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

[Alexander Graf]

> Am 12.06.2014 um 12:50 schrieb Greg Kurz <address@hidden>:
> 
> On Thu, 12 Jun 2014 12:39:27 +0200
> Alexander Graf <address@hidden> wrote:
> 
>> 
>> 
>>> 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.
> 
> I agree with the fact that the value does not change and should not be 
> migrated in the first place.
> I am just worried about the size of the active ports bitmap that is streamed 
> in the for loop... it
> is only 32 bit as of today, because we are limited to 32 ports. How would 
> this work if the limit is
> raised ? How can the destination machine know how many bits have to be read ?

The destination has be be executed in compat mode to the older qemu version via 
a versioned machine type. This should ensure that limits are kept.


Alex