intention was to call it from the leaf classes: > > commit 51e5ef459eca045d7e8afe880ee60190f0b75b26 > Author: Marc-André Lureau > Date: Tue Nov 27 12:55:59 2018 +0400 > > arm: replace instance_post_init() > > Replace arm_cpu_post_init() instance callback by calling it from leaf > classes, to avoid potential ordering issue with other post_init callbacks. > > Signed-off-by: Marc-André Lureau > Suggested-by: Igor Mammedov > Reviewed-by: Igor Mammedov > Acked-by: Eduardo Habkost > > > but then we end up calling it multiple times in the class hierarch, which is a recipe for bugs, and makes it difficult to understand what arm_cpu_post_init() > even means, what calling this function is supposed to do. > > For a "max" or "host" cpu on AArch64, this function is called: > > for the ARM CPU base class, TYPE_ARM_CPU, in > > cpu.c::arm_cpu_instance_init, > > then later again for the TYPE_AARCH64_CPU class, child of TYPE_ARM_CPU, in > > cpu64.c::aarch64_cpu_instance_init, > > then later again for the TYPE_ARM_HOST_CPU class, child of TYPE_AARCH64_CPU, in > > cpu.c::arm_host_initfn. > > Same for "max". > > When looking at 32bit CPUs instead, only the ARM CPU base class ends up calling arm_cpu_post_init. > "Leaf" classes do not do it (see cpu_tcg.c). > > What is then arm_cpu_post_init even supposed to mean? And why do we have a separate arm_cpu_finalize_features()? Nothing in the ARM cpu classes initializations ever seems to be "final" to me. > > Thanks, > > Claudio > > >>>> >>>> In the case of cpu host instead, >>>> >>>> "-cpu host" calls kvm_arm_set_cpu_features_from_host(), same as max, then calls aarch64_add_sve_properties() but does NOT add "svw-max-vq". >>>> >>>> Is this a bug? >> >> It was left out intentionally. More below. >> >>> >>> Maybe; that's a question for Richard or Drew... >>> >>>> Are "max" and "host" for KVM supposed to be the same like with x86? >> >> Yes, but my understanding of "max" == "host" for KVM is that that only >> applies to the perspective of the guest. What CPU and what CPU features >> the guest can see should be exactly the same with either "max" or "host", >> depending on the enabling/disabling of any optional CPU properties. >> >> The question here seems to be that, if one has a CPU property, does that >> imply the other should have the same? Which would effectively allow the >> two to be aliases (when KVM is enabled). I don't know, does x86 ensure >> 100% property compatibility? >> >> I opted not to support sve-max-vq for "host" because I consider it a >> legacy CPU property, one I didn't want to propagate. Indeed it may >> make more sense to depreciate sve-max-vq than to "fix" this issue >> by adding it to "host". Note, we can already create equivalent SVE >> CPUs. The following are the same from the perspective of the guest >> >> -accel kvm -cpu host,sve512=on >> -accel kvm -cpu max,sve512=on >> >> And, for TCG, these are the same from the perspective of the guest >> >> -accel tcg -cpu max,sve512=on >> -accel tcg -cpu max,sve-max-vq=4 >> >> So we already don't need sve-max-vq. >> >> Thanks, >> drew >> >