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 2F657C27C53 for ; Wed, 12 Jun 2024 11:39:17 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1sHMJB-0004Gi-3Y; Wed, 12 Jun 2024 07:38:29 -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 1sHMJ9-0004GY-I3 for qemu-devel@nongnu.org; Wed, 12 Jun 2024 07:38:27 -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 1sHMJ7-0001FR-PY for qemu-devel@nongnu.org; Wed, 12 Jun 2024 07:38:27 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1718192304; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=5fD2pYojMAcurxn3tkreeI+FXSJvVJd7z1KH+PQ8ek8=; b=YEO/FI7ryu691hr4QHLN8HJaRbN+FKu8vuhia6BSHDlqN/z0+c3Y6aYpmw3ZRB/vIMIqmt hEfSY2+FIfh0LBk/IGc8FG5ySzQxRoNd5YYundQSh0NJhddXf5chY+ZIvdWpGVpCArTm4b 94zLo4tNJg3cAGOwOv+CtsJfQKD2bxo= 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-589-tdsDymAeNCuRkt4T1SaJaA-1; Wed, 12 Jun 2024 07:38:21 -0400 X-MC-Unique: tdsDymAeNCuRkt4T1SaJaA-1 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (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 A241C195608B; Wed, 12 Jun 2024 11:38:19 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.115]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 9CC2419560B1; Wed, 12 Jun 2024 11:38:17 +0000 (UTC) Date: Wed, 12 Jun 2024 12:38:14 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Paolo Bonzini Cc: Alexander Monakov , qemu-devel@nongnu.org, Richard Henderson Subject: Re: [PATCH 0/5] Reinstate ability to use Qemu on pre-SSE4.1 x86 hosts Message-ID: References: <20240612105525.8795-1-amonakov@ispras.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.12 (2023-09-09) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 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: 11 X-Spam_score: 1.1 X-Spam_bar: + X-Spam_report: (1.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.143, 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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=no 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, Jun 12, 2024 at 01:12:43PM +0200, Paolo Bonzini wrote: > On Wed, Jun 12, 2024 at 1:04 PM Daniel P. Berrangé wrote: > > > > On Wed, Jun 12, 2024 at 01:55:20PM +0300, Alexander Monakov wrote: > > > Hello, > > > > > > I'm sending straightforward reverts to recent patches that bumped minimum > > > required x86 instruction set to SSE4.2. The older chips did not stop working, > > > and people still test and use new software on older hardware: > > > https://sourceware.org/bugzilla/show_bug.cgi?id=31867 > > > > > > Considering the very minor gains from the baseline raise, I'm honestly not > > > sure why it happened. It seems better to let distributions handle that. > > > > Indeed distros are opinionated about the x86_64 baseline they want > > to target. > > > > While RHEL-9 switched to a x86_64-v2 baseline, Fedora has repeatedly > > rejected the idea of moving to an x86_64-v2 baseline, wanting to retain > > full backwards compat. So this assumption in QEMU is preventing the > > distros from satisfying their chosen build target goals. > > I didn't do this because of RHEL9, I did it because it's silly that > QEMU cannot use POPCNT and has to waste 2% of the L1 d-cache to > compute the x86 parity flag (and POPCNT was introduced at the same > time as SSE4.2). > > Intel x86_64-v2 processors have been around for about 15 years, AMD > for a little less (2011). I'd rather hear from users about the > usecases for running QEMU on such old processors before reverting, as > this does not get in the way of booting/installing distros on old > machines. Unless QEMU is run from within the installation media, which > it isn't, requiring a particular processor family does not prevent > Fedora from being installable on pre-v2 processors. This isn't anything to do with the distro installer. The use case is that the distro wants all its software to be able to run on the x86_64 baseline it has chosen to build with. If we want to use POPCNT in the TCG code, can we not do a runtime check and selectively build pieces of code with __attribute__((target("popcnt"))), as we've done historically for the bufferiszero.c code, rather than changing the entire QEMU baseline ? 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 :|