From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:35407) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WxHXT-00042h-62 for qemu-devel@nongnu.org; Wed, 18 Jun 2014 11:12:46 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1WxHXM-0003SR-Pq for qemu-devel@nongnu.org; Wed, 18 Jun 2014 11:12:39 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55411) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WxHXM-0003SM-FI for qemu-devel@nongnu.org; Wed, 18 Jun 2014 11:12:32 -0400 Date: Wed, 18 Jun 2014 18:12:49 +0300 From: "Michael S. Tsirkin" Message-ID: <20140618151249.GA1697@redhat.com> References: <20140613111703.22108.14322.stgit@bahia.local> <20140617073631.GL16768@stefanha-thinkpad.redhat.com> <539FF0E3.6040407@suse.de> <20140618103814.GG14030@stefanha-thinkpad.redhat.com> <20140618143521.3941fd4f@bahia.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140618143521.3941fd4f@bahia.local> Subject: Re: [Qemu-devel] [PATCH v8 00/20] virtio endian-ambivalent target List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Greg Kurz Cc: Kevin Wolf , Peter Maydell , rusty@ozlabs.au.ibm.com, Anthony Liguori , Juan Quintela , Stefan Hajnoczi , Rusty Russell , Alexander Graf , qemu-devel@nongnu.org, aneesh.kumar@linux.vnet.ibm.com, Stefan Hajnoczi , Amit Shah , Paolo Bonzini , Andreas =?iso-8859-1?Q?F=E4rber?= On Wed, Jun 18, 2014 at 02:35:21PM +0200, Greg Kurz wrote: > On Wed, 18 Jun 2014 18:38:14 +0800 > Stefan Hajnoczi wrote: > > > On Tue, Jun 17, 2014 at 09:40:19AM +0200, Alexander Graf wrote: > > > > > > On 17.06.14 09:36, Stefan Hajnoczi wrote: > > > >On Fri, Jun 13, 2014 at 01:18:00PM +0200, Greg Kurz wrote: > > > >>This version merges the changes requested during the v7 review, remarks from > > > >>ppc64 dump support review (yes, we talked about virtio there) and the work on > > > >>virtio subsections migration. Also two new patches have been added: > > > >>- patch #1 is a preliminary fix for virtio-serial posted by Alexander Graf > > > >>- patch #9 prepares the work on the virtio_is_big_endian() helper > > > >> > > > >>The most significant changes are: > > > >>- introduction of a new CPU method for virtio > > > >>- endianness is taken from CPU that resets the device > > > >>- fastpath virtio memory accessors for fixed endian targets > > > >>- VMState based virtio subsections (compatibility friendly) > > > >I'm surprised it's not enough for the virtio device to have an > > > >endianness field (big/little). It seems these patches make endianness > > > >depend on the CPUState through which the device is being accessed. > > > > > > > >Can you explain why it's necessary to check the CPUState? > > > > > > They only check CPUState at the point in time of reset, as that's the only > > > case where we can derive the implicit endian configuration from :). > > > > What bothers me is that real hardware can't do this. Given that VIRTIO > > 1.0 is always little-endian I guess this is just a temporary hack for > > ppc little-endian. Would be nice to add a comment so it's clear why > > this approach is being taken instead of a cleaner solution. > > > > Stefan > > Hi Stefan, > > Previous versions of this patch set had such comments: > > "virtio data structures are defined as "target endian", which assumes > that's a fixed value. In fact, that actually means it's platform-specific. > The OASIS virtio 1.0 spec will fix this, by making all little endian. > > We need to support both implementations and we want to share as much code > as possible." > > but these lines got lost between v6 and v7... my bad... :-\ > > I agree all of this is a hack but: > - it has been on the table for nearly a year > - ppc LE distros are already available > - the memory accessors part makes sense for 1.0 > > and, speaking of 1.0, I couldn't find any clue about when QEMU would support > this (Cc'ing Rusty for his input), but we (IBM and distro partners) need > ppc LE support now. > > Cheers. QEMU 2.2 I think. One disadvantage of this work is that it removes some of the stimulus for people to work on 1.0, replacing it with hacks. Hmm. > -- > Gregory Kurz kurzgreg@fr.ibm.com > gkurz@linux.vnet.ibm.com > Software Engineer @ IBM/Meiosys http://www.ibm.com > Tel +33 (0)562 165 496 > > "Anarchy is about taking complete responsibility for yourself." > Alan Moore.