From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:44572) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z5Efe-0008Tz-1n for qemu-devel@nongnu.org; Wed, 17 Jun 2015 10:50:31 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Z5EfZ-0003DZ-Vk for qemu-devel@nongnu.org; Wed, 17 Jun 2015 10:50:29 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42609) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z5EfZ-0003DR-Q9 for qemu-devel@nongnu.org; Wed, 17 Jun 2015 10:50:25 -0400 Received: from int-mx13.intmail.prod.int.phx2.redhat.com (int-mx13.intmail.prod.int.phx2.redhat.com [10.5.11.26]) by mx1.redhat.com (Postfix) with ESMTPS id 6B0FB36B4C1 for ; Wed, 17 Jun 2015 14:50:25 +0000 (UTC) Date: Wed, 17 Jun 2015 16:50:22 +0200 From: "Michael S. Tsirkin" Message-ID: <20150617164437-mutt-send-email-mst@redhat.com> References: <20150617094829-mutt-send-email-mst@redhat.com> <55812BC4.3000600@redhat.com> <20150617121148-mutt-send-email-mst@redhat.com> <55815B06.5030508@redhat.com> <20150617133816-mutt-send-email-mst@redhat.com> <55815FA7.3070205@redhat.com> <20150617135340-mutt-send-email-mst@redhat.com> <55816039.3080103@redhat.com> <20150617135716-mutt-send-email-mst@redhat.com> <5581660E.2090807@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5581660E.2090807@redhat.com> Subject: Re: [Qemu-devel] [PATCH] Migration compatibility for serial List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: amit.shah@redhat.com, qemu-devel@nongnu.org, "Dr. David Alan Gilbert (git)" , quintela@redhat.com On Wed, Jun 17, 2015 at 02:20:30PM +0200, Paolo Bonzini wrote: > > > On 17/06/2015 13:58, Michael S. Tsirkin wrote: > > > No, it doesn't. The guest misbehaves maybe, but the migration format is > > > not affected. > > > > I just tried, set prog_if to different values, sure it failed. > > How so? It's just a byte in config space. But even then, fixing > migration is just a side effect of keeping config space consistent for a > given machine type (i.e. not changing hardware type under the guest's feet). David's patches are also guest visible, are they not? We are losing state guest can indirectly observe, right? > > Here's another one, at random: > > > > Author: Michael S. Tsirkin > > Date: Thu Feb 14 19:11:27 2013 +0200 > > > > e1000: unbreak the guest network migration to 1.3 > > > > QEMU 1.3 does not emulate the link auto negotiation, so if migrate to a > > 1.3 machine during link auto negotiation, the guest link will be set to down. > > Fix this by just disabling auto negotiation for 1.3 and older. > > > > Signed-off-by: Michael S. Tsirkin > > Okay, that's an interesting one, and there's a similar one for e1000 > interrupt mitigation. > > The interesting point is that in both cases the bug compatibility > extends to other behavior of the device, i.e. more than just migration. But the guest registers are exactly the same. It is only guest-visible indirectly, as timing of link up events. So why not keep auto-negotiation running correctly? Because we can't keep it running across migration. > I would even say that bug-compatibility of migration is just a side > effect, not the primary end. For interrupt mitigation, it was not > enabled on older machine types in the first place, because it could > break guests. Keeping backwards migration working was just a side > effect; simply, checking "s->compat_flags & E1000_FLAG_MIT" is the only > sensible way to write e1000_mit_state_needed. Auto negotiation should > have been done the same way, which is what your patch did. > > Paolo True, David's patches only trigger if migration happens. I am guessing there is no need to touch other paths, but I am not very familiar with the hardware in question. -- MST