From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:55478) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1goVKj-0002HL-CP for qemu-devel@nongnu.org; Tue, 29 Jan 2019 10:30:10 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1goVKV-0007ol-SH for qemu-devel@nongnu.org; Tue, 29 Jan 2019 10:29:49 -0500 Received: from mx1.redhat.com ([209.132.183.28]:39030) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1goVKV-0007Q5-K1 for qemu-devel@nongnu.org; Tue, 29 Jan 2019 10:29:39 -0500 Date: Tue, 29 Jan 2019 15:28:12 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20190129152811.GF2650@work-vm> References: <20190125114155.32062-1-vkuznets@redhat.com> <20190125114155.32062-5-vkuznets@redhat.com> <20190125124738.GB30730@rkaganb.sw.ru> <87bm44onnh.fsf@vitty.brq.redhat.com> <20190128113017.GA2435@rkaganb.sw.ru> <87lg34zy37.fsf@vitty.brq.redhat.com> <20190128182229.GF2585@work-vm> <877eenze0u.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <877eenze0u.fsf@vitty.brq.redhat.com> Subject: Re: [Qemu-devel] [PATCH RFC 4/8] i386/kvm: Implement 'hv-all' pass-through mode List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Vitaly Kuznetsov Cc: Roman Kagan , "qemu-devel@nongnu.org" , Paolo Bonzini , Richard Henderson , Eduardo Habkost , Marcelo Tosatti * Vitaly Kuznetsov (vkuznets@redhat.com) wrote: > "Dr. David Alan Gilbert" writes: > > > I'm not sure what the equivalent bear traps are in the Hyper-V world, > > but I'd be surprised if there weren't any; for example what happens > > when someone upgrades one of their hosts to some minor version that > > adds/removes a feature? > > Here we're talking about Hyper-V emulation in KVM, features only get > added there, but even if it gets removed it will be detected by libvirt ... OK, but then you do get the same behaviour; upgrade a host to a new kernel/qemu and get a new enlightenment, and you can't back migrate to the older one (possibly with no warning). > > > > Also, how does libvirt figure out that the features are actually the > > same - does it need a bunch of detection code? > > ... as I *think* it compares Feature CPUID words (and all Hyper-V > features which we enable with hv-all are there). Not too bad if it does, but also look at the scary command lines we get generated full of -cpu ...+feature,+feature,-feature,.... Dave > -- > Vitaly -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK