CP-46757: advertise feature-flush-cache

Windows guests do not request BLKIF_OP_WRITE_BARRIER and only support
BLKIF_OP_FLUSH_DISKCACHE, so advertise and support that to allow for
explicit cache and data integrity management.

Signed-off-by: Mark Syms <mark.syms@citrix.com>

Author: MarkSymsCtx

drivers/td-ctx.c

@@ -230,7 +230,8 @@ __xenio_blkif_get_requests(struct td_xenblkif * const blkif,
 
         xenio_blkif_get_request(blkif, dst, rc);
 
-		if (unlikely(dst->operation == BLKIF_OP_WRITE_BARRIER))
+		if (unlikely(dst->operation == BLKIF_OP_WRITE_BARRIER ||
+			     dst->operation == BLKIF_OP_FLUSH_DISKCACHE))
 			barrier = true;
     }
 
@@ -278,7 +279,8 @@ xenio_blkif_get_requests(struct td_xenblkif * const blkif,
 
         n += __xenio_blkif_get_requests(blkif, reqs + n, count - n);
 
-		if (unlikely(n && reqs[(n - 1)]->operation == BLKIF_OP_WRITE_BARRIER))
+		if (unlikely(n && (reqs[(n - 1)]->operation == BLKIF_OP_WRITE_BARRIER ||
+				   reqs[(n - 1)]->operation == BLKIF_OP_FLUSH_DISKCACHE)))
 			break;
 
     } while (1);
@@ -327,8 +329,8 @@ tapdisk_xenio_ctx_process_ring(struct td_xenblkif *blkif,
 		limit -= n_reqs;
         final = 1;
 
-		if (unlikely(reqs[(n_reqs - 1)]->operation ==
-					BLKIF_OP_WRITE_BARRIER)) {
+		if (unlikely(reqs[(n_reqs - 1)]->operation == BLKIF_OP_WRITE_BARRIER ||
+			     reqs[(n_reqs - 1)]->operation == BLKIF_OP_FLUSH_DISKCACHE)) {
 			ASSERT(!blkif->barrier.msg);
 			blkif->barrier.msg = reqs[(n_reqs - 1)];
 			blkif->barrier.io_done = false;

drivers/td-req.c

@@ -467,8 +467,9 @@ tapdisk_xenblkif_complete_request(struct td_xenblkif * const blkif,
 
 	depth++;
 
-	processing_barrier_message =
-		tapreq->msg.operation == BLKIF_OP_WRITE_BARRIER;
+	processing_barrier_message = (
+		tapreq->msg.operation == BLKIF_OP_WRITE_BARRIER ||
+		tapreq->msg.operation == BLKIF_OP_FLUSH_DISKCACHE);
 
 	/*
 	 * If a barrier request completes, check whether it's an I/O completion
@@ -763,6 +764,7 @@ tapdisk_xenblkif_make_vbd_request(struct td_xenblkif * const blkif,
         break;
     case BLKIF_OP_WRITE:
     case BLKIF_OP_WRITE_BARRIER:
+    case BLKIF_OP_FLUSH_DISKCACHE:
         if (likely(blkif->stats.xenvbd))
 			blkif->stats.xenvbd->st_wr_req++;
 	if (likely(blkif->vbd_stats.stats))
@@ -783,7 +785,8 @@ tapdisk_xenblkif_make_vbd_request(struct td_xenblkif * const blkif,
      * Check that the number of segments is sane.
      */
     if (unlikely((tapreq->msg.nr_segments == 0 &&
-                tapreq->msg.operation != BLKIF_OP_WRITE_BARRIER) ||
+                (tapreq->msg.operation != BLKIF_OP_WRITE_BARRIER &&
+                 tapreq->msg.operation != BLKIF_OP_FLUSH_DISKCACHE)) ||
             tapreq->msg.nr_segments > BLKIF_MMAX_SEGMENTS_PER_REQUEST)) {
         RING_ERR(blkif, "req %lu: bad number of segments in request (%d)\n",
                 tapreq->msg.id, tapreq->msg.nr_segments);

tapback/frontend.c

@@ -300,6 +300,12 @@ connect_frontend(vbd_t *device) {
             break;
         }
 
+	if ((err = tapback_device_printf(device, xst, "feature-flush-cache", true,
+                        "%d", 1))) {
+		WARN(device, "failed to write feature-flush-cache: %s\n", strerror(-err));
+		break;
+        }
+
         if ((err = tapback_device_printf(device, xst, "sector-size", true,
                         "%u", device->sector_size))) {
             WARN(device, "failed to write sector-size: %s\n", strerror(-err));