blkif: Clarify units for 'sector'-sized blkif request params.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

diff --git a/xen/include/public/io/blkif.h b/xen/include/public/io/blkif.h
index 2380066b8cb53aa8b106fb8bbe2176c644d49a19..3e0c5abae3957dada775027ed288a86f9b709f9e 100644 (file)
--- a/xen/include/public/io/blkif.h
+++ b/xen/include/public/io/blkif.h
@@ -84,6 +84,13 @@
  */
 #define BLKIF_MAX_SEGMENTS_PER_REQUEST 11
 
+/* 
+ * NB. first_sect and last_sect in blkif_request_segment, as well as
+ * sector_number in blkif_request, are always expressed in 512-byte units.
+ * However they must be properly aligned to the real sector size of the
+ * physical disk, which is reported in the "sector-size" node in the backend
+ * xenbus info. Also the xenbus "sectors" node is expressed in 512-byte units.
+ */
 struct blkif_request_segment {
     grant_ref_t gref;        /* reference to I/O buffer frame        */
     /* @first_sect: first sector in frame to transfer (inclusive).   */