Eduardo Habkost <ehabk...@redhat.com> writes: > On Mon, Sep 14, 2015 at 09:09:12AM -0600, Eric Blake wrote: >> On 09/13/2015 03:28 AM, Michael S. Tsirkin wrote: >> > On Fri, Sep 11, 2015 at 03:44:47PM -0300, Eduardo Habkost wrote: >> >> Ping? >> >> >> >> So, what's the reason we are still keeping those old machines in the >> >> code? >> > >> > Victor also wanted to clean out some very old machine types for >> > the PIIX, too. >> > >> > But if someone created a machine with libvirt, these machine types >> > are now written in the XML. Failing to start guests isn't nice. >> >> New qemu with old libvirt isn't always guaranteed to work. And new >> libvirt can be patched to automatically update any old hard-coded >> machine name to a newer safe alternative, _when such update is proven >> safe_ (we've done it in the past for Fedora: if I recall correctly, >> Fedora 13 branched its own machine type, then in Fedora 15 qemu decided >> to quit supporting the branch, so libvirt was patched downstream in >> Fedora 15-17 to rewrite the old name into its safe upstream counterpart. >> The downstream patch was dropped in Fedora 18 since F15 was EOL by then >> so no more new machines would have been created with the old spelling, >> and since a year was deemed long enough for people to have either run >> their guest to pick up the automatic update, or that their guest was so >> infrequently run that they could read the error message and act on it >> themselves). >> >> Failing to start guests isn't nice, but it also isn't the end of the >> world, when there is no choice but to break ABI. An explicit ABI break >> (by making the user rewrite machine type) is better than silent change >> in behavior with a potential for broken guests. > > What I get from this, is that this is an user interface and usability > problem, and it's better to solve it at the appropriate layer (which is > not QEMU). > > If QEMU can't emulate those machines anymore, QEMU's job is to just tell > that to libvirt (so libvirt and management layers can decide what's the > best way to help the user cope with it), instead of lying to them and > say that we still emulate the same machine when we actually don't.
Seconded. The story for direct use by humans is at least as convincing.
