Question about virtio-scsi max_segments setting

[Lin Ma]

Hi all,

We know that the seg_max of virtio-scsi respects to virtqueue size:
scsiconf->seg_max is set to virtqueue size - 2.

Some of my scsi HBAs have max_segments as low as 64 in host(max_sectors_kb is 
256).
When I use scsi lun passthrough disk in guest, In case of default
virtqueue size(256), The max_segments of the disk in guest is 254.

In this scenario:
host:~ # sg_vpd --page=bl /dev/sdb
Block limits VPD page (SBC):
  Write same non-zero (WSNZ): 0
  Maximum compare and write length: 0 blocks [Command not implemented]
  Optimal transfer length granularity: 512 blocks
  Maximum transfer length: 512 blocks
  Optimal transfer length: 512 blocks

host:~ # cat /sys/block/sdb/queue/max_sectors_kb
256
host:~ # cat /sys/block/sdb/queue/max_hw_sectors_kb
256
host:~ # cat /sys/block/sdb/queue/max_segments
64
host:~ # cat /sys/block/sdb/queue/max_hw_sectors_kb
256

guest:~ # sg_vpd --page=bl /dev/sdb
Block limits VPD page (SBC):
  Write same non-zero (WSNZ): 0
  Maximum compare and write length: 0 blocks [Command not implemented]
  Optimal transfer length granularity: 512 blocks
  Maximum transfer length: 512 blocks
  Optimal transfer length: 512 blocks

guest:~ # cat /sys/block/sdb/queue/max_sectors_kb 
256
guest:~ # cat /sys/block/sdb/queue/max_hw_sectors_kb
32767
guest:~ # cat /sys/block/sdb/queue/max_segments
254
guest:~ # cat /sys/block/sdb/queue/max_segment_size
65536

Performing "mkfs.xfs -f /dev/sdb" in guest triggers I/O errors.
The error message in stdout:
mkfs.xfs: libxfs_device_zero write failed: Remote I/O error

And the error message in dmesg:
[  887.867763] sd 6:0:0:2: [sdb] tag#147 FAILED Result: hostbyte=DID_OK 
driverbyte=DRIVER_SENSE
[  887.867766] sd 6:0:0:2: [sdb] tag#147 Sense Key : Illegal Request [current] 
[  887.867773] sd 6:0:0:2: [sdb] tag#147 Add. Sense: Invalid field in cdb
[  887.867775] sd 6:0:0:2: [sdb] tag#147 CDB: Write(10) 2a 00 0f a3 00 40 00 02 
00 00
[  887.867778] blk_update_request: critical target error, dev sdb, sector 
262340672 op 0x1:(WRITE) flags 0x8800 phys_seg 65 prio class 0


I found 2 ways to workaround the I/O error:
* Reduce the max_sectors_kb value in guest. E.g: 256->255 or lower.
* Modify virtio-scsi code to assign scsiconf->seg_max = max_hw_iov - 2
  (in my scenario, the max_hw_iov is 64)

I'm wondering that the seg_max always respects to virtqueue size, Is it
reasonable?
In case the max_segments of host scsi HBA is very low, say 64, Does the
max_segments=254 in guest make sense? Obviously the value exceed the
host limit, Does it cause I/O issue?

Thanks,
Lin