Re: [Qemu-devel] [PATCH v5 1/2 (for 2.10)] docs: document support lifetime for features

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Jul 24, 2017 at 03:49:29PM +0200, Paolo Bonzini wrote:
> On 19/07/2017 13:56, Thomas Huth wrote:
> > +@node Machine types
> > +@section Machine types
> > +
> > +For architectures which aim to support live migration compatibility
> > +across releases, each release will introduce a new versioned machine
> > +type. For example, the 2.8.0 release introduced machine types
> > +``pc-i440fx-2.8'' and ``pc-q35-2.8'' for the x86_64/i686 architectures.
> > +
> > +To allow live migration of a guest running on a 2.8.0 release to a
> > +2.9.0, the QEMU 2.9.0 version must support the ``pc-i440fx-2.8'' and
> > +``pc-q35-2.8''. To allow users live migrating VMs to skip multiple
> > +intermediate releases when upgrading, new releases of QEMU will
> > +support machine types from many previous versions.
> > +
> > +The supported lifetime for versioned machine types is 12 releases,
> > +which is equivalent to 4 years worth of previous QEMU releases.
> 
> I think there's still no consensus on this.

Indeed, which is exactly why I sent this patch - we need to come up
with a sensible policy here, so we can stop repeating the same debate
over & over & over each time some proposes a patch to kill off some
random old machine type.

The 12 release / 4 year figure was a fairly arbitrary starting
point to which I'd be hoping to see critical reviewer feedback
on (with possible counterproposals) so we can try to get something
documented, to put an end to the repeated debates in this area
each time someone proposes a patch.

>                                              The first two paragraphs
> should be added to the documentation for -machine in qemu-options.hx,
> since "-machine [type=]foo" is currently not documented at all.

I'll happily send a patch for the docs for qemu-options.hx.

Regards,
Daniel
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