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 84336CEDD86 for ; Wed, 9 Oct 2024 11:35:58 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1syUyk-0003mi-W5; Wed, 09 Oct 2024 07:35:43 -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 1syUyi-0003hl-QG for qemu-devel@nongnu.org; Wed, 09 Oct 2024 07:35:41 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1syUyf-0005M2-Nk for qemu-devel@nongnu.org; Wed, 09 Oct 2024 07:35:40 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1728473735; h=from:from:reply-to: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=EZtQjOBbOrnR4gX4Fi3E7vZosSLGFmcS8MahuTPEqlU=; b=VSvzIgQ0x0itluOoVqBLuYnAxDtGOSrpdhvo6mJ7pWI2PzEMgIddnYa3LbYPjEWn0wjnbv 5Pf9ep/tXzEDafklzULsOmimwRbqu1d+Q4o42oQLorv6VAkur9IWdxDmYZ5oghhajIuGjp WTItfdJL9yYr22ujh1WaqsL83kCrEdI= Received: from mx-prod-mc-02.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-509-uasPVkghOCCVkOqzIQRrFw-1; Wed, 09 Oct 2024 07:35:34 -0400 X-MC-Unique: uasPVkghOCCVkOqzIQRrFw-1 Received: from mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.15]) (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-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 993061953951; Wed, 9 Oct 2024 11:35:32 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.112]) by mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 9D5B11956086; Wed, 9 Oct 2024 11:35:30 +0000 (UTC) Date: Wed, 9 Oct 2024 12:35:26 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Markus Armbruster Cc: qemu-devel@nongnu.org, Paolo Bonzini , Richard Henderson , Eduardo Habkost Subject: Re: x86 CPU properties "family", "model", "stepping", "tsc-freq" Message-ID: References: <87wmihr14b.fsf@pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <87wmihr14b.fsf@pond.sub.org> User-Agent: Mutt/2.2.12 (2023-09-09) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.15 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.151, 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_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Wed, Oct 09, 2024 at 01:01:40PM +0200, Markus Armbruster wrote: > QOM properties serve several purposes: initial configuration (external > interface), run time control and monitoring (external interface), and > internal purposes like versioning. Which purpose(s) a property serves > is often unclear. > > The x86 CPU properties "family", "model", and "stepping" are used > internally; see target/i386/cpu.c and hw/i386/pc.c. I figure changing > them at run time makes no sense. What about configuration? Can the > user set arbitrary CPU properties? If yes, are these properties meant > to be set by the user? The named CPU models have associated family/model/stepping (FMS), and the string "model-i"' info defined against them, that match the some arbitrary choice of silicon. QEMU doesn't support all possible values though. For example, we've seen real Skylake-Server CPUs with stepping in the range 0-4, but QEMU always reports 4 for stepping, so altering that is potentially interesting if wanting to check guest behaviour with a very specific FMS value. More generally QEMU doesn't support every possible CPU model that exists. You can invent entirely new CPUs by turning on/off individual CPUID feature flags, along with setting the FMS and the model-id string. $ ./make-tiny-image.py --run "cat /proc/cpuinfo" 6.9.9-200.fc40.x86_64 $ qemu-system-x86_64 -nodefaults -display none -serial stdio -accel kvm -m 1024 -kernel /lib/modules/6.10.8-200.fc40.x86_64/vmlinuz -initrd tiny-initrd.img -append 'console=ttyS0 quiet' -cpu Skylake-Server | grep -E '(model|family|stepping)' cpu family : 6 model : 85 model name : Intel Xeon Processor (Skylake) stepping : 4 Invent my own CPU... $ qemu-system-x86_64 -nodefaults -display none -serial stdio -accel kvm -m 1024 -kernel /lib/modules/6.10.8-200.fc40.x86_64/vmlinuz -initrd tiny-initrd.img -append 'console=ttyS0 quiet' -cpu Skylake-Server,family=13,model=42,model-id="Fish Food",stepping=3 | grep -E '(model|family|stepping)' cpu family : 13 model : 42 model name : Fish Food stepping : 1 Whether or not the requested FMS & model-id choices actually make conceptual sense is upto the user to figure out :-) Setting these values is certainly niche, but still valid IMHO. > Property "tsc-freq" seems not to be used internally. I figure changing > it at run time makes no sense. Looks like it is be meant to be set by > the user: x86_cpu_parse_featurestr() seems to parse it from the argument > of -cpu. Correct? Note that -cpu help doesn't mention it. "tsc-freq" is a back compat alias for 'tsc-frequency' AFAICT, and we need the latter if you want to guarantee fixed TSC freq across migration. eg commit 561dbb41b1d752098249128d8462aaadc56fd15d With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|