From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56381) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YYF53-0004BM-CC for qemu-devel@nongnu.org; Wed, 18 Mar 2015 10:36:25 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YYF4z-0004uz-9z for qemu-devel@nongnu.org; Wed, 18 Mar 2015 10:36:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48474) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YYF4z-0004uv-1M for qemu-devel@nongnu.org; Wed, 18 Mar 2015 10:36:17 -0400 Date: Wed, 18 Mar 2015 15:36:03 +0100 From: "Michael S. Tsirkin" Message-ID: <20150318153207-mutt-send-email-mst@redhat.com> References: <18492b2679b3194f92b1ceefdbe4355bb840e660.1426155432.git.chen.fan.fnst@cn.fujitsu.com> <20150318142205-mutt-send-email-mst@redhat.com> <1426687346.3643.357.camel@redhat.com> <20150318150358-mutt-send-email-mst@redhat.com> <1426688101.3643.367.camel@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1426688101.3643.367.camel@redhat.com> Subject: Re: [Qemu-devel] [PATCH v5 7/7] pc: add PC_I440FX_COMPAT to disable aercap for vifo device List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Alex Williamson Cc: Chen Fan , izumi.taku@jp.fujitsu.com, qemu-devel@nongnu.org On Wed, Mar 18, 2015 at 08:15:01AM -0600, Alex Williamson wrote: > On Wed, 2015-03-18 at 15:05 +0100, Michael S. Tsirkin wrote: > > On Wed, Mar 18, 2015 at 08:02:26AM -0600, Alex Williamson wrote: > > > On Wed, 2015-03-18 at 14:23 +0100, Michael S. Tsirkin wrote: > > > > typo in subject: vfio, not vifo. > > > > > > > > On Thu, Mar 12, 2015 at 06:23:59PM +0800, Chen Fan wrote: > > > > > for piix4 chipset, we don't need to expose aer, so introduce > > > > > PC_I440FX_COMPAT for all piix4 machines to disable aercap, > > > > > and add HW_COMPAT_2_2 to disable aercap for all lower > > > > > than 2.3. > > > > > > > > > > Signed-off-by: Chen Fan > > > > > > > > Well vfio is never migrated ATM. > > > > So why is compat code needed at all? > > > > > > It's not for migration, it's to maintain current behavior on existing > > > platforms. If someone gets an uncorrected AER error on q35 machine type > > > today, the VM stops. With this change, AER would be exposed to the > > > guest and the guest could handle it. The compat change therefore > > > maintains the stop VM behavior on existing q35 machine types. > > > > If stop VM behaviour is useful, expose it to users. > > If not, then don't. > > I don't see why does it have to be tied to machine types. > > Because q35-2.2 machine type will currently do a stop VM on uncorrected > AER error. If we don't tie that to a machine option then q35-2.2 would > suddenly start exposing the error to the guest. That's a fairly > significant change in behavior for a static machine type. I don't think you can classify it as a behaviour change. VM stop is not guest visible behaviour. Are you worrying about guests misbehaving when they see these errors? Then you want this as user-controlled, supported option. In other words: we only tie things to machine types when we have to. This code gets almost no testing, and is a lot of work to test. This one sounds like "just in case" is not a good motivation. -- MST