[1/2] scsi:core:Add sysfs interface to control if skip lun with PQ=1

commit 948e922fc4461 ("scsi: core: map PQ=1, PDT=other values to
SCSI_SCAN_TARGET_PRESENT") returns SCSI_SCAN_TARGET_PRESENT if inquiry
returns PQ=1.

According to the SPC, PQ=1 means the addressed logical unit having the
indicated device type is not accessible, it does not mean the addressed
logical unit is invalid. We still can map this lun to an sg device.

In some conditions, we do not want to skip these devices, for example
with iSCSI:

When iSCSI initiator logged in target, the target attached none valid
lun but lun0. lun0 is not an valid disk, while it would response
inquiry command with PQ=1 and other general scsi commands like probe lun.
The others luns of target is added/removed dynamicly.

We want the lun0 to be mapped to an sg device in initiator, so we can
probe luns of target based on lun0.

I add an sysfs interface named no_skip_pq1 in each Scsi_Host to
control if to skip lun which return PQ=1 for inquiry.

The default behavior is not changed, which means we would still skip
add lun if inquiry returns PQ=1. We can set host's no_skip_pq1  in
specific drivers or via sysfs.

Signed-off-by: Wenchao Hao <haowenchao@huawei.com>
---
 drivers/scsi/scsi_scan.c  |  9 ++++++---
 drivers/scsi/scsi_sysfs.c | 29 +++++++++++++++++++++++++++++
 include/scsi/scsi_host.h  |  3 +++
 3 files changed, 38 insertions(+), 3 deletions(-)

On 12/14/22 1:08 AM, Wenchao Hao wrote:
> 
> When iSCSI initiator logged in target, the target attached none valid
> lun but lun0. lun0 is not an valid disk, while it would response
> inquiry command with PQ=1 and other general scsi commands like probe lun.
> The others luns of target is added/removed dynamicly.
> 
> We want the lun0 to be mapped to an sg device in initiator, so we can
> probe luns of target based on lun0.

What do you want to do exactly? Is the idea with your patch we would create
an sg device, then in userpsace you would do some scan related commands. If
you find devices then you use sysfs to have scsi-ml scan/add a specific device
like

echo 5 0 0 8 > host5/scan

?

It's not really clear to me why you need the sg device, and can't just do?

echo - - - > .../hostN/scan

? Do you only want to add specific devices like you are doing some sort of
LUN masking on the initiator side?

Is the issue that you need the sg device there, so you can detect when a device
is no longer present on the target and then you will have userspace remove the
device via the sysfs delete file?

On 2022/12/19 5:37, Mike Christie wrote:
> On 12/14/22 1:08 AM, Wenchao Hao wrote:
>>
>> When iSCSI initiator logged in target, the target attached none valid
>> lun but lun0. lun0 is not an valid disk, while it would response
>> inquiry command with PQ=1 and other general scsi commands like probe lun.
>> The others luns of target is added/removed dynamicly.
>>
>> We want the lun0 to be mapped to an sg device in initiator, so we can
>> probe luns of target based on lun0.
> 
> What do you want to do exactly? Is the idea with your patch we would create
> an sg device, then in userpsace you would do some scan related commands. If
> you find devices then you use sysfs to have scsi-ml scan/add a specific device
> like
> 
> echo 5 0 0 8 > host5/scan
> 
> ?
> 
> It's not really clear to me why you need the sg device, and can't just do?
> 
> echo - - - > .../hostN/scan
>
We do not directly echo - - - > .../hostN/scan. Instead, we use rescan-scsi-bus.sh
defined in sg3_utils to scan luns. If no sg device mapped to this host, the default
scan operations could not recognize luns.

We can simulate this with scsi_debug by following steps:

1. build scsi debug with following changes;

diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
index 8553277effb3..36dcdd2c3fe4 100644
--- a/drivers/scsi/scsi_debug.c
+++ b/drivers/scsi/scsi_debug.c
@@ -836,6 +836,7 @@ static int dif_errors;
 static bool sdeb_zbc_in_use;   /* true for host-aware and host-managed disks */
 static int sdeb_zbc_zone_cap_mb;
 static int sdeb_zbc_zone_size_mb;
+static int sdeb_pq_type;
 static int sdeb_zbc_max_open = DEF_ZBC_MAX_OPEN_ZONES;
 static int sdeb_zbc_nr_conv = DEF_ZBC_NR_CONV_ZONES;
 
