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

From: Paolo Bonzini
Subject: Re: [Qemu-ppc] [Qemu-devel] [QEMU RFC PATCH v2 4/6] Migration: migrate QTAILQ
Date: Wed, 25 May 2016 15:22:34 -0400 (EDT)

> 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

> 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.

> 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.



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