bitkeeper revision 1.1159.1.357 (41894672voUQPrsrEPix8QJ3tJJH5Q)

Merge ssh://srg//auto/groups/xeno/BK/xeno.bk
into equilibrium.research:/export/scratch/xeno-docs.bk

1 files changed, 17 insertions, 9 deletions

diff --git a/docs/misc/blkif-drivers-explained.txt b/docs/misc/blkif-drivers-explained.txt
index adf1d3738a..7f50494b63 100644
--- a/docs/misc/blkif-drivers-explained.txt
+++ b/docs/misc/blkif-drivers-explained.txt
@@ -50,7 +50,7 @@ solution.
 The structure is like this:
 
    +--------------------------+  +--------------------------+
-   | Domain 0 (privileged)    |  | Domain 1 (unprivileged) |
+   | Domain 0 (privileged)    |  | Domain 1 (unprivileged)  |
    |                          |  |                          |
    | Xend ( Application )     |  |                          |
    | Blkif Backend Driver     |  | Blkif Frontend Driver    |
@@ -107,7 +107,7 @@ front and backend domains over the control channel.  The end that is
 passed to a domain is just an integer "port" uniquely identifying the
 event channel's local connection to that domain.  An example of this
 setup code is in linux-2.6.x/drivers/xen/blkfront/blkfront.c in
-blkif_status_change, which receives several status change events as
+blkif_connect(), which receives several status change events as
 the driver starts up.  It is passed an event channel end in a
 BLKIF_INTERFACE_STATUS_CONNECTED message, and patches it in like this:
 
@@ -136,7 +136,7 @@ static inline void flush_requests(void)
 }
 }}}
 
-notify_via_evtchn issues a hypercall to set the event waiting flag on
+notify_via_evtchn() issues a hypercall to set the event waiting flag on
 the other domain's end of the channel.
 
 ===== Communication Rings =====
@@ -151,9 +151,13 @@ used, they are however worth discussing as they are used in many
 places in the current code and are a useful model for communicating
 across a shared page.
 
-A shared page is set up by a guest first allocating and passing the
-address of a page in its own address space to the backend driver.  
+A shared page is set up by a front end guest first allocating and passing 
+the address of a page in its own address space to the backend driver.  
 
+Consider the following code, also from blkfront.c.  Note:  this code
+is in blkif_disconnect().  The driver transitions from STATE_CLOSED
+to STATE_DISCONNECTED before becoming CONNECTED.  The state automata
+is in blkif_status().
 
    blk_ring = (blkif_ring_t *)__get_free_page(GFP_KERNEL);
    blk_ring->req_prod = blk_ring->resp_prod = resp_cons = req_prod = 0;
@@ -222,20 +226,20 @@ is described by four pointers into a circular buffer of fixed-size
 records.  Pointers may only advance, and may not pass one another.
 
 
-                          rsp_cons----+
+                         resp_cons----+
                                       V
            +----+----+----+----+----+----+----+
-           |    |    |    free      |RSP1|RSP2|
+           |    |    |  free(A)     |RSP1|RSP2|
            +----+----+----+----+----+----+----+
  req_prod->|    |       -------->        |RSP3|
            +----+                        +----+
-           |REQ8|                        |    |<-rsp_prod
+           |REQ8|                        |    |<-resp_prod
            +----+                        +----+
            |REQ7|                        |    |
            +----+                        +----+
            |REQ6|       <--------        |    |
            +----+----+----+----+----+----+----+
-           |REQ5|REQ4|    free      |    |    |
+           |REQ5|REQ4|    free(B)   |    |    |
            +----+----+----+----+----+----+----+
   req_cons---------^
 
@@ -299,6 +303,10 @@ ring are simple.
    exclusive access to the messages between its consumer and producer,
    and should absolutely not read or write outside this region.
 
+   Thus the front end has exclusive access to the free(A) region 
+   in the figure above, and the back end driver has exclusive
+   access to the free(B) region.
+
 In general, drivers keep a private copy of their producer pointer and
 then set the shared version when they are ready for the other end to
 process a set of messages.  Additionally, it is worth paying attention