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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id BE15AD0C61D for ; Fri, 25 Oct 2024 14:53:33 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1t4Lgd-0002xZ-EL; Fri, 25 Oct 2024 10:53:11 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1t4LgZ-0002x8-8s for qemu-devel@nongnu.org; Fri, 25 Oct 2024 10:53:07 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1t4LgU-00054T-Aj for qemu-devel@nongnu.org; Fri, 25 Oct 2024 10:53:03 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1729867981; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=oCATtNYUN/GH6SdVzjWLD+NRphY1brBLxdU6nbHifzE=; b=MI/UxZZ/yB+uCu6/Ya6CrDcdl6FjZaxDAnN3/GDmw2REIixje8YwNtZd1tC+jMRA8ZYnTc 5NANnouD5vgejOB/LkYen8G5BJ/PWXReQP0gJDJPj8WMUXFoI9TxpXa8Xe9okHkcYxpdQZ F1sW4L4BClRy9eztD4Mbqt6PTqK/zTU= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-149-vvtCLRQwPH6-Gn7xQefQPA-1; Fri, 25 Oct 2024 10:51:32 -0400 X-MC-Unique: vvtCLRQwPH6-Gn7xQefQPA-1 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id BA9EC19560A6; Fri, 25 Oct 2024 14:51:30 +0000 (UTC) Received: from pinwheel (unknown [10.39.208.23]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 09383300018D; Fri, 25 Oct 2024 14:51:22 +0000 (UTC) Date: Fri, 25 Oct 2024 16:51:19 +0200 From: Kashyap Chamarthy To: Eric Auger Cc: eric.auger.pro@gmail.com, cohuck@redhat.com, qemu-devel@nongnu.org, qemu-arm@nongnu.org, kvmarm@lists.linux.dev, peter.maydell@linaro.org, richard.henderson@linaro.org, alex.bennee@linaro.org, maz@kernel.org, oliver.upton@linux.dev, sebott@redhat.com, shameerali.kolothum.thodi@huawei.com, armbru@redhat.com, berrange@redhat.com, abologna@redhat.com, jdenemar@redhat.com, shahuang@redhat.com, mark.rutland@arm.com, philmd@linaro.org, pbonzini@redhat.com Subject: Re: [RFC 00/21] kvm/arm: Introduce a customizable aarch64 KVM host model Message-ID: References: <20241025101959.601048-1-eric.auger@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20241025101959.601048-1-eric.auger@redhat.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kchamart@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.454, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=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: 2.1.29 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-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Fri, Oct 25, 2024 at 12:17:19PM +0200, Eric Auger wrote: Hi Eric, I'm new to Arm, so please bear with my questions :) > This RFC series introduces a KVM host "custom" model. (a) On terminology: as we know, in the x86 world, QEMU uses these terms[1]: - Host passthrough - Named CPU models - Then there's the libvirt abstraction, "host-model", that aims to provide the best of 'host-passthrough' + named CPU models. Now I see the term "host 'custom' model" here. Most management-layer tools and libvirt users are familiar with the classic terms "host-model" or "custom". If we now say "host 'custom' model", it can create confusion. I hope we can settle on one of the existing terms, or create a new term if need be. (I'll share one more thought on how layers above libvirt tend to use the term "custom", as a reply to patch 21/21, "arm/cpu-features: Document custom vcpu model".) (b) The current CPU features doc[2] for Arm doesn't mention "host passthrough" at all. It is only implied by the last part of this paragraph, from the section titled "A note about CPU models and KVM"[3]: "Named CPU models generally do not work with KVM. There are a few cases that do work [...] but mostly if KVM is enabled the 'host' CPU type must be used." Related: in your reply[4] to Dan in this series, you write: "Having named models is the next thing". So named CPU models will be a thing in Arm, too? Then the above statement in the Arm 'cpu-features' will need updating :-) [...] > - the QEMU layer does not take care of IDREG field value consistency. > The kernel neither. I imagine this could be the role of the upper > layer to implement a vcpu profile that makes sure settings are > consistent. Here we come to "named" models. What should they look > like on ARM? Are there reasons why they can't be similar to how x86 reports in `qemu-system-x86 -cpu help`? E.g. If it's an NVIDIA "Grace A02" (Neoverse-V2) host, it can report: [gracehopper] $> qemu-kvm -cpu help Available CPUs: gracehopper-neoverse-v2 cortex-a57 (deprecated) host max Or whatever is the preferred nomenclature for ARM. It also gives users of both x86 and ARM deployments a consistent expectation. Currently on a "Grace A02" ("Neoverse-V2") machine, it reports: [gracehopper] $> qemu-kvm -cpu help Available CPUs: cortex-a57 (deprecated) host max I see it's because there are no named models yet on ARM :-) [...] [1] https://www.qemu.org/docs/master/system/i386/cpu.html [2] https://www.qemu.org/docs/master/system/arm/cpu-features.html [3] https://www.qemu.org/docs/master/system/arm/cpu-features.html#a-note-about-cpu-models-and-kvm [4] https://lists.nongnu.org/archive/html/qemu-arm/2024-10/msg00891.html -- /kashyap