From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from list by lists.gnu.org with archive (Exim 4.71) id 1fXqYN-0005j2-FR for mharc-qemu-trivial@gnu.org; Tue, 26 Jun 2018 12:10:51 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40483) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fXqYL-0005he-4c for qemu-trivial@nongnu.org; Tue, 26 Jun 2018 12:10:50 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fXqYK-0001EM-0y for qemu-trivial@nongnu.org; Tue, 26 Jun 2018 12:10:49 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:46360 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fXqYF-0001Bu-Qx; Tue, 26 Jun 2018 12:10:43 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 45960401EF16; Tue, 26 Jun 2018 16:10:42 +0000 (UTC) Received: from redhat.com (unknown [10.33.36.56]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5E6F0213ED6A; Tue, 26 Jun 2018 16:10:40 +0000 (UTC) Date: Tue, 26 Jun 2018 17:10:38 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Eduardo Habkost Cc: Paolo Bonzini , Thomas Huth , zhang.zhanghailiang@huawei.com, Ben Warren , qemu-trivial@nongnu.org, Cornelia Huck , Markus Armbruster , qemu-devel@nongnu.org, Stefan Hajnoczi Message-ID: <20180626161038.GF32319@redhat.com> Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= References: <743d495e-51cd-8c8e-293e-026b8dece74a@redhat.com> <87lgb3xsv8.fsf@dusky.pond.sub.org> <4292432f-5f26-6925-4c14-ad1370e36e5b@redhat.com> <20180625173056.GM7451@localhost.localdomain> <97504b42-9ebe-45c5-6ff3-b43102c8374a@redhat.com> <20180625195126.GN7451@localhost.localdomain> <20180626122908.GQ7451@localhost.localdomain> <577768c0-5205-2fb4-209f-845332fe158d@redhat.com> <20180626160623.GT7451@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20180626160623.GT7451@localhost.localdomain> User-Agent: Mutt/1.10.0 (2018-05-17) X-Scanned-By: MIMEDefang 2.78 on 10.11.54.6 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.5]); Tue, 26 Jun 2018 16:10:42 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.5]); Tue, 26 Jun 2018 16:10:42 +0000 (UTC) for IP:'10.11.54.6' DOMAIN:'int-mx06.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'berrange@redhat.com' RCPT:'' X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 66.187.233.73 Subject: Re: [Qemu-trivial] [Qemu-devel] [RFC PATCH 4/4] qemu-options: Do not show -enable-kvm and -enable-hax in the docs anymore X-BeenThere: qemu-trivial@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 26 Jun 2018 16:10:50 -0000 On Tue, Jun 26, 2018 at 01:06:23PM -0300, Eduardo Habkost wrote: > On Tue, Jun 26, 2018 at 03:05:33PM +0200, Paolo Bonzini wrote: > > On 26/06/2018 14:29, Eduardo Habkost wrote: > > > On Tue, Jun 26, 2018 at 07:57:18AM +0200, Paolo Bonzini wrote: > > >> On 25/06/2018 21:51, Eduardo Habkost wrote: > > >>> In either case, I'm not arguing (yet) for changing the default > > >>> upstream. I'm just arguing for upstream QEMU to not make any > > >>> promises about the default. > > >> > > >> It would be a guest ABI breakage for TCG guests, so it would only apply > > >> to new machine types. I don't think it's worth the complication. > > > > > > That's exactly the point: I want to stop promising a stable guest > > > ABI when the accelerator is omitted, because I see no benefit in > > > wasting energy on this. > > > > On the other hand I see no benefit in changing a default that people are > > obviously not using (since most people use KVM, not TCG). Distros will > > keep shipping, and people will keep using qemu-kvm even if we change the > > default. > > Not changing the default is different from promising we will keep > ABI compatibility if the accelerator is omitted. I just want to > get rid of the latter. I guess the key question is what is the risk of causing problems if we switch from tcg to kvm:tcg when accelerator is omitted ? Based on what I've seen the likely troublespot would be people who are using QEMU inside a guest with nested-virt enabled. Some nested-virt impls are buggy and will cause L2 guest hangs, or worse L1 host crashes. Then again we're not causing that brokenness - just revealing what already exists. 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 :|