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=-5.4 required=3.0 tests=FROM_EXCESS_BASE64, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS, URIBL_BLOCKED,USER_AGENT_MUTT 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 E4F0DC04E53 for ; Wed, 15 May 2019 08:44:04 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id BC5D12089E for ; Wed, 15 May 2019 08:44:04 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BC5D12089E 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 ([127.0.0.1]:33491 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hQpW7-0000sg-Sf for qemu-devel@archiver.kernel.org; Wed, 15 May 2019 04:44:03 -0400 Received: from eggs.gnu.org ([209.51.188.92]:42259) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hQpVI-0000bb-Sn for qemu-devel@nongnu.org; Wed, 15 May 2019 04:43:18 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hQpVH-00047a-Ht for qemu-devel@nongnu.org; Wed, 15 May 2019 04:43:12 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54820) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hQpVH-000475-9l for qemu-devel@nongnu.org; Wed, 15 May 2019 04:43:11 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 85791307D977; Wed, 15 May 2019 08:43:10 +0000 (UTC) Received: from redhat.com (ovpn-112-73.ams2.redhat.com [10.36.112.73]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 72E0564425; Wed, 15 May 2019 08:42:45 +0000 (UTC) Date: Wed, 15 May 2019 09:42:43 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Message-ID: <20190515084243.GC10067@redhat.com> References: <155786484688.13873.6037015630912983760.stgit@gimli.home> <20190515061503.GF16681@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190515061503.GF16681@xz-x1> User-Agent: Mutt/1.11.4 (2019-03-13) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.48]); Wed, 15 May 2019 08:43:10 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [PATCH for-4.1 v2] q35: Revert to kernel irqchip X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: ehabkost@redhat.com, mst@redhat.com, Cornelia Huck , qemu-devel@nongnu.org, Alex Williamson , pbonzini@redhat.com, rth@twiddle.net Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, May 15, 2019 at 02:15:03PM +0800, Peter Xu wrote: > On Tue, May 14, 2019 at 02:14:41PM -0600, Alex Williamson wrote: > > Commit b2fc91db8447 ("q35: set split kernel irqchip as default") changed > > the default for the pc-q35-4.0 machine type to use split irqchip, which > > turned out to have disasterous effects on vfio-pci INTx support. KVM > > resampling irqfds are registered for handling these interrupts, but > > these are non-functional in split irqchip mode. We can't simply test > > for split irqchip in QEMU as userspace handling of this interrupt is a > > significant performance regression versus KVM handling (GeForce GPUs > > assigned to Windows VMs are non-functional without forcing MSI mode or > > re-enabling kernel irqchip). > > > > The resolution is to revert the change in default irqchip mode in the > > pc-q35-4.1 machine and create a pc-q35-4.0.1 machine for the 4.0-stable > > branch. The qemu-q35-4.0 machine type should not be used in vfio-pci > > configurations for devices requiring legacy INTx support without > > explicitly modifying the VM configuration to use kernel irqchip. > > > > Link: https://bugs.launchpad.net/qemu/+bug/1826422 > > Fixes: b2fc91db8447 ("q35: set split kernel irqchip as default") > > Signed-off-by: Alex Williamson > > Hi, Alex, > > I have two (probably naive) questions about the patch, possibly due to > lack of context of previous discussions so please let me know if > there's any upstream discussion that I can read. > > Firstly, could I ask why we need this 4.0.1 machine type specific for > fixing this problem? Asked because this seems to be the first time > QEMU introduces the X.Y.Z machine type in master. Could it be somehow > delayed to the release of QEMU 4.1? From the planning page I see that > it's releasing on Aug 06th/13th, a bit far away but not really that > much imho. I'm perfectly fine with this, but I just want to make sure > I have the correct understanding of the motivations. > The second question is about our previous decision to introduce QEMU > 4.1 machine type before it's released (which is not related to the > patch at all). Is it really correct to do so before releasing of 4.1? > So now even with a development QEMU 4.0 branch the user will be able > to create 4.1 machines using "-M pc-q35-4.1", then what if the user > migrated a real 4.1 machine (with the to-be-released QEMU 4.1 binary) > to some 4.1 machine that was run with such an old 4.0 QEMU binary? > The problem is we can add more compatible properties into > pc_q35_4_1_machine_options and future pc_compat_4_1 array before QEMU > 4.1 is finally released and then "-M pc-q35-4.1" will actually have > different combination of properties IMHO, which seems to break > compatibility. Am I wrong somewhere? You are correct - we can't release a pc-q35-4.1 machine type to stable because this machine type may change arbitrarily again before release. Alex's suggestion of a pc-q35-4.0.1 is best thought of as releasing a point in time snapshot of the pc-q35-4.1 machine type to stable. 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 :|