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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jun 12, 2014 at 12:14:40PM +0200, Greg Kurz wrote:
> On Thu, 12 Jun 2014 11:43:20 +0200
> Alexander Graf <agraf@suse.de> wrote:
> 
> > 
> > On 12.06.14 11:39, Paolo Bonzini wrote:
> > > Il 12/06/2014 11:37, Michael S. Tsirkin ha scritto:
> > >> Maybe just drop unnecessary stuff for new machine types?
> > >> Then we won't need hacks to migrate it.
> > >
> > > For any machine type it's trivial to migrate it.  All machine types 
> > > including old ones can disregard the migrated values.
> > 
> > How about a patch like this before the actual LE awareness ones? With 
> > this we should make virtio-serial config space completely independent of 
> > live migration.
> > 
> > Also since QEMU versions that do read these swapped values during 
> > migration are not bi-endian aware, we can never get into a case where a 
> > cross-endian save needs to be considered ;).
> > 
> > 
> > Alex
> > 
> > 
> > diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c
> > index 2b647b6..73cb9b7 100644
> > --- a/hw/char/virtio-serial-bus.c
> > +++ b/hw/char/virtio-serial-bus.c
> > @@ -670,6 +670,7 @@ static int virtio_serial_load(QEMUFile *f, void 
> > *opaque, int version_id)
> >       uint32_t max_nr_ports, nr_active_ports, ports_map;
> >       unsigned int i;
> >       int ret;
> > +    uint32_t tmp;
> > 
> >       if (version_id > 3) {
> >           return -EINVAL;
> > @@ -685,17 +686,12 @@ static int virtio_serial_load(QEMUFile *f, void 
> > *opaque, int version_id)
> >           return 0;
> >       }
> > 
> > -    /* 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)) {
> > -        /* Source could have had more ports than us. Fail migration. */
> > -        return -EINVAL;
> > -    }
> > +    /* Unused */
> > +    qemu_get_be16s(f, &tmp);
> > +    qemu_get_be16s(f, &tmp);
> > +    qemu_get_be32s(f, &tmp);
> > 
> > +    max_nr_ports = tswap32(s->config.max_nr_ports);
> >       for (i = 0; i < (max_nr_ports + 31) / 32; i++) {
> >           qemu_get_be32s(f, &ports_map);
> > 
> > 
> 
> For the moment, we have 0 < max_nr_ports < 32 so the source
> machine only stores a single 32 bit value... If this limit
> gets raised, we can end up sending more than that... and
> only the source machine max_nr_ports value can give the
> information...

I don't think we need to worry.
We won't be able to change max_nr_ports in compat machine types.


> -- 
> 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.