Re: [Qemu-devel] [PATCH] define qemukvm-1.2 machine type

[Paolo Bonzini]

> On Fri, Jan 18, 2013 at 12:34:57PM -0500, Paolo Bonzini wrote:
> > > > version_id and minimum_version_id should be changed to 2, as the
> > > > comment indicates.
> > > 
> > > But won't that mean we have to carry that patch forever, and
> > > while we carry
> > > that patch we can never migrate from Fedora qemu to an upstream
> > > qemu instance?
> > > I'd like to avoid carrying any incompatibility forward, if
> > > possible.
> > 
> > To some extent you have to choose between backwards- and forwards-
> > compatibility.  But I think that you can achieve what you want
> > by leaving version_id to 3, while setting minimum_version_id to 2.
> 
> What about
> 
> "Problem is it uses acpi_load_old, when reading from qemu-kvm 1.2 (which
> advertises format as V2), which reads 4*16 bits (instead of 16 bits)
> for en/sts fields. So it can corrupt incoming migration data."

That's if minimum_version_id==3.

But if you set minimum_version_id==2, you fix incoming migration from
qemu-kvm 1.2 (and break it from upstream QEMU 1.2).  acpi_load_old will
only be called for version 1.

At the same time, because version_id==3 you will still have working
migration to upstream QEMU 1.3 and future releases (and break backwards
migration to qemu-kvm 1.2, but that's not a problem).

Paolo