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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Mon, Feb 11, 2019 at 11:22:27PM +0100, Samuel Thibault wrote:
> > Marc-André Lureau, le lun. 11 févr. 2019 12:34:47 +0100, a ecrit:
> > > On Mon, Feb 11, 2019 at 12:09 PM Daniel P. Berrangé <berrange@redhat.com> 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.
> > 
> > Mmm, but do we guarantee that the current version of slirp will be able
> > to read blobs produced by future versions of slirp?
> > 
> > Future extensions of the format would have to be so that the current
> > version could discard their content without issues.
> > 
> > Perhaps qemu should actually explicitly pass 4 to
> > register_savevm_live(), for that function to only record that format
> > (and thus get compatibility of course) and only bump to greater values
> > when qemu is modified to make use of a functionality which requires
> > extending the blob format, which then makes it unreadable by older qemu
> > releases, but that is fine for qemu.
> 
> If QEMU wants to make use of functionality that requires the new blob
> format, it would have to restrict that based on machine type. That way
> it can ensure full compatibility with old  QEMU.

Library compatibility for migration blobs is messy;  we've had the same
problem with usbredir.

It's very difficult to keep track of; for example lets say that libslirp
gets used by 3 or 4 different projects, and the package maintainer for
libslirp in your favorite distro updates it - it's not necessarily
obvious to them that doing so could break qemu migration.

The ideal way is to work in terms of features and not versions; so that
a machine type can enable libslirp-feature-foo in the new machine type
and only then will the migration contents change.

Dave

> Regards,
> Daniel
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK