Module Name: src
Committed By: jym
Date: Mon Jul 25 00:06:49 UTC 2011
Modified Files:
src/sys/arch/xen/xen: xbd_xenbus.c
Log Message:
And... explain xbd(4).
To generate a diff of this commit:
cvs rdiff -u -r1.46 -r1.47 src/sys/arch/xen/xen/xbd_xenbus.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/arch/xen/xen/xbd_xenbus.c
diff -u src/sys/arch/xen/xen/xbd_xenbus.c:1.46 src/sys/arch/xen/xen/xbd_xenbus.c:1.47
--- src/sys/arch/xen/xen/xbd_xenbus.c:1.46 Mon Jul 25 00:02:38 2011
+++ src/sys/arch/xen/xen/xbd_xenbus.c Mon Jul 25 00:06:49 2011
@@ -1,4 +1,4 @@
-/* $NetBSD: xbd_xenbus.c,v 1.46 2011/07/25 00:02:38 jym Exp $ */
+/* $NetBSD: xbd_xenbus.c,v 1.47 2011/07/25 00:06:49 jym Exp $ */
/*
* Copyright (c) 2006 Manuel Bouyer.
@@ -25,8 +25,32 @@
*
*/
+/*
+ * The file contains the xbd frontend code required for block-level
+ * communications (similar to hard disks) between two Xen domains.
+ *
+ * We are not supposed to receive solicitations spontaneously from backend. The
+ * protocol is therefore fairly simple and uses only one ring to communicate
+ * with backend: frontend posts requests to the ring then wait for their
+ * replies asynchronously.
+ *
+ * xbd follows NetBSD's disk(9) convention. At any time, a LWP can schedule
+ * an operation request for the device (be it open(), read(), write(), ...).
+ * Calls are typically processed that way:
+ * - initiate request: xbdread/write/open/ioctl/..
+ * - depending on operation, it is handled directly by disk(9) subsystem or
+ * goes through physio(9) first.
+ * - the request is ultimately processed by xbdstart() that prepares the
+ * xbd requests, post them in the ring I/O queue, then signal the backend.
+ *
+ * When a response is available in the queue, the backend signals the frontend
+ * via its event channel. This triggers xbd_handler(), which will link back
+ * the response to its request through the request ID, and mark the I/O as
+ * completed.
+ */
+
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: xbd_xenbus.c,v 1.46 2011/07/25 00:02:38 jym Exp $");
+__KERNEL_RCSID(0, "$NetBSD: xbd_xenbus.c,v 1.47 2011/07/25 00:06:49 jym Exp $");
#include "opt_xen.h"
#include "rnd.h"
