Re: [Qemu-devel] [PATCH v2 1/3] Add support for 64bit ARM system registers

[Peter Maydell]

On 14 March 2012 19:38, Michael Roth <address@hidden> wrote:
> On Wed, Mar 14, 2012 at 10:09:09PM +0300, Alexey Starikovskiy wrote:
>> Do I need to do anything beside following or not?
>
> Not sure, ARM folks?
>
> My suggestion would be to lose the the catch-all -EINVAL error we
> throw in machine.c:cpu_load() when version_id != CPU_SAVE_VERSION
> and only conditionally load 64-bit registers if the source's
> version_id >= CPU_SAVE_VERSION. But we've never done that in the
> past, and we broke old->new as recently as 2 months ago so I'm not
> sure it's worth it since there hasn't been a release since then.

My position at the moment is that old->new migration on ARM is
not supported and further that we make no attempt to avoid version
bumps. At some point as KVM-on-ARM gets towards being complete
we'll have to start worrying about compatibility, but at the moment
I really don't think that either (a) all the devices have tested
and working migration state or (b) we have a CPUState that's actually
in good enough shape[*] that we can reasonably start to insist on
not breaking cross-version migration.

So migration-wise I have no problem with the patch as it stands.
(I have other issues with it I suspect but haven't got to reviewing
it yet.)

[*] as an example of the kind of problem I'd like to see a solution
for, there's no reason that adding support for a new feature like
LPAE should require the migration state for non-LPAE CPUs to change,
but as things stand it does.

-- PMM