diff --git a/xen/include/public/io/blkif.h b/xen/include/public/io/blkif.h
index 22f1eef0c0ca..9b00d633d372 100644
--- a/xen/include/public/io/blkif.h
+++ b/xen/include/public/io/blkif.h
@@ -237,12 +237,16 @@
  * sector-size
  *      Values:         <uint32_t>
  *
- *      The logical block size, in bytes, of the underlying storage. This
- *      must be a power of two with a minimum value of 512.
+ *      The logical block size, in bytes, of the underlying storage. This must
+ *      be a power of two with a minimum value of 512.  The sector size should
+ *      only be used for request segment length and alignment.
  *
- *      NOTE: Because of implementation bugs in some frontends this must be
- *            set to 512, unless the frontend advertizes a non-zero value
- *            in its "feature-large-sector-size" xenbus node. (See below).
+ *      When exposing a device that uses a logical sector size of 4096, the
+ *      only difference xenstore wise will be that 'sector-size' (and possibly
+ *      'physical-sector-size' if supported by the backend) will be 4096, but
+ *      the 'sectors' node will still be calculated using 512 byte units.  The
+ *      sector base units in the ring requests fields will all be 512 byte
+ *      based despite the logical sector size exposed in 'sector-size'.
  *
  * physical-sector-size
  *      Values:         <uint32_t>
@@ -254,9 +258,9 @@
  * sectors
  *      Values:         <uint64_t>
  *
- *      The size of the backend device, expressed in units of "sector-size".
- *      The product of "sector-size" and "sectors" must also be an integer
- *      multiple of "physical-sector-size", if that node is present.
+ *      The size of the backend device, expressed in units of 512b.  The
+ *      product of "sectors" * 512 must also be an integer multiple of
+ *      "physical-sector-size", if that node is present.
  *
  *****************************************************************************
  *                            Frontend XenBus Nodes
@@ -338,6 +342,7 @@
  * feature-large-sector-size
  *      Values:         0/1 (boolean)
  *      Default Value:  0
+ *      Notes:          DEPRECATED, 12
  *
  *      A value of "1" indicates that the frontend will correctly supply and
  *      interpret all sector-based quantities in terms of the "sector-size"
@@ -411,6 +416,11 @@
  *(10) The discard-secure property may be present and will be set to 1 if the
  *     backing device supports secure discard.
  *(11) Only used by Linux and NetBSD.
+ *(12) Possibly only ever implemented by the QEMU Qdisk backend and the Windows
+ *     PV block frontend.  Other backends and frontends supported 'sector-size'
+ *     values greater than 512 before such feature was added.  Frontends should
+ *     not expose this node, neither should backends make any decisions based
+ *     on it being exposed by the frontend.
  */
 
 /*
@@ -619,11 +629,14 @@
 #define BLKIF_MAX_INDIRECT_PAGES_PER_REQUEST 8
 
 /*
- * NB. 'first_sect' and 'last_sect' in blkif_request_segment, as well as
- * 'sector_number' in blkif_request, blkif_request_discard and
- * blkif_request_indirect are sector-based quantities. See the description
- * of the "feature-large-sector-size" frontend xenbus node above for
- * more information.
+ * NB. 'first_sect' and 'last_sect' in blkif_request_segment are all in units
+ * of 512 bytes, despite the 'sector-size' xenstore node possibly having a
+ * value greater than 512.
+ *
+ * The value in 'first_sect' and 'last_sect' fields must be setup so that the
+ * resulting segment offset and size is aligned to the logical sector size
+ * reported by the 'sector-size' xenstore node, see 'Backend Device Properties'
+ * section.
  */
 struct blkif_request_segment {
     grant_ref_t gref;        /* reference to I/O buffer frame        */
@@ -634,6 +647,10 @@ struct blkif_request_segment {
 
 /*
  * Starting ring element for any I/O request.
+ *
+ * The 'sector_number' field is in units of 512b, despite the value of the
+ * 'sector-size' xenstore node.  Note however that the offset in
+ * 'sector_number' must be aligned to 'sector-size'.
  */
 struct blkif_request {
     uint8_t        operation;    /* BLKIF_OP_???                         */
@@ -648,6 +665,10 @@ typedef struct blkif_request blkif_request_t;
 /*
  * Cast to this structure when blkif_request.operation == BLKIF_OP_DISCARD
  * sizeof(struct blkif_request_discard) <= sizeof(struct blkif_request)
+ *
+ * The 'sector_number' field is in units of 512b, despite the value of the
+ * 'sector-size' xenstore node.  Note however that the offset in
+ * 'sector_number' must be aligned to 'sector-size'.
  */
 struct blkif_request_discard {
     uint8_t        operation;    /* BLKIF_OP_DISCARD                     */
@@ -660,6 +681,11 @@ struct blkif_request_discard {
 };
 typedef struct blkif_request_discard blkif_request_discard_t;
 
+/*
+ * The 'sector_number' field is in units of 512b, despite the value of the
+ * 'sector-size' xenstore node.  Note however that the offset in
+ * 'sector_number' must be aligned to 'sector-size'.
+ */
 struct blkif_request_indirect {
     uint8_t        operation;    /* BLKIF_OP_INDIRECT                    */
     uint8_t        indirect_op;  /* BLKIF_OP_{READ/WRITE}                */
