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

[Daniel P . Berrangé]

On Mon, Feb 11, 2019 at 12:34:47PM +0100, Marc-André Lureau wrote:
> Hi
> 
> On Mon, Feb 11, 2019 at 12:09 PM Daniel P. Berrangé <address@hidden> wrote:
> >
> > 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.
> 
> 
> Right, this is all supported currently with the proposed patch set,
> since it is effectively the same code.
> 
> So register_savevm_live() get passed slirp_state_version() (currently == 4)
> 
> & slirp_state_load() get the version_id from 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 ?
> 
> Is this relevant for slirp? I don't see any version tied to machine
> type. Am I missing something?

Old to new would work provided new slirp can read & understand all previous
slirp state versions. It is entirely possible no one ever tested new -> old
live migration with slirp. I doubt we would have tested it in RHEL, since we
don't really care very much about slirp in context of migration. 

Regards,
Daniel
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