From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40453) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fXqYI-0005gw-Mb for qemu-devel@nongnu.org; Tue, 26 Jun 2018 12:10:47 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fXqYG-0001CL-07 for qemu-devel@nongnu.org; Tue, 26 Jun 2018 12:10:46 -0400 Date: Tue, 26 Jun 2018 17:10:38 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= 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> Subject: Re: [Qemu-devel] [RFC PATCH 4/4] qemu-options: Do not show -enable-kvm and -enable-hax in the docs anymore List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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 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 :|