From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCHv4 3/4] cpuid: disable pv eoi for 1.1 and older compat types Date: Wed, 29 Aug 2012 17:29:52 +0300 Message-ID: <20120829142952.GA7003@redhat.com> References: <877gshu8f5.fsf@codemonkey.ws> <20120829134021.GB32728@redhat.com> <873935yelv.fsf@codemonkey.ws> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Gleb Natapov , avi@redhat.com, mtosatti@redhat.com, qemu-devel@nongnu.org, kvm@vger.kernel.org, Jan Kiszka , Blue Swirl To: Anthony Liguori Return-path: Received: from mx1.redhat.com ([209.132.183.28]:28028 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751658Ab2H2O2j (ORCPT ); Wed, 29 Aug 2012 10:28:39 -0400 Content-Disposition: inline In-Reply-To: <873935yelv.fsf@codemonkey.ws> Sender: kvm-owner@vger.kernel.org List-ID: On Wed, Aug 29, 2012 at 09:09:16AM -0500, Anthony Liguori wrote: > Gleb Natapov writes: > > > On Wed, Aug 29, 2012 at 08:36:30AM -0500, Anthony Liguori wrote: > >> "Michael S. Tsirkin" writes: > >> > >> > In preparation for adding PV EOI support, disable PV EOI by default for > >> > 1.1 and older machine types, to avoid CPUID changing during migration. > >> > > >> > PV EOI can still be enabled/disabled by specifying it explicitly. > >> > Enable for 1.1 > >> > -M pc-1.1 -cpu kvm64,+kvm_pv_eoi > >> > Disable for 1.2 > >> > -M pc-1.2 -cpu kvm64,-kvm_pv_eoi > >> > > >> > Signed-off-by: Michael S. Tsirkin > >> > >> As best I can tell, we're papering over an ABI breakage in KVM. > >> > >> If an old QEMU attempts to do a live migration on a new kernel, > >> migrating to an QEMU on a different box with an older kernel, it will > >> fail because there is state that isn't being migrated. > >> > >> This ought to be fixed in the kernel by making these features > >> whitelisted by userspace. > >> > > What do you mean. Userspace and only userspace decides what cpuid bits > > will be seen by a guest. Currently userspace enables all PV cpuid bits > > it finds. > > Right, I misunderstood from the commit message. I see now that the > problem is that bfee7546df51c08e395dc8a7676a5c7f20186fee unconditionally > enabled kvm_pv_eoi without taking into account migration support. All that commit does it let user disable pv eoi. Don't see how reverting it will help ... > I think for 1.2 we should simply revert the above commit and then we can > restore it for 1.3 with proper support for migration. > > Regards, > > Anthony Liguori > > > > > -- > > Gleb.