Re: [Qemu-devel] machines and versions - why so many?

[Paolo Bonzini <pbonzini@redhat.com>]

Il 23/06/2014 23:35, Alexey Kardashevskiy ha scritto:
> nec-usb-xhci is off for PC_COMPAT_1_3 because of what? PIIX emulation was
> broken in 1.3? Or nec-usb-xhci?

nec-usb-xhci is not "off".

             .driver   = "nec-usb-xhci",\
             .property = "msi",\
             .value    = "off",\

What's off is MSI (and MSI-X), because it wasn't implemented.

> If it is the latter, why is the tweak limited by PC?

Because at the time nobody cared about migration compatibility (even now 
only pSeries is starting to care).

> Looks like I must copy PC_COMPAT_X_X as PSERIES_COMPAT_X_X starting 1.6 (or
> 1.7 - whichever starts supporting migration well enough on pseries) because
> pretty much of what they do is tweaking PCI devices and we can have all of
> these devices on pseries.

It's not necessary to do this retroactively, unless you're planning to 
test forward migration from 1.6 to 2.1.  Just start with 2.2 (and add 
pseries-2.1 now as an alias).

Note that firmware interfaces and CPU features should also be versioned 
if they're guest visible.  That's the point of functions like pc_compat_1_6.

> And then keep an eye on what is happening in PC
> world to copy same tweaks to pseries as they come. Is that correct?

Yes.  We should come up sooner or later with a way to share the tweaks 
though.

> Others (ARM, s390) do not do that because they do not support PCI? Or
> migration? Or some other reason?

This is not limited to PCI.  Others do not support migration across 
different QEMU versions.

Paolo