xml encoding conflict with my defult emacs language configuartion

[niDapeng]

I set my emacs environment as Chinese(GB2312), which is my common use. 
When I edit XML by UTF-8, I input some Chinese words( GB2312 encoding). When I 
try to save, emacs prompt there is conflict between them.Message as following:
======================================================
Selected encoding chinese-iso-8bit-dos disagrees with utf-8 specified by file 
contents.  Really save (else edit coding cookies and try again)? (yes or no) 
======================================================


I try to use "C-x RET-c UTF-8 C-x C-s" to save text, it prompt some message and 
also save some corrupt characters, and message as following:
=================================================
These default coding systems were tried to encode text
in the buffer `a.xml':
  utf-8
However, each of them encountered these problematic characters:
  utf-8: ÖÐ ÎÄ
The first problematic character is at point in the displayed buffer,
and C-u C-x = will give information about it.

Select one of the following safe coding systems, or edit the buffer:
  gb2312 hz-gb-2312 iso-2022-7bit
Or specify any other coding system
at the risk of losing the problematic characters.
=================================================


I guess so: because my buffer text is GB2312 ( my input method only accept 
GB2312). Emacs can't convert GB2312 to UTF-8 automatically, it only enforce 
GB2312 to UTF-8.

Is there a easy way to approach my reqired? I need: firstly, it can save GB2312 
to UTF-8 correctly, secondly, it's better there aren't so many message to let 
me confirm, "silent save" is better.


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