question about io path in the front/backend

[tgh <wwwwww4187@sina.com.cn>]

hi
  I have read some documents and wiki about split driver in xen,and I am 
confused about the I/O path ,in which a sys_read() pass through the domU 
and dom0,does sys_read() in the domU pass through vfs and ,say ,ext3fs 
in domU,and insert request into the requeest_queue of the 
frontend-driver,is it right? 
  and then ,say domU sets up with a *.img file in the dom0, then what 
does frontend and backend driver do?
  does frontend transmit the request to the backend ,is it right?
  and then what does backend driver do ? does backend transfer the 
request to the phyiscal driver in the dom0 ,is it right?
 or does backend transfer the request into some read()operation ,and 
submit it to the vfs and ,say,ext3fs in dom0, and do another relatively 
complete io path in the dom0,is it right?  
 or  if  backend  transfer the request to physical driver directly, how 
does the backend deal with the request's virtual address ,and how does 
backend manage bio buffer ,does physical driver and backend and frontend 
share the bio buffer in  some way, or what does xen deal with it ?


 I am confused about all that ,could someone help me
Thanks in advance