On Mon, Jun 04, 2018 at 08:17:23PM +0300, Michael S. Tsirkin wrote: > On Mon, Jun 04, 2018 at 10:26:24AM -0300, Eduardo Habkost wrote: > > On Mon, Jun 04, 2018 at 02:01:29PM +0100, Daniel P. Berrangé wrote: > > > On Mon, Jun 04, 2018 at 09:54:15AM -0300, Eduardo Habkost wrote: > > > > On Mon, Jun 04, 2018 at 04:38:22AM +0300, Michael S. Tsirkin wrote: > > > > > On Sun, Jun 03, 2018 at 12:27:49PM +0300, Marcel Apfelbaum wrote: > > > > > > Moving to QEMU 3.0 seems like a good opportunity for such a change. > > > > > > > > > > > > I440FX is really old and does not support modern features like > > > > > > IOMMU. > > > > > > Q35's SATA emulation is faster than pc's IDE, native PCI express > > > > > > hotplug > > > > > > is cleaner than ACPI based one and so on... > > > > > > > > > > > > Also the libvirt guys added very good support for the Q35 machine > > > > > > (thanks!). > > > > > > > > > > > > Management software should always specify the machine type and for > > > > > > the > > > > > > current setups, adding '-machine pc' to the command line is not > > > > > > such a > > > > > > big deal. > > > > > > > > > > > > In time the pc machine will fade out and we will probably stop > > > > > > adding > > > > > > new versions at some point. > > > > > > > > > > > > Signed-off-by: Marcel Apfelbaum <marcel.apfelb...@gmail.com> > > > > > > > > > > For command line users, I think changing the default isn't nice. > > > > > > > > > > Yes it's easy to add -machine pc but there's no documentation > > > > > that tells you to do so. Add to that shortcuts like -cdrom > > > > > stop working, hotplug needs extra bridges to work, and one > > > > > can see that while management tool users benefit from q35, > > > > > command line users will suffer. > > > > > > > > > > Can't we add a tag for management without changing the command line > > > > > default? How about "management-default"? "recommended"? "latest"? > > > > > > > > We could add new aliases if they are useful for management > > > > software, but we would need a well-defined use case and set of > > > > requirements+expectations for the new alias. > > > > > > I'm not convinced by the idea of adding a distinct default "for mgmt". All > > > the problems described wrt 'q35' vs 'pc' apply equally to management apps > > > as they do to humans. It just happens that one common mgmt layer (libvirt) > > > knows how to handle some of the complexity of q35. Other mgmt apps though > > > are just as likely to be hurt by the change as humans are. So effectively > > > the proposed "for mgmt" is actually "for libvirt >= some version", which > > > feels like a layering violation to me. > > > > This means the new alias would be used only if requested > > explicitly by management software (not used automatically by > > libvirt). > > > > Taking that into account, I still don't see what exactly would be > > the use case here, and what exactly users can/can't expect when > > using the new alias. > > Let's see what we have now first: > > 1. We have a requirement for the user to save the machine type on install > and maintain it with the image (a separate thread discusses saving that > as part of a qcow2 image). > > 2. If you use an alias instead you are supposed to resolve it > and save the resolved value. If you save the alias instead, > you can't do cross-version live migration. > > 3. If you don't specify anything you get a machine tagged default. You are > supposed to find it and save the value found. If you don't and just > keep using the default, you can't do cross-version live migration. > > --- > > So now we would like to relax 3 to say > "If you don't and just keep using the default, some images might not > boot". > > unfortunately we probably can't change 3 like this. > So what I propose instead is simply: > > 4. If you find a machine type value tagged "qmp-default" you must save > the value found. If you don't and just keep using the qmp-default each > time, then existing guest images won't boot. > This relaxed compatibility requirement allows for advanced features > as compared to default.
What problems would this system solve? Who would prefer to use the "qmp-default" machine-type instead of the "pc" or "q35" aliases? -- Eduardo
