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=-5.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,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 A3BE9C433E6 for ; Tue, 19 Jan 2021 18:59:59 +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 1298B20776 for ; Tue, 19 Jan 2021 18:59:59 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1298B20776 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]:57378 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1l1tuy-0003Qc-Ka for qemu-devel@archiver.kernel.org; Tue, 19 Jan 2021 11:31:44 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:45346) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l1tsA-0001qS-Mn for qemu-devel@nongnu.org; Tue, 19 Jan 2021 11:28:50 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:27459) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1l1ts6-00066r-SR for qemu-devel@nongnu.org; Tue, 19 Jan 2021 11:28:49 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1611073725; 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=RkHhbZ7jW+orGH7b6AsSi4at9n5Vaf/mFPDrTzdAeL4=; b=Q8EF7Fb4c3b79BPa2bpHrvo8zKc2gVq7fABjm5y3UjrphI/EupNah5s5GZJTRlvM77rfdp C7jF7UuhzdFLLlzuTeQfg5xOQwKtpoYnq9f6m3nKUMmZur81nAcGEJRyqHjet6j5yVIl0K Pxkuhjrit+aaKoZkKGzKe5JJdPZ2nxc= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-447-P1hRYP-YMUSKeToQ8L22vw-1; Tue, 19 Jan 2021 11:28:40 -0500 X-MC-Unique: P1hRYP-YMUSKeToQ8L22vw-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7EB6B8066E6; Tue, 19 Jan 2021 16:28:39 +0000 (UTC) Received: from redhat.com (ovpn-112-84.ams2.redhat.com [10.36.112.84]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E1B1160C0F; Tue, 19 Jan 2021 16:28:33 +0000 (UTC) Date: Tue, 19 Jan 2021 16:28:26 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: David Edmondson Subject: Re: [RFC PATCH 0/2] x86 CPU feature +/- fiddling and +kvm-no-defaults Message-ID: <20210119162826.GL2335568@redhat.com> References: <20210119142207.3443123-1-david.edmondson@oracle.com> MIME-Version: 1.0 In-Reply-To: <20210119142207.3443123-1-david.edmondson@oracle.com> User-Agent: Mutt/1.14.6 (2020-07-11) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.195, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, 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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Laurent Vivier , Thomas Huth , Eduardo Habkost , "Michael S. Tsirkin" , Richard Henderson , qemu-devel@nongnu.org, Markus Armbruster , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, Jan 19, 2021 at 02:22:05PM +0000, David Edmondson wrote: > Currently "-cpu -feature,+feature" will disable -feature, which seems > contrary to the intention of the user. Fix this such that the later > flag wins. There are no changes to the interaction of +/- and =on/=off. The -feature/+feature syntax is the legacy way of configuring features, with feature=on|off being the preferred, since that matches the general QEMU standard for boolean properties. Your proposed change in ordering of + vs - makes conceptual sense, but it is none the less a semantic change in behaviour that may well cause breakage for existing deployed VMs. This impacts guest ABI so could particularly cause live migration problems. IOW, we should have implemented it the way you propose in the first place, but I don't think it is safe to change it now, unless you can tie that new semantic to a machine type version. Before we consider that though, Paolo has just deprecated many of the legacy approaches for boolean properties in this: https://lists.nongnu.org/archive/html/qemu-devel/2021-01/msg04341.html I'm inclined to say that we just follow on from that and finally deprecate the +feature/-feature CPU syntax which we're already considering legacy. This would remove the need to care about changing its behaviour 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 :|