@@ -1583,6 +1584,7 @@ static int resp_inquiry(struct scsi_cmnd *scp, struct sdebug_dev_info *devip)
        u32 alloc_len, n;
        int ret;
        bool have_wlun, is_disk, is_zbc, is_disk_zbc;
+       struct scsi_device *sdev = scp->device;
 
        alloc_len = get_unaligned_be16(cmd + 3);
        arr = kzalloc(SDEBUG_MAX_INQ_ARR_SZ, GFP_ATOMIC);
@@ -1598,7 +1600,11 @@ static int resp_inquiry(struct scsi_cmnd *scp, struct sdebug_dev_info *devip)
                pq_pdt = 0x7f;  /* not present, PQ=3, PDT=0x1f */
        else
                pq_pdt = (sdebug_ptype & 0x1f);
-       arr[0] = pq_pdt;
+       if (sdev->lun == 0) {
+               arr[0] = pq_pdt | (sdeb_pq_type << 5);
+       } else {
+               arr[0] = pq_pdt;
+       }
        if (0x2 & cmd[1]) {  /* CMDDT bit set */
                mk_sense_invalid_fld(scp, SDEB_IN_CDB, 1, 1);
                kfree(arr);
@@ -5883,6 +5889,7 @@ module_param_named(zone_cap_mb, sdeb_zbc_zone_cap_mb, int, S_IRUGO);
 module_param_named(zone_max_open, sdeb_zbc_max_open, int, S_IRUGO);
 module_param_named(zone_nr_conv, sdeb_zbc_nr_conv, int, S_IRUGO);
 module_param_named(zone_size_mb, sdeb_zbc_zone_size_mb, int, S_IRUGO);
+module_param_named(pq_type, sdeb_pq_type, int, S_IRUGO);
 
 MODULE_AUTHOR("Eric Youngdale + Douglas Gilbert");
 MODULE_DESCRIPTION("SCSI debug adapter driver");
@@ -7825,6 +7832,7 @@ static int sdebug_driver_probe(struct device *dev)
                error = -ENODEV;
                return error;
        }
+       //hpnt->no_skip_pq1 = 1;
        if (submit_queues > nr_cpu_ids) {
                pr_warn("%s: trim submit_queues (was %d) to nr_cpu_ids=%u\n",
                        my_name, submit_queues, nr_cpu_ids);

2. insmod scsi_debug with modparam max_luns=4 pq_type=1, we would get following scsi devices

# lsscsi -g
[0:0:0:1]    disk    Linux    scsi_debug       0191  /dev/sda   /dev/sg0 
[0:0:0:2]    disk    Linux    scsi_debug       0191  /dev/sdb   /dev/sg1 
[0:0:0:3]    disk    Linux    scsi_debug       0191  /dev/sdc   /dev/sg2 

3. remove the scsi_debug disks

echo 1 > /sys/block/sda/device/delete
echo 1 > /sys/block/sdb/device/delete
echo 1 > /sys/block/sdc/device/delete

4. scan with rescan-scsi-bus.sh would failed to discovery luns.

# rescan-scsi-bus.sh 
Scanning SCSI subsystem for new devices
Scanning host 0 for  SCSI target IDs 0 1 2 3 4 5 6 7, all LUNs
0 new or changed device(s) found.          
0 remapped or resized device(s) found.
0 device(s) removed.

Did not add the luns. If do not comment the line hpnt->no_skip_pq1 = 1 in
scsi_debug.c, the scan operations would found these luns succeed.

> ? Do you only want to add specific devices like you are doing some sort of
> LUN masking on the initiator side?
> 
> Is the issue that you need the sg device there, so you can detect when a device
> is no longer present on the target and then you will have userspace remove the
> device via the sysfs delete file?
> .

Yes, this is one scenarios. luns on the target are removed or added dynamically,
the only fixed one is LUN0 which response PQ=1.

If luns are removed from target after been added by initiator, we could know which
one should be removed from initiator via LUN0.

We can think LUN0 with PQ=1 is a dummy lun, which is fixed and can tell luns info on
the target.

According to the SPC, PQ=1 means the addressed logical unit having the
indicated device type is not accessible, it does not mean the addressed
logical unit is invalid. So I think we still can map this lun to an sg device.