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

[Paolo Bonzini]

On 26/05/2016 18:43, Jianjun Duan wrote:
>>> 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_*.
> 
> I thought every data pointer can be converted to and from void type of
> pointer. Anyway we can make that assumption here.

Yes, you can do that.

> How about alignment? Is it OK to assume pointers are always aligned at
> 4byte for 32 bit and 8byte for 64 bit? If yes then we can just add the
> size of a void pointer to first to get last in QTAILQ head. We will get
> something like:
> first=0
> last = sizeof(void *)
> next=0
> prev=sizeof(void *)

Yes, that's my point.  These four are constants.

Thanks,

Paolo