[PATCH 3/4] virtio-blk: avoid function call in the fast path

[Chaitanya Kulkarni <kch@nvidia.com>]

We can avoid a function call virtblk_map_data() in the fast path if
block layer request has no physical segments by moving the call
blk_rq_nr_phys_segments() from virtblk_map_data() to virtio_queue_rq().

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
---
 drivers/block/virtio_blk.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index d038800474c2..74c3a48cd1e5 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -178,9 +178,6 @@ static int virtblk_map_data(struct blk_mq_hw_ctx *hctx, struct request *req,
 {
 	int err;
 
-	if (!blk_rq_nr_phys_segments(req))
-		return 0;
-
 	vbr->sg_table.sgl = vbr->sg;
 	err = sg_alloc_table_chained(&vbr->sg_table,
 				     blk_rq_nr_phys_segments(req),
@@ -303,7 +300,7 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
 	struct request *req = bd->rq;
 	struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
 	unsigned long flags;
-	int num;
+	int num = 0;
 	int qid = hctx->queue_num;
 	bool notify = false;
 	blk_status_t status;
@@ -315,10 +312,12 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
 
 	blk_mq_start_request(req);
 
-	num = virtblk_map_data(hctx, req, vbr);
-	if (unlikely(num < 0)) {
-		virtblk_cleanup_cmd(req);
-		return BLK_STS_RESOURCE;
+	if (blk_rq_nr_phys_segments(req)) {
+		num = virtblk_map_data(hctx, req, vbr);
+		if (unlikely(num < 0)) {
+			virtblk_cleanup_cmd(req);
+			return BLK_STS_RESOURCE;
+		}
 	}
 
 	spin_lock_irqsave(&vblk->vqs[qid].lock, flags);
-- 
2.29.0