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

[Paolo Bonzini]

Il 23/06/2014 16:15, Alexey Kardashevskiy ha scritto:
> Hi!
>
> I have been hearing recently that we (server PPC) should have more that
> just one pseries machine in QEMU because this is what everybody else does :)

If you want to support live upgrades from QEMU 2.1 to QEMU 2.2, you 
should indeed prepare by adding a stable name such as pseries-2.1 
already in QEMU 2.1.

> My current understanding is that multiple machines (like
> pc-i440fx-1.4..2.1, and many others) are needed:
>
> 3) for devices which are created explicitly and for which we want some
> capabilities be disabled and we do not want to bother about this every time
> we run QEMU;

This is not the exact reason.  What you wrote _could_ be a reason to use 
compat_properties, but it is not why x86 has multiple machine types.

The reason is because when you migrate from QEMU 1.7 to QEMU 2.1, the 
machine should behave exactly the same as it did in QEMU 1.7.  Any 
guest-visible change introduced between QEMU 1.7 and QEMU 2.1 (*) must 
be removed from QEMU 2.1.  This is the reason why x86 has one machine 
type per version.

    (*) X got faster is not a guest-visible change.  A guest-visible
        change is something like:

        - a register gets a new default value

        - a register that was read-only is now read-write

        - the size of a memory region changed (e.g. a ROM BAR grew
          from 128k to 256k)

        Sometimes, if no known guest is using the register (e.g. only
        firmware uses it) it is okay to skip the creation of the
        compat property.  However, if in doubt, better add it.

> ok, this one makes some sense for "pseries" to have (and upcoming
> endianness register on VGA seems to be the case) but it seems that adding a
> "compat" or "feature" property to the VGA device (and other devices which
> deal with this kind of compatibility) is still more architecturally correct
> thing to do, and let libvirt deal with the rest.

Yes, this sounds sensible.

Paolo

> Since I (almost) always miss the bigger picture, what do I miss now? :) Thanks!