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[Qemu-devel] Re: [PATCH v2 1/2] hw/arm_sysctl.c: Add the Versatile Expre

From: Peter Maydell
Subject: [Qemu-devel] Re: [PATCH v2 1/2] hw/arm_sysctl.c: Add the Versatile Express system registers
Date: Tue, 22 Mar 2011 20:32:48 +0000

On 22 March 2011 19:53, Juan Quintela <address@hidden> wrote:
> Peter Maydell <address@hidden> wrote:
>> Migration from the old version to the new version can be supported
>> if it is OK for the new fields to remain in their default state
>> [XXX is this right? are they zeroed, or do they get the value
>> the device's reset function sets them to, or something else?]
> You can initialize in your init function at the value that you want, or
> use foo_post_load() function (that receives the version as a parameter)
> to assign to any correct values that you need.

To check I understand this, this means an incoming migration is
always done to a fresh, never-been-used-before device that has had
its init called but not its reset?

>> when the state of an old-version snapshot is loaded. To implement
>> this you need to use the VMSTATE_*_V macros which let you specify
>> the version in which a field was introduced, for instance:
>>  VMSTATE_UINT32_V(sys_cfgdata, arm_sysctl_state, 2)
>> for a field introduced in version 2. You should also increment
>> the version_id, but leave the minimum_version_id unchanged.
>> Newly added VMSTATE_*_V fields should go at the end of the
>> VMState description.
> Just to make things more complicated, this has been "deprecated" O:-)

It has? Your examples below still use it...

> - We know that old device was wrong, and that there is no way we can
>  load (reliabely) from version 0.  Then we just increase the version:

If you're increasing the version can you also clean up by
converting any old VMSTATE_*_V() into plain VMSTATE_*() at this
point, since we can't migrate from those old versions any more?

> - We know that we can load from v1.  But that we want to always sent
>  bar2 for migration, then we just increase versions to:
> const VMStateDescription vmstate_foo = {
>    .name = "foo",
>    .version_id = 2,
>    .minimum_version_id = 1,
>    .minimum_version_id_old = 1,
>    .fields      = (VMStateField []) {
>        VMSTATE_INT32(bar, FOOState),
>        VMSTATE_INT32_V(bar2, FOOState, 1),
>    }
> };
> And we are done.  We are able to receive state 0 and 1, and we would
> always sent version 1.

Your numbers in the struct and the text don't seem to match?
My guess is you meant to write version_id = 1, minimum_version* = 0 ?

> Have I manage to explain myself a little bit?

Yes, thanks, that's very helpful.

-- PMM

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