Re: [Qemu-devel] [PATCH 2/5] exec: qemu_ram_alloc_device, qemu_ram_resize

[Juan Quintela <quintela@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> wrote:

> Am I missing a part of the discussion?  When we migrate, the second QEMU
> uses the BIOS from the first.
>
> So:
>
> qemu-2.0 -M pc-2.0 -> qemu-2.2 -M pc-2.0
>
>    uses 2.0 BIOS
>
> qemu-2.2 -M pc-2.0 -> qemu-2.0 -M pc-2.0
>
>    uses 2.2 BIOS
>
> Both should work, in general.  BIOS is rarely the reason for
> incompatibilities.  However, breakage can happen, for example I know
> that RHEL7 SeaBIOS does not work on RHEL6.  RHEL6 SeaBIOS works on
> RHEL7, but it needs a couple workarounds.
>
> Shipping a separate BIOS for different machine types is unrealistic and
> pointless.  It would also be a good terrain for bug reports, unless you
> also do things like "forbid creating -device megasas-gen2 on 2.1 because
> it was introduced in 2.2".

And I agree with that.  If it got introduced on 2.2, it should not be
allowed on pc-2.1.  It just makes things more complicated.  We don't
have infrastructure to enforce that.  And I am claining that is the
problem.  We are just papering over this problem each time that it
happens.  I honestely think that the only way to really fix
compatibility is enforcing that machine types are stable.  right now
they are now, and we ended nothing it.

> Remember that libvirt keeps the same machine
> type for the whole life of a virtual machine definition, even if other
> parts of the hardware (e.g. disks or NICs) change.

There is a way to "upgrade" the machine type of a specific machine.  If
you want to update it, just do it.  If you want "it is not broking, so
not mess with it", it means just that, not changing anything that we can
avoid to change.

Later, Juan.