[PATCH] scsi: storvsc: Correct sysfs parameters as per Hyper-V storvsc requirement

[Saurabh Sengar <ssengar@linux.microsoft.com>]

This patch corrects 3 parameters:
1. Correct the sysfs entry for maximum hardware transfer limit of single
   transfer (max_hw_sectors_kb) by setting max_sectors, this was set to
   default value 512kb before.
2. Correct SGL memory offset alignment as per Hyper-V page size.
3. Correct sg_tablesize which accounts for max SGL segments entries in a
   single SGL.

Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
---
 drivers/scsi/storvsc_drv.c | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
index ca3530982e52..3e032660ae36 100644
--- a/drivers/scsi/storvsc_drv.c
+++ b/drivers/scsi/storvsc_drv.c
@@ -1844,7 +1844,7 @@ static struct scsi_host_template scsi_driver = {
 	.cmd_per_lun =		2048,
 	.this_id =		-1,
 	/* Ensure there are no gaps in presented sgls */
-	.virt_boundary_mask =	PAGE_SIZE-1,
+	.virt_boundary_mask =	HV_HYP_PAGE_SIZE - 1,
 	.no_write_same =	1,
 	.track_queue_depth =	1,
 	.change_queue_depth =	storvsc_change_queue_depth,
@@ -1969,11 +1969,31 @@ static int storvsc_probe(struct hv_device *device,
 	/* max cmd length */
 	host->max_cmd_len = STORVSC_MAX_CMD_LEN;
 
+	/* max_hw_sectors_kb */
+	host->max_sectors = (stor_device->max_transfer_bytes) >> 9;
 	/*
-	 * set the table size based on the info we got
-	 * from the host.
+	 * There are 2 requirements for Hyper-V storvsc sgl segments,
+	 * based on which the below calculation for max segments is
+	 * done:
+	 *
+	 * 1. Except for the first and last sgl segment, all sgl segments
+	 *    should be align to HV_HYP_PAGE_SIZE, that also means the
+	 *    maximum number of segments in a sgl can be calculated by
+	 *    dividing the total max transfer length by HV_HYP_PAGE_SIZE.
+	 *
+	 * 2. Except for the first and last, each entry in the SGL must
+	 *    have an offset that is a multiple of HV_HYP_PAGE_SIZE,
+	 *    whereas the complete length of transfer may not be aligned
+	 *    to HV_HYP_PAGE_SIZE always. This can result in 2 cases:
+	 *    Example for unaligned case: Let's say the total transfer
+	 *    length is 6 KB, the max segments will be 3 (1,4,1).
+	 *    Example for aligned case: Let's say the total transfer length
+	 *    is 8KB, then max segments will still be 3(2,4,2) and not 4.
+	 *    4 (read next higher value) segments will only be required
+	 *    once the length is at least 2 bytes more then 8KB (read any
+	 *    HV_HYP_PAGE_SIZE aligned length).
 	 */
-	host->sg_tablesize = (stor_device->max_transfer_bytes >> PAGE_SHIFT);
+	host->sg_tablesize = ((stor_device->max_transfer_bytes - 2) >> HV_HYP_PAGE_SHIFT) + 2;
 	/*
 	 * For non-IDE disks, the host supports multiple channels.
 	 * Set the number of HW queues we are supporting.
-- 
2.25.1