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

[Peter Maydell <peter.maydell@linaro.org>]

On 5 March 2011 16:50, Peter Maydell <peter.maydell@linaro.org> wrote:
> I've appended a draft of a suggested extra section for
> docs/migration.txt so you can tell me if I've misunderstood
> it all :-)

Ping? In particular if somebody can fill in the [XXX] bit for
me (and correct any other mistakes!) I'll submit this as a proper
patch to the docs.

> ---begin---
> === Adding state fields to a device ===
>
> If you make a bugfix or enhancement to a device which requires the
> addition of extra state, you need to add these new state fields
> to the VMStateDescription so that:
>  (1) they are saved and loaded correctly
>  (2) migration between the new and old versions either works
>     or fails cleanly.
>
> If the change is such that migration between versions would not
> work anyway, you can just add the new fields using the usual
> VMSTATE_* macros, increment the version_id and set the
> minimum_version_id to be the same as the version_id.
>
> 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?]
> 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.
>
> ---endit---

thanks
-- PMM