Re: [Qemu-devel] [QEMU RFC PATCH v2 4/6] Migration: migrate QTAILQ

[Jianjun Duan]

On 05/25/2016 12:22 PM, Paolo Bonzini wrote:
>> 1 QTAILQ should only be accessed using the interfaces defined in
>> queue.h. Its structs should not be directly used. So I created
>> interfaces in queue.h to query about its layout. If the implementation
>> is changed, these interfaces should be changed accordingly. Code using
>> these interfaces should not break.
> 
> You don't need to query the layout, as long as the knowledge
> remains hidden in QTAILQ_RAW_* macros.  And because QTAILQ_*_OFFSET
> returns constant values, you can just put the knowledge of the offsets
> directly in QTAILQ_RAW_FOREACH and QTAILQ_RAW_INSERT_TAIL.
> 
>> 2 Based on point 1, vmstate_load_state/vmstate_put_state in vmstate.c
>> doesn't exactly know the structs of QTAILAQ head and entry. So pointer
>> arithmetic is needed to put/get a QTAILQ. To do it, we need those 6
>> parameters to be passed in. So it is not redundant if we only want to
>> only use the interfaces.
> 
> No, you only need two.  The other four are internal to qemu/queue.h.
> Just like QTAILQ users do not know about tqh_* and tqe_*, they need not
> know about their offsets, only the fields that contain them.
> 

In vmstate_load/put_state usually we use a VMSD to describe the type for
dump/load purpose. The metadata for the QTAILQ is for the same purpose.
Of course we can encode the information in a VMSD or VMStateField if
we don't have to change much.

The user may only care the position of head and entry. But to
implement QTAILQ_RAW_***, we need more offset information than that.
If we don't query the offsets using something like offset() and store
it in a metadata, we have to make the assumption that all the pointer
types have the same size. Since in vmstate_load/put_state we don't have
any type information about the QTAILQ instance, we cannot use offset()
in QTAILQ_RAW_*** macros. May have to stick the constants there for
first/last/next/prev in QTAILQ_RAW_***. Not sure if it will work for
all arches.
>> 3 At this moment, vmstate_load_state/vmstate_put_state couldn't handle a
>> queue, or a list, or another recursive structure. To make it
>> extensible, I think a metadata is needed. The idea is for any
>> structure which needs special handling, customized metadata/put/get
>> should provide enough flexibility to hack around.
> 
> I think your solution is a bit overengineered.  If the metadata can
> fit in the VMStateField, you can use VMStateField.
> 
> Thanks,
> 
> Paolo
> 

Thanks,
Jianjun