On 25/05/2016 22:17, Jianjun Duan wrote: > > > 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.
A VMSD describes the "sub-structure" of the field. Here, the substructure of a QTAILQ is the structure of each entry. > 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. We make this assumption elsewhere anyway. _You_ are making it by doing loads and stores through void** in QTAILQ_RAW_*. Thanks, Paolo
