target/rd: Add ramdisk bit for NULLIO operation

This patch adds a rd_nullio parameter that allows RAMDISK_MCP backends
to function in NULLIO mode, where all se_cmd I/O is immediately completed
in rd_execute_rw() without actually performing the SGL memory copy.

This is useful for performance testing when the ramdisk SGL memory copy
begins to eat lots of cycles during heavy small block workloads, so allow
this bit to be enabled when necessary on a per rd_dev basis.

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_rd.c

@@ -291,6 +291,11 @@ rd_execute_rw(struct se_cmd *cmd)
 	u32 src_len;
 	u64 tmp;
 
+	if (dev->rd_flags & RDF_NULLIO) {
+		target_complete_cmd(cmd, SAM_STAT_GOOD);
+		return 0;
+	}
+
 	tmp = cmd->t_task_lba * se_dev->dev_attrib.block_size;
 	rd_offset = do_div(tmp, PAGE_SIZE);
 	rd_page = tmp;
@@ -373,11 +378,12 @@ rd_execute_rw(struct se_cmd *cmd)
 }
 
 enum {
-	Opt_rd_pages, Opt_err
+	Opt_rd_pages, Opt_rd_nullio, Opt_err
 };
 
 static match_table_t tokens = {
 	{Opt_rd_pages, "rd_pages=%d"},
+	{Opt_rd_nullio, "rd_nullio=%d"},
 	{Opt_err, NULL}
 };
 
@@ -408,6 +414,14 @@ static ssize_t rd_set_configfs_dev_params(struct se_device *dev,
 				" Count: %u\n", rd_dev->rd_page_count);
 			rd_dev->rd_flags |= RDF_HAS_PAGE_COUNT;
 			break;
+		case Opt_rd_nullio:
+			match_int(args, &arg);
+			if (arg != 1)
+				break;
+
+			pr_debug("RAMDISK: Setting NULLIO flag: %d\n", arg);
+			rd_dev->rd_flags |= RDF_NULLIO;
+			break;
 		default:
 			break;
 		}
@@ -424,8 +438,9 @@ static ssize_t rd_show_configfs_dev_params(struct se_device *dev, char *b)
 	ssize_t bl = sprintf(b, "TCM RamDisk ID: %u  RamDisk Makeup: rd_mcp\n",
 			rd_dev->rd_dev_id);
 	bl += sprintf(b + bl, "        PAGES/PAGE_SIZE: %u*%lu"
-			"  SG_table_count: %u\n", rd_dev->rd_page_count,
-			PAGE_SIZE, rd_dev->sg_table_count);
+			"  SG_table_count: %u  nullio: %d\n", rd_dev->rd_page_count,
+			PAGE_SIZE, rd_dev->sg_table_count,
+			!!(rd_dev->rd_flags & RDF_NULLIO));
 	return bl;
 }
 

drivers/target/target_core_rd.h

@@ -22,6 +22,7 @@ struct rd_dev_sg_table {
 } ____cacheline_aligned;
 
 #define RDF_HAS_PAGE_COUNT	0x01
+#define RDF_NULLIO		0x02
 
 struct rd_dev {
 	struct se_device dev;