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

[Alexander Graf]

> Am 12.06.2014 um 12:14 schrieb Greg Kurz <address@hidden>:
> 
> On Thu, 12 Jun 2014 11:43:20 +0200
> Alexander Graf <address@hidden> 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...

Why? This value only ever gets set in realize, so it will not change during the 
lifetime of the device - which means we don't need to migrate it.


Alex