Re: [Qemu-devel] [PATCH RFC 1/1] virtio: migrate config_vector

[Christian Borntraeger <borntraeger@de.ibm.com>]

Am 14.05.2015 um 19:00 schrieb Dr. David Alan Gilbert:
> * Christian Borntraeger (borntraeger@de.ibm.com) wrote:
>> Am 14.05.2015 um 11:36 schrieb Michael S. Tsirkin:
>>> On Thu, May 14, 2015 at 11:22:13AM +0200, Christian Borntraeger wrote:
>>>> Am 13.05.2015 um 23:47 schrieb Michael S. Tsirkin:
>>>>> On Wed, May 13, 2015 at 08:57:00PM +0200, Christian Borntraeger wrote:
>>>>>> Am 13.05.2015 um 18:14 schrieb Michael S. Tsirkin:
>>>>>>>> - AFAICS, there's no easy way to add transport-specific subsections -
>>>>>>>>   and simply adding config_vector in ccw would break compatibility
>>>>>>>
>>>>>>> subsections break compatibility too.  The only way around that is to set
>>>>>>> a flag to skip migrating config_vector for old machine types.
>>>>>>
>>>>>> My main concern is about undetected compatibility issues. A subsection will 
>>>>>> tell the user that something went wrong. What happens if we just add a new
>>>>>> qemu_put_byte in the stream. Will the savevm core always detect that we have
>>>>>> too many or not enough bytes? If yes, adding new stuff in the stream will
>>>>>> always be detected in some way as error we can go with just adding
>>>>>> qemu_put_be16/qemu_get_be16 in virtio_ccw_save_config/virtio_ccw_load_config.
>>>>>> Old/new QEMUs will then not be compatible - but thats probably ok as long as it
>>>>>> errors out.
>>>>>>
>>>>>> My understanding was that we do not have a guarentee that this will be
>>>>>> detected all the time and having random junk in some variables is a debugging
>>>>>> nightmare. Is that correct?
>>>>>>
>>>>>>
>>>>>> Christian
>>>>>
>>>>> It's not too bad - normally there's a bunch of strings that
>>>>> helps you find out what's going on.
>>>>> But if you really care about debuggability of migration streams, help move
>>>>> forward dgilbert's RFC that switched to a self-delimiting format.
>>>>> Just piling up random hacks in virtio seems like a wrong approach.
>>>>>
>>>>
>>>> Thats not my question. PLEASE try to understand my question.
>>>> I want a hard stop if migration changes in incompatible ways.
>>>> If adding a qemu_put_byte in virtio_ccw gets detected we can just fix
>>>> virtio_ccw AS YOU SUGGESTED. I just want to know if I can rely on that 
>>>> or not.
>>>>
>>>> Christian 
>>>
>>> I answered exactly this question but let me try to spell the answer
>>> out a bit more.
>>>
>>> There are three answers:
>>> 1.  Yes, it's sure to get detected because everything gets shifted
>>>     and then you get an unexpected string instead of next device name.
>>
>> Ok got it. So simply adding a qemu_put/get_byte will always fail the migration and we
>> can just fixup virtio-ccw.c at the cost of being not migrateable between versions before/after that change. 
> 
> Gahhh!  No!   Adding an extra byte into the stream causes random horrible failures
> that get very very confusing.  Yes, it will probably fail, but how it will
> fail and what error you get is just guess work.
> (And note, it's strictly a 'probably fail' - if you happen to land with
> a zero byte where you expect the start of the next section the migration
> code will think it's the end of the migration stream and blissfully start
> the CPUs).

As Conny is away today, I will drive the dicussion a bit further :-)
So we really want a feature that detects this change and prevents migration.
I think its totally  fine to not be able to migrate between todays QEMUs and
a fixed version for  s390 as there no supported environment today I am aware
of. What would be the preferred way to go?
a: Connies approach of a subsection that is only migrated if necessary 
(config vector != 0xffff)
b: change virtio-ccw.c with put/get_be16 and make a new version of the
s390-ccw machine? The old version will set a property to not migrate the config
vector. (like Michaels 2nd suggestion)
c: ?

Christian