vscsiif: allow larger segments-per-request values

At least certain tape devices require fixed size blocks to be operated
upon, i.e. breaking up of I/O requests is not permitted. Consequently
we need an interface extension that (leaving aside implementation
limitations) doesn't impose a limit on the number of segments that can
be associated with an individual request.

This, in turn, excludes the blkif extension FreeBSD folks implemented,
as that still imposes an upper limit (the actual I/O request still
specifies the full number of segments - as an 8-bit quantity -, and
subsequent ring slots get used to carry the excess segment
descriptors).

The alternative therefore is to allow the frontend to pre-set segment
descriptors _before_ actually issuing the I/O request. I/O will then
be done by the backend for the accumulated set of segments.

To properly associate segment preset operations with the main request,
the rqid-s between them should match (originally I had hoped to use
this to avoid producing individual responses for the pre-set
operations, but that turned out to violate the underlying shared ring
implementation).

Negotiation of the maximum number of segments a particular backend
implementation supports happens through a new "segs-per-req" xenstore
node.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/include/public/io/vscsiif.h b/xen/include/public/io/vscsiif.h
index 76d96937084e28749a5ad05e2efe55a5b3a0f1dd..c84643a19c9486f18874a18bcb1b930a6b78bf55 100644 (file)
--- a/xen/include/public/io/vscsiif.h
+++ b/xen/include/public/io/vscsiif.h
@@ -34,6 +34,7 @@
 #define VSCSIIF_ACT_SCSI_CDB         1    /* SCSI CDB command */
 #define VSCSIIF_ACT_SCSI_ABORT       2    /* SCSI Device(Lun) Abort*/
 #define VSCSIIF_ACT_SCSI_RESET       3    /* SCSI Device(Lun) Reset*/
+#define VSCSIIF_ACT_SCSI_SG_PRESET   4    /* Preset SG elements */
 
 /*
  * Maximum scatter/gather segments per request.
@@ -50,6 +51,12 @@
 #define VSCSIIF_MAX_COMMAND_SIZE         16
 #define VSCSIIF_SENSE_BUFFERSIZE         96
 
+struct scsiif_request_segment {
+    grant_ref_t gref;
+    uint16_t offset;
+    uint16_t length;
+};
+typedef struct scsiif_request_segment vscsiif_segment_t;
 
 struct vscsiif_request {
     uint16_t rqid;          /* private guest value, echoed in resp  */
@@ -66,18 +73,26 @@ struct vscsiif_request {
                                          DMA_NONE(3) requests  */
     uint8_t nr_segments;              /* Number of pieces of scatter-gather */
 
-    struct scsiif_request_segment {
-        grant_ref_t gref;
-        uint16_t offset;
-        uint16_t length;
-    } seg[VSCSIIF_SG_TABLESIZE];
+    vscsiif_segment_t seg[VSCSIIF_SG_TABLESIZE];
     uint32_t reserved[3];
 };
 typedef struct vscsiif_request vscsiif_request_t;
 
+#define VSCSIIF_SG_LIST_SIZE ((sizeof(vscsiif_request_t) - 4) \
+                              / sizeof(vscsiif_segment_t))
+
+struct vscsiif_sg_list {
+    /* First two fields must match struct vscsiif_request! */
+    uint16_t rqid;          /* private guest value, must match main req */
+    uint8_t act;            /* VSCSIIF_ACT_SCSI_SG_PRESET */
+    uint8_t nr_segments;    /* Number of pieces of scatter-gather */
+    vscsiif_segment_t seg[VSCSIIF_SG_LIST_SIZE];
+};
+typedef struct vscsiif_sg_list vscsiif_sg_list_t;
+
 struct vscsiif_response {
     uint16_t rqid;
-    uint8_t padding;
+    uint8_t act;               /* valid only when backend supports SG_PRESET */
     uint8_t sense_len;
     uint8_t sense_buffer[VSCSIIF_SENSE_BUFFERSIZE];
     int32_t rslt;