[Qemu-devel] Re: new->old version migration

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/07/2011 10:07 AM, Michael S. Tsirkin wrote:
> New thread stated intentionally, the original patch is Message-ID:
> <349e93a4cfc6e1effc1b681cae53f805fdb9624e.1296713825.git.amit.shah@redhat.com>
>
> On Thu, Feb 03, 2011 at 11:47:08AM +0530, Amit Shah wrote:
>    
>> Add a compat property for older machine types.  When this is used (via
>> -M pc-0.13, for example), the new flow control mechanisms will not be
>> used.  This is done to keep migration from a machine started with older
>> type on a pc-0.14+ qemu to an older machine working.
>>
>> The property is named 'flow_control' and defaults to on.
>>
>> Reported-by: Alex Williamson<alex.williamson@redhat.com>
>> Signed-off-by: Amit Shah<amit.shah@redhat.com>
>>      
> So, I think there are two things that need to be agreed on:
>
> - Can we commit to support migration from new qemu version to an old one?
>    We haven't in the past but downstreams do want this,
>    so it makes sense to have the infrastructure upstream.
>    

Only within a stable release series and only when it's possible without 
sacrificing integrity.  I know some downstreams disagree with this but I 
don't think this is a business we want to get into.

Regards,

Anthony Liguori

> - The infrastructure/command line option for such support.
>    We have the -M flags to describe the machine that
>    we are running, but that abstracts away guest-visible machine,
>    which the migration format is not.
>    Also, same qemu could migrate to any older version.
>    So I think we would have to add a flag (call it -V for now)
>    to savevm/migrate commands to specify the format to be used.
>    Naturally some machines would be incompatible with
>    specific -V values, that's nothing new.
>
> Pls comment.
>
>