Re: [Qemu-devel] [PATCH RFC 8/8] virtio: add endian-ambivalent support to VirtIODevice

[Alexander Graf]

On 12.06.14 11:07, Michael S. Tsirkin wrote:
> On Thu, Jun 12, 2014 at 10:55:40AM +0200, Alexander Graf wrote:
> > On 12.06.14 09:54, Michael S. Tsirkin wrote:
> > > On Thu, Jun 12, 2014 at 09:43:51AM +0200, Greg Kurz wrote:
> > > > On Thu, 29 May 2014 12:16:26 +0200
> > > > Paolo Bonzini <address@hidden> wrote:
> > > > > Il 29/05/2014 11:12, Greg Kurz ha scritto:
> > > > > > int virtio_load(VirtIODevice *vdev, QEMUFile *f)
> > > > > > {
> > > > > > [...]
> > > > > >              nheads = vring_avail_idx(&vdev->vq[i]) - 
> > > > > > vdev->vq[i].last_avail_idx;
> > > > > >                         ^^^^^^^^^^^
> > > > > >              /* Check it isn't doing very strange things with descriptor 
> > > > > > numbers. */
> > > > > >              if (nheads > vdev->vq[i].vring.num) {
> > > > > > [...]
> > > > > > }
> > > > > >
> > > > > > and
> > > > > >
> > > > > > static int virtio_serial_load(QEMUFile *f, void *opaque, int version_id)
> > > > > > {
> > > > > > [...]
> > > > > >      /* The config space */
> > > > > >      qemu_get_be16s(f, &s->config.cols);
> > > > > >      qemu_get_be16s(f, &s->config.rows);
> > > > > >
> > > > > >      qemu_get_be32s(f, &max_nr_ports);
> > > > > >      tswap32s(&max_nr_ports);
> > > > > >       ^^^^^^
> > > > > >      if (max_nr_ports > tswap32(s->config.max_nr_ports)) {
> > > > > > [...]
> > > > > > }
> > > > > >
> > > > > > If we stream subsections after the device descriptor as it is done
> > > > > > in VMState, these two will break because the device endian is stale.
> > > > > >
> > > > > > The first one can be easily dealt with: just defer the sanity check
> > > > > > to a post_load function.
> > > > > Good, we're lucky here.
> > > > >
> > > > > > The second is a bit more tricky: the
> > > > > > virtio serial migrates its config space (target endian) and the
> > > > > > active ports bitmap. The load code assumes max_nr_ports from the
> > > > > > config space tells the size of the ports bitmap... that means the
> > > > > > virtio migration protocol is also contaminated by target endianness. :-\
> > > > > Ouch.
> > > > >
> > > > > I guess we could break migration in the case of host endianness !=
> > > > > target endianness, like this:
> > > > >
> > > > >       /* These three used to be fetched in target endianness and then
> > > > >        * stored as big endian.  It ended up as little endian if host and
> > > > >        * target endianness doesn't match.
> > > > >        *
> > > > >        * Starting with qemu 2.1, we always store as big endian.  The
> > > > >        * version wasn't bumped to avoid breaking backwards compatibility.
> > > > >        * We check the validity of max_nr_ports, and the incorrect-
> > > > >        * endianness max_nr_ports will be huge, which will abort migration
> > > > >        * anyway.
> > > > >        */
> > > > >       uint16_t cols = tswap16(s->config.cols);
> > > > >       uint16_t rows = tswap16(s->config.rows);
> > > > >       uint32_t max_nr_ports = tswap32(s->config.max_nr_ports);
> > > > >
> > > > >       qemu_put_be16s(f, &cols);
> > > > >       qemu_put_be16s(f, &rows);
> > > > >       qemu_put_be32s(f, &max_nr_ports);
> > > > >
> > > > > ...
> > > > >
> > > > >       uint16_t cols, rows;
> > > > >
> > > > >       qemu_get_be16s(f, &cols);
> > > > >       qemu_get_be16s(f, &rows);
> > > > >       qemu_get_be32s(f, &max_nr_ports);
> > > > >
> > > > >       /* Convert back to target endianness when storing into the config
> > > > >        * space.
> > > > >        */
> > > > Paolo,
> > > >
> > > > The patch set to support endian changing targets adds a device_endian
> > > > field to the VirtIODevice structure to be used instead of the default
> > > > target endianness as it happens with tswap() macros. It also introduces
> > > > virtio_tswap() helpers for this purpose, but they can only be used when
> > > > the device_endian field has been restored... in a subsection after the
> > > > device descriptor... :-\
> > > Store it earlier then, using plain put/get.
> > > You can still add a section conditionally to cause
> > > a cleaner failure in broken cross-version scenarios.
> > >
> > > > If the scenario is ppc64le-on-ppc64: tswap() macros don't do anything
> > > > and we cannot convert back to LE...
> > > >
> > > > >       s->config.cols = tswap16(cols);
> > > > >       s->config.rows = tswap16(rows);
> > > > Since cols and rows are not involved in the protocol, we can safely
> > > > defer the conversion to post load.
> > > >
> > > > >       if (max_nr_ports > tswap32(s->config.max_nr_ports) {
> > > > >           ...
> > > > >       }
> > > > Since we know that 0 < max_nr_ports < 32,  is it acceptable to guess
> > > > the correct endianness with a heuristic ?
> > > >
> > > > if (max_nr_ports > tswap32(s->config.max_nr_ports)) {
> > > >         max_nr_ports = bswap32(max_nr_ports);
> > > > }
> > > >
> > > > if (max_nr_ports > tswap32(s->config.max_nr_ports)) {
> > > >         return -EINVAL;
> > > > }
> > > >
> > > > > > In the case the answer for above is "legacy virtio really sucks" then,
> > > > > > is it acceptable to not honor bug-compatibility with older versions and
> > > > > > fix the code ? :)
> > > > > As long as the common cases don't break, yes.  The question is what are
> > > > > the common cases.  Here I think the only non-obscure case that could
> > > > > break is x86-on-PPC, and it's not that common.
> > > > >
> > > > > Paolo
> > > > Thanks.
> > > One starts doubting whether all this hackery is worth it.  virtio 1.0
> > > should be out real soon now, it makes everything LE so the problem goes
> > > away. It's not like PPC LE is so popular that we must support old drivers
> > > at all costs.  Won't time be better spent backporting virtio 1.0 drivers?
> > There are already released and working Linux distributions (Ubuntu,
> > openSUSE, maybe others) that don't have virtio 1.0 drivers. Putting our
> > heads into the sand is not an option ;).
> >
> >
> > Alex
> I don't get it. Does virtio work there at the moment?

With the LE enable patch, yes, sure. Imagine the same would happen for 
Windows and e1000 - would you still argue the same way?


Alex