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

From: Christian Borntraeger
Subject: Re: [Qemu-devel] [PATCH RFC 1/1] virtio: migrate config_vector
Date: Thu, 14 May 2015 11:22:13 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

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.


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