Re: [Qemu-devel] [PATCH RFC] hw/pc: set q35 as the default x86 machine

[Eduardo Habkost <ehabkost@redhat.com>]

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.apfelbaum@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