nvmet: support fabrics sq flow control

Technical proposal 8005 "fabrics SQ flow control" introduces a mode
where a host and controller agree to omit sq_head pointer updates
when sending nvme completions.

In case the host indicated desire to operate in this mode (connect attribute)
the controller will return back a connect completion with sq_head value
of 0xffff as indication that it will omit sq_head pointer updates.

This mode saves us an atomic update in the I/O path.
Reviewed-by: Hannes Reinecke <hare@suse.com>
[hch: suggested better implementation]
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/nvme/target/core.c

@@ -597,26 +597,28 @@ struct nvmet_ns *nvmet_ns_alloc(struct nvmet_subsys *subsys, u32 nsid)
 	return ns;
 }
 
-static void __nvmet_req_complete(struct nvmet_req *req, u16 status)
+static void nvmet_update_sq_head(struct nvmet_req *req)
 {
+	if (req->sq->size) {
 		u32 old_sqhd, new_sqhd;
-	u16 sqhd;
 
-	if (status)
-		nvmet_set_status(req, status);
-
-	if (req->sq->size) {
 		do {
 			old_sqhd = req->sq->sqhd;
 			new_sqhd = (old_sqhd + 1) % req->sq->size;
 		} while (cmpxchg(&req->sq->sqhd, old_sqhd, new_sqhd) !=
 					old_sqhd);
 	}
-	sqhd = req->sq->sqhd & 0x0000FFFF;
-	req->rsp->sq_head = cpu_to_le16(sqhd);
+	req->rsp->sq_head = cpu_to_le16(req->sq->sqhd & 0x0000FFFF);
+}
+
+static void __nvmet_req_complete(struct nvmet_req *req, u16 status)
+{
+	if (!req->sq->sqhd_disabled)
+		nvmet_update_sq_head(req);
 	req->rsp->sq_id = cpu_to_le16(req->sq->qid);
 	req->rsp->command_id = req->cmd->common.command_id;
-
+	if (status)
+		nvmet_set_status(req, status);
 	if (req->ns)
 		nvmet_put_namespace(req->ns);
 	req->ops->queue_response(req);
@@ -765,6 +767,7 @@ bool nvmet_req_init(struct nvmet_req *req, struct nvmet_cq *cq,
 	req->sg_cnt = 0;
 	req->transfer_len = 0;
 	req->rsp->status = 0;
+	req->rsp->sq_head = 0;
 	req->ns = NULL;
 
 	/* no support for fused commands yet */

drivers/nvme/target/fabrics-cmd.c

@@ -115,6 +115,12 @@ static u16 nvmet_install_queue(struct nvmet_ctrl *ctrl, struct nvmet_req *req)
 	/* note: convert queue size from 0's-based value to 1's-based value */
 	nvmet_cq_setup(ctrl, req->cq, qid, sqsize + 1);
 	nvmet_sq_setup(ctrl, req->sq, qid, sqsize + 1);
+
+	if (c->cattr & NVME_CONNECT_DISABLE_SQFLOW) {
+		req->sq->sqhd_disabled = true;
+		req->rsp->sq_head = cpu_to_le16(0xffff);
+	}
+
 	return 0;
 }
 

drivers/nvme/target/nvmet.h

@@ -106,6 +106,7 @@ struct nvmet_sq {
 	u16			qid;
 	u16			size;
 	u32			sqhd;
+	bool			sqhd_disabled;
 	struct completion	free_done;
 	struct completion	confirm_done;
 };

include/linux/nvme.h

@@ -1044,6 +1044,10 @@ struct nvmf_disc_rsp_page_hdr {
 	struct nvmf_disc_rsp_page_entry entries[0];
 };
 
+enum {
+	NVME_CONNECT_DISABLE_SQFLOW	= (1 << 2),
+};
+
 struct nvmf_connect_command {
 	__u8		opcode;
 	__u8		resv1;