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

["Michael S. Tsirkin" <mst@redhat.com>]

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.

> -- 
> Eduardo