xen/blkfront: Implement FUA with BLKIF_OP_WRITE_BARRIER

The BLKIF_OP_WRITE_BARRIER is a full ordered barrier, so we can use it
to implement FUA as well as a plain FLUSH.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Christoph Hellwig <hch@lst.de>

1 files changed, 12 insertions, 2 deletions

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 31c8a643d109..76b874a79175 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -286,8 +286,18 @@ static int blkif_queue_request(struct request *req)
 
         ring_req->operation = rq_data_dir(req) ?
                 BLKIF_OP_WRITE : BLKIF_OP_READ;
-        if (req->cmd_flags & REQ_FLUSH)
+
+        if (req->cmd_flags & (REQ_FLUSH | REQ_FUA)) {
+                /*
+                 * Ideally we could just do an unordered
+                 * flush-to-disk, but all we have is a full write
+                 * barrier at the moment.  However, a barrier write is
+                 * a superset of FUA, so we can implement it the same
+                 * way.  (It's also a FLUSH+FUA, since it is
+                 * guaranteed ordered WRT previous writes.)
+                 */
                 ring_req->operation = BLKIF_OP_WRITE_BARRIER;
+        }
 
         ring_req->nr_segments = blk_rq_map_sg(req->q, req, info->sg);
         BUG_ON(ring_req->nr_segments > BLKIF_MAX_SEGMENTS_PER_REQUEST);
@@ -1065,7 +1075,7 @@ static void blkfront_connect(struct blkfront_info *info)
         info->feature_flush = 0;
 
         if (!err && barrier)
-                info->feature_flush = REQ_FLUSH;
+                info->feature_flush = REQ_FLUSH | REQ_FUA;
 
         err = xlvbd_alloc_gendisk(sectors, info, binfo, sector_size);
         if (err) {