From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53480) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fXVKU-0006wY-O1 for qemu-devel@nongnu.org; Mon, 25 Jun 2018 13:31:07 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fXVKR-0007Vh-EB for qemu-devel@nongnu.org; Mon, 25 Jun 2018 13:31:06 -0400 Date: Mon, 25 Jun 2018 14:30:56 -0300 From: Eduardo Habkost Message-ID: <20180625173056.GM7451@localhost.localdomain> References: <20180613151942.GC19901@redhat.com> <87k1r24quk.fsf@dusky.pond.sub.org> <20180619171539.0adc90d6.cohuck@redhat.com> <20180622181108.GY7451@localhost.localdomain> <20180622193522.GI7451@localhost.localdomain> <743d495e-51cd-8c8e-293e-026b8dece74a@redhat.com> <87lgb3xsv8.fsf@dusky.pond.sub.org> <4292432f-5f26-6925-4c14-ad1370e36e5b@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4292432f-5f26-6925-4c14-ad1370e36e5b@redhat.com> 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: Paolo Bonzini Cc: Markus Armbruster , Thomas Huth , zhang.zhanghailiang@huawei.com, Ben Warren , qemu-trivial@nongnu.org, Cornelia Huck , qemu-devel@nongnu.org, Stefan Hajnoczi On Mon, Jun 25, 2018 at 12:28:34PM +0200, Paolo Bonzini wrote: > On 25/06/2018 08:50, Markus Armbruster wrote: > > Paolo Bonzini writes: > > > >> On 22/06/2018 21:35, Eduardo Habkost wrote: > >>>>> Why is this better than using KVM by default if it's available? > >>>> The answer is (as almost always): Compatibility with migration. Nobody > >>>> dares to sacrifice that chicken :-( > >>> We can now kill it if we announce the feature as deprecated a > >>> couple of releases in advance. > >>> > >>> If we declare that compatibility when the accelerator is omitted > >>> is deprecated in 3.0, in QEMU 3.3 we will be free to choose a > >>> different default accelerator. > >> > >> We can, we don't necessarily want it. > >> > >> The status quo is that people using KVM are invoking qemu as qemu-kvm, > >> people using TCG are invoking qemu as qemu-system-*. All distros are > >> shipping a qemu-kvm or more rarely kvm binary, which is invariably a > >> wrapper script except for RHEL because RHEL doesn't have a qemu-system-* > >> binary at all. > >> > >> By the way, changing qemu-system-*'s default to e.g. RHEL's "kvm or tcg" > >> would not help distros that have "-accel kvm" in their /usr/bin/qemu-kvm > >> script. > > > > It wouldn't hurt them, either. > > Right; to sum up, it does make things a little less consistent for their > users in two ways: > > - qemu-system- behaves differently from qemu-system-. > For example, for ARM the default CPU model might not work for KVM, so > you would have to add a "-cpu xxx" option. > > - qemu-system- would still need an accelerator option on OS X or > Windows, where there is not quite parity between TCG and the native > accelerator, in terms of either features or stability. Because of this > we wouldn't be able to change the default to "whatever virtualizing > accelerators are available followed by TCG". > > > Attentive distros could even replace the wrapper script by a link. > > If they are okay with replacing the "KVM only" semantics with "KVM or > TCG", which I think is generally worse. If we can't get agreement on what's the right default for each QEMU binary, I think that's yet another reason to document that upstream QEMU won't guarantee ABI compatibility if -accel is omitted. If downstream distributions want to keep promising ABI compatibility, it will be up to them. -- Eduardo