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Re: [Qemu-ppc] [Qemu-devel] [PATCH] target-ppc: Add @cpu_dt_id into migr

From: Peter Maydell
Subject: Re: [Qemu-ppc] [Qemu-devel] [PATCH] target-ppc: Add @cpu_dt_id into migration stream
Date: Thu, 10 Apr 2014 15:49:47 +0100

On 10 April 2014 15:35, Alexey Kardashevskiy <address@hidden> wrote:
> Then what is the purpose of many, many VMSTATE_.*_EQUAL?

Often it's backwards compatibility with a previous vmstate
or save/load function set which incorrectly sent data it didn't
need to.

> And I do not want to send configuration by the proposed patch, I want to
> make sure that the new guest is able to continue. Why exactly is this bad?

It's not bad, but as several people have now pointed out to you,
you're trying to fix a tiny tiny corner of the real, larger
problem, in a way which isn't going to generalise to actually
fixing the larger problem. So if we took your change then
(a) we still wouldn't be able to support detection of migration
between two systems with mismatched configuration, so it doesn't
really achieve anything
(b) if we ever did manage to fix that we'd have to remove your
change (because that bit of config checking would now be handled
via whatever generic mechanism we implemented), except we probably
couldn't remove it since that would break migration version
compatibility, so we'd end up with a wart we have to carry
around forever

-- PMM

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