Re: [Qemu-devel] [v3 PATCH] target-mips: implement UserLocal Register

[Peter Maydell]

On 29 May 2014 15:29, James Hogan <address@hidden> wrote:
> On 29/05/14 14:26, Peter Maydell wrote:
>> On 29 May 2014 14:22, James Hogan <address@hidden> wrote:
>>> loadvm/migration will write Config3, possibly with a different value if
>>> the snapshot being loaded is an older one without userlocal support.
>>
>> Migration load always happens with a freshly reset system,
>> so this isn't a problem.
>
> Why does that make a difference? The snapshot may have been saved on a
> slightly different version without that bit set (if that isn't supported
> then we may as well not bother trying to support loading different
> snapshot versions).

A key requirement of migration or vm restore is that the
machine configuration (including all command line parameters)
must be exactly identical at both ends. (We don't attempt to
catch this user error, which typically results in weird stuff
happening or in a migration failure with an obscure message).
So you'll never get a migration from a bit-set config to a
bit-cleared config. Note that this is different from a cross
version migration, which would be "from QEMU 2.0 with config-X
to QEMU 2.1 with config-X".

> Also loadvm can happen at any time, not just after
> reset.

The "loadvm" monitor command is implemented as "first reset;
then load state".

thanks
-- PMM