Re: [Qemu-devel] [PATCH 1/1] RFC: net/slirp: link with libslirp

[Daniel P . Berrangé]

On Fri, Feb 08, 2019 at 07:12:26PM +0100, Marc-André Lureau wrote:
> Once libslirp has received its first release, we can link with the
> external libslirp library.
> 
> The migration data should be compatible with current and older qemu
> versions (same compatibility as today). See "slirp: add state
> saving/loading" patch. However, the content should be treated as a
> blob, as the format may change eventually in the future.

How are we going to manage live migration compat if libslirp changes
the blob content ?

Bear in mind that we need to support all existing QEMU releases live
migrating to effectively all future QEMU releases, with all future
libslirp releases, in *both* directions. ie arbitrarily newer
libslirp needs to be able to emit a blob format that can be read
by arbitrarily older slirp inside QEMU.

Normally we tie data format changes to the machine type. How are
we going to achieve such machine type associations with an external
libslirp ?

It seems to me that if a libslirp wants to change live migration
format, it will need to support both the old and new formats
indefinitely, and the new format must be an opt-in. QEMU can
then tell libslirp which migraiton format based on the machine
type.



Regards,
Daniel
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