blkif: add indirect descriptors interface to public headers

Indirect descriptors introduce a new block operation
(BLKIF_OP_INDIRECT) that passes grant references instead of segments
in the request. This grant references are filled with arrays of
blkif_request_segment_aligned, this way we can send more segments in a
request.

This interface is already implemented in Linux >= 3.11.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>

xen/include/public/io/blkif.h

@@ -467,6 +467,30 @@
  */
 #define BLKIF_OP_DISCARD           5
 
+/*
+ * Recognized if "feature-max-indirect-segments" in present in the backend
+ * xenbus info. The "feature-max-indirect-segments" node contains the maximum
+ * number of segments allowed by the backend per request. If the node is
+ * present, the frontend might use blkif_request_indirect structs in order to
+ * issue requests with more than BLKIF_MAX_SEGMENTS_PER_REQUEST (11). The
+ * maximum number of indirect segments is fixed by the backend, but the
+ * frontend can issue requests with any number of indirect segments as long as
+ * it's less than the number provided by the backend. The indirect_grefs field
+ * in blkif_request_indirect should be filled by the frontend with the
+ * grant references of the pages that are holding the indirect segments.
+ * This pages are filled with an array of blkif_request_segment_aligned
+ * that hold the information about the segments. The number of indirect
+ * pages to use is determined by the maximum number of segments
+ * an indirect request contains. Every indirect page can contain a maximum
+ * of 512 segments (PAGE_SIZE/sizeof(blkif_request_segment_aligned)),
+ * so to calculate the number of indirect pages to use we have to do
+ * ceil(indirect_segments/512).
+ *
+ * If a backend does not recognize BLKIF_OP_INDIRECT, it should *not*
+ * create the "feature-max-indirect-segments" node!
+ */
+#define BLKIF_OP_INDIRECT          6
+
 /*
  * Maximum scatter/gather segments per request.
  * This is carefully chosen so that sizeof(blkif_ring_t) <= PAGE_SIZE.
@@ -474,6 +498,11 @@
  */
 #define BLKIF_MAX_SEGMENTS_PER_REQUEST 11
 
+/*
+ * Maximum number of indirect pages to use per request.
+ */
+#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, are always expressed in 512-byte units.
@@ -517,6 +546,28 @@ struct blkif_request_discard {
 };
 typedef struct blkif_request_discard blkif_request_discard_t;
 
+struct blkif_request_indirect {
+    uint8_t        operation;    /* BLKIF_OP_INDIRECT                    */
+    uint8_t        indirect_op;  /* BLKIF_OP_{READ/WRITE}                */
+    uint16_t       nr_segments;  /* number of segments                   */
+    uint64_t       id;           /* private guest value, echoed in resp  */
+    blkif_sector_t sector_number;/* start sector idx on disk (r/w only)  */
+    blkif_vdev_t   handle;       /* same as for read/write requests      */
+    grant_ref_t    indirect_grefs[BLKIF_MAX_INDIRECT_PAGES_PER_REQUEST];
+#ifdef __i386__
+    uint64_t       pad;          /* Make it 64 byte aligned on i386      */
+#endif
+};
+typedef struct blkif_request_indirect blkif_request_indirect_t;
+
+struct blkif_request_segment_aligned {
+    grant_ref_t gref;            /* reference to I/O buffer frame        */
+    /* @first_sect: first sector in frame to transfer (inclusive).   */
+    /* @last_sect: last sector in frame to transfer (inclusive).     */
+    uint8_t     first_sect, last_sect;
+    uint16_t    _pad; /* padding to make it 8 bytes, so it's cache-aligned */
+};
+
 struct blkif_response {
     uint64_t        id;              /* copied from request */
     uint8_t         operation;       /* copied from request */