scsi: core: Support allocating a pseudo SCSI device

Allocate a pseudo SCSI device if 'nr_reserved_cmds' has been set. Pseudo
SCSI devices have the SCSI ID <max_id>:U64_MAX so they won't clash with
any devices the LLD might create. Pseudo SCSI devices are excluded from
scanning and will not show up in sysfs. Additionally, pseudo SCSI
devices are skipped by shost_for_each_device(). This prevents that the
SCSI error handler tries to submit a reset to a non-existent logical
unit.

Do not allocate a budget map for pseudo SCSI devices since the
cmd_per_lun limit does not apply to pseudo SCSI devices.

Do not perform queue depth ramp up / ramp down for pseudo SCSI devices.

Pseudo SCSI devices will be used to send internal commands to a storage
device.

[ bvanassche: edited patch description / renamed host_sdev into
  pseudo_sdev / unexported scsi_get_host_dev() / modified error path in
  scsi_get_pseudo_dev() / skip pseudo devices in __scsi_iterate_devices()
  and also when calling sdev_init(), sdev_configure() and sdev_destroy().
  See also
  https://lore.kernel.org/linux-scsi/20211125151048.103910-2-hare@suse.de/ ]

Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://patch.msgid.link/20251031204029.2883185-5-bvanassche@acm.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

1 files changed, 66 insertions, 1 deletions

diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
index de039efef290..7acbfcfc2172 100644
--- a/drivers/scsi/scsi_scan.c
+++ b/drivers/scsi/scsi_scan.c
@@ -349,6 +349,9 @@ static struct scsi_device *scsi_alloc_sdev(struct scsi_target *starget,
 
 	scsi_sysfs_device_initialize(sdev);
 
+	if (scsi_device_is_pseudo_dev(sdev))
+		return sdev;
+
 	depth = sdev->host->cmd_per_lun ?: 1;
 
 	/*
@@ -1070,6 +1073,9 @@ static int scsi_add_lun(struct scsi_device *sdev, unsigned char *inq_result,
 
 	sdev->sdev_bflags = *bflags;
 
+	if (scsi_device_is_pseudo_dev(sdev))
+		return SCSI_SCAN_LUN_PRESENT;
+
 	/*
 	 * No need to freeze the queue as it isn't reachable to anyone else yet.
 	 */
@@ -1213,6 +1219,12 @@ static int scsi_probe_and_add_lun(struct scsi_target *starget,
 	if (!sdev)
 		goto out;
 
+	if (scsi_device_is_pseudo_dev(sdev)) {
+		if (bflagsp)
+			*bflagsp = BLIST_NOLUN;
+		return SCSI_SCAN_LUN_PRESENT;
+	}
+
 	result = kmalloc(result_len, GFP_KERNEL);
 	if (!result)
 		goto out_free_sdev;
@@ -2084,12 +2096,65 @@ void scsi_forget_host(struct Scsi_Host *shost)
  restart:
 	spin_lock_irqsave(shost->host_lock, flags);
 	list_for_each_entry(sdev, &shost->__devices, siblings) {
-		if (sdev->sdev_state == SDEV_DEL)
+		if (scsi_device_is_pseudo_dev(sdev) ||
+		    sdev->sdev_state == SDEV_DEL)
 			continue;
 		spin_unlock_irqrestore(shost->host_lock, flags);
 		__scsi_remove_device(sdev);
 		goto restart;
 	}
 	spin_unlock_irqrestore(shost->host_lock, flags);
+
+	/*
+	 * Remove the pseudo device last since it may be needed during removal
+	 * of other SCSI devices.
+	 */
+	if (shost->pseudo_sdev)
+		__scsi_remove_device(shost->pseudo_sdev);
 }
 
+/**
+ * scsi_get_pseudo_sdev() - Attach a pseudo SCSI device to a SCSI host
+ * @shost: Host that needs a pseudo SCSI device
+ *
+ * Lock status: None assumed.
+ *
+ * Returns:     The scsi_device or NULL
+ *
+ * Notes:
+ *	Attach a single scsi_device to the Scsi_Host. The primary aim for this
+ *	device is to serve as a container from which SCSI commands can be
+ *	allocated. Each SCSI command will carry a command tag allocated by the
+ *	block layer. These SCSI commands can be used by the LLDD to send
+ *	internal or passthrough commands without having to manage tag allocation
+ *	inside the LLDD.
+ */
+struct scsi_device *scsi_get_pseudo_sdev(struct Scsi_Host *shost)
+{
+	struct scsi_device *sdev = NULL;
+	struct scsi_target *starget;
+
+	guard(mutex)(&shost->scan_mutex);
+
+	if (!scsi_host_scan_allowed(shost))
+		goto out;
+
+	starget = scsi_alloc_target(&shost->shost_gendev, 0, shost->max_id);
+	if (!starget)
+		goto out;
+
+	sdev = scsi_alloc_sdev(starget, U64_MAX, NULL);
+	if (!sdev) {
+		scsi_target_reap(starget);
+		goto put_target;
+	}
+
+	sdev->borken = 0;
+
+put_target:
+	/* See also the get_device(dev) call in scsi_alloc_target(). */
+	put_device(&starget->dev);
+
+out:
+	return sdev;
+}