xml encoding conflict with my defult emacs language configuartion

["niDapeng"<steveneo@21cn.com>]

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.


----------------------------------------------
ÍæÓÎÏ·Ó®ÊýÂëÏà»ú¡¢MP3¡¢UÅÌ 
http://mail.21cn.com/huodong/0310/ 
ÖÐÃÀÈÕº«¸ßÖÐÉúÐÔµ÷²é:ÈÕÅ®ÉúÐÔ¹ÛÄî×·Å 
http://news.21cn.com/huahua/ 
¹ã¶«ADSLÓû§ÓÐ×ÅÊý£¬21CNÉÌÎñÓÊÃâ·ÑÁì 
http://mail.21cn.com/vip 
Ï£À°Ç°×ÜÀíÓëÃÀÑÞ¿Õ½ãÄÖ³öÌÒÉ«ÐÔ³óÎÅ(ͼ) 
http://news.21cn.com/social/