scsi: core: Don't memset() the entire scsi_cmnd in scsi_init_command()

Replace the big fat memset that requires saving and restoring various
fields with just initializing those fields that need initialization.

All the clearing to 0 is moved to scsi_prepare_cmd() as scsi_ioctl_reset()
alreadly uses kzalloc() to allocate a pre-zeroed command.

This is still conservative and can probably be optimized further.

Link: https://lore.kernel.org/r/20220224175552.988286-3-hch@lst.de
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_lib.c

@@ -1163,45 +1163,16 @@ static void scsi_cleanup_rq(struct request *rq)
 /* Called before a request is prepared. See also scsi_mq_prep_fn(). */
 void scsi_init_command(struct scsi_device *dev, struct scsi_cmnd *cmd)
 {
-	void *buf = cmd->sense_buffer;
-	void *prot = cmd->prot_sdb;
 	struct request *rq = scsi_cmd_to_rq(cmd);
-	unsigned int flags = cmd->flags & SCMD_PRESERVED_FLAGS;
-	unsigned long jiffies_at_alloc;
-	int retries, to_clear;
-	bool in_flight;
-	int budget_token = cmd->budget_token;
 
-	if (!blk_rq_is_passthrough(rq) && !(flags & SCMD_INITIALIZED)) {
-		flags |= SCMD_INITIALIZED;
+	if (!blk_rq_is_passthrough(rq) && !(cmd->flags & SCMD_INITIALIZED)) {
+		cmd->flags |= SCMD_INITIALIZED;
 		scsi_initialize_rq(rq);
 	}
 
-	jiffies_at_alloc = cmd->jiffies_at_alloc;
-	retries = cmd->retries;
-	in_flight = test_bit(SCMD_STATE_INFLIGHT, &cmd->state);
-	/*
-	 * Zero out the cmd, except for the embedded scsi_request. Only clear
-	 * the driver-private command data if the LLD does not supply a
-	 * function to initialize that data.
-	 */
-	to_clear = sizeof(*cmd) - sizeof(cmd->req);
-	if (!dev->host->hostt->init_cmd_priv)
-		to_clear += dev->host->hostt->cmd_size;
-	memset((char *)cmd + sizeof(cmd->req), 0, to_clear);
-
 	cmd->device = dev;
-	cmd->sense_buffer = buf;
-	cmd->prot_sdb = prot;
-	cmd->flags = flags;
 	INIT_LIST_HEAD(&cmd->eh_entry);
 	INIT_DELAYED_WORK(&cmd->abort_work, scmd_eh_abort_handler);
-	cmd->jiffies_at_alloc = jiffies_at_alloc;
-	cmd->retries = retries;
-	if (in_flight)
-		__set_bit(SCMD_STATE_INFLIGHT, &cmd->state);
-	cmd->budget_token = budget_token;
-
 }
 
 static blk_status_t scsi_setup_scsi_cmnd(struct scsi_device *sdev,
@@ -1586,10 +1557,35 @@ static blk_status_t scsi_prepare_cmd(struct request *req)
 	struct scsi_cmnd *cmd = blk_mq_rq_to_pdu(req);
 	struct scsi_device *sdev = req->q->queuedata;
 	struct Scsi_Host *shost = sdev->host;
+	bool in_flight = test_bit(SCMD_STATE_INFLIGHT, &cmd->state);
 	struct scatterlist *sg;
 
 	scsi_init_command(sdev, cmd);
 
+	cmd->eh_eflags = 0;
+	cmd->allowed = 0;
+	cmd->prot_type = 0;
+	cmd->prot_flags = 0;
+	cmd->submitter = 0;
+	cmd->cmd_len = 0;
+	cmd->cmnd = NULL;
+	memset(&cmd->sdb, 0, sizeof(cmd->sdb));
+	cmd->underflow = 0;
+	cmd->transfersize = 0;
+	cmd->host_scribble = NULL;
+	cmd->result = 0;
+	cmd->extra_len = 0;
+	cmd->state = 0;
+	if (in_flight)
+		__set_bit(SCMD_STATE_INFLIGHT, &cmd->state);
+
+	/*
+	 * Only clear the driver-private command data if the LLD does not supply
+	 * a function to initialize that data.
+	 */
+	if (!shost->hostt->init_cmd_priv)
+		memset(cmd + 1, 0, shost->hostt->cmd_size);
+
 	cmd->prot_op = SCSI_PROT_NORMAL;
 	if (blk_rq_bytes(req))
 		cmd->sc_data_direction = rq_dma_dir(req);