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

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 06/24/2014 03:15 PM, Paolo Bonzini wrote:
> 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".

Yes, my bad, I understood (but explained this wrong) that XHCI is enabled
but without MSI.


> 
>             .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).


"only pSeries"? There are plenty of x86 machines already and if it is not
for migration support, then what are they all for? :)


>> 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.

Yeah, I got that now. Thanks!



-- 
Alexey