From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:43429) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1T6ixl-0005Z1-01 for qemu-devel@nongnu.org; Wed, 29 Aug 2012 10:09:50 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1T6ixa-000716-Bq for qemu-devel@nongnu.org; Wed, 29 Aug 2012 10:09:44 -0400 Received: from mail-ob0-f173.google.com ([209.85.214.173]:41899) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1T6ixa-000711-7M for qemu-devel@nongnu.org; Wed, 29 Aug 2012 10:09:34 -0400 Received: by obbta14 with SMTP id ta14so954705obb.4 for ; Wed, 29 Aug 2012 07:09:33 -0700 (PDT) From: Anthony Liguori In-Reply-To: <20120829134021.GB32728@redhat.com> References: <877gshu8f5.fsf@codemonkey.ws> <20120829134021.GB32728@redhat.com> Date: Wed, 29 Aug 2012 09:09:16 -0500 Message-ID: <873935yelv.fsf@codemonkey.ws> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Subject: Re: [Qemu-devel] [PATCHv4 3/4] cpuid: disable pv eoi for 1.1 and older compat types List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gleb Natapov Cc: kvm@vger.kernel.org, "Michael S. Tsirkin" , mtosatti@redhat.com, qemu-devel@nongnu.org, Blue Swirl , Jan Kiszka , avi@redhat.com 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. 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.