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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: Sat, 5 Mar 2011 16:50:52 +0000

On 5 March 2011 14:59, Paolo Bonzini <address@hidden> wrote:
> On 03/05/2011 01:34 PM, Peter Maydell wrote:
>> Can you give an example/explanation? docs/migration.txt doesn't
>> seem to cover this...
> Sure, sorry for being terse. It simply needs to be:
>        VMSTATE_UINT32_V(sys_cfgdata, arm_sysctl_state, 2),
>        VMSTATE_UINT32_V(sys_cfgctrl, arm_sysctl_state, 2),
>        VMSTATE_UINT32_V(sys_cfgstat, arm_sysctl_state, 2),
> Also, minimum_version_id needs to remain 1 since you do support loading
> version 1 saved virtual machines.

Ah. I was just following Juan's suggestion:
> - if you don't care about backward compatibility, just add +1 to all the
> version fields and you are done.

but I guess if it's that trivial we might as well support it.
(My guess is that basically nobody is doing any kind of
migration of ARM TCG VMs, so I think it's all a bit moot.)

What do the new fields get set to when you do a load from
a v1 snapshot?

Other random snapshot/vmstate questions while I'm here:

(1) Is there supposed to be any kind of guard on trying to
do a vmsave on a system with devices that don't implement
save/load? IME it just produces a snapshot which doesn't
work when you reload it...
(2) How do you track incompatible changes at the machine
level? For instance, suppose we accidentally forgot to
model a NOT gate in an IRQ line, so we add a qemu_irq_invert()
to the machine init function. None of the devices have
changed, but you can't load the state of an old version
of the machine into a new version, because then the
devices on either end of the inverter would be inconsistent
about the state of the line. But there's no version number
for a machine as far as I can see.

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 :-)

=== 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.


-- PMM

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