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

[Eduardo Habkost]

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 <address@hidden>
> > > 
> > > 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.

-- 
Eduardo