port of linux patch for hyperv storvsc -- works but how?

[Andriy Gapon]

I would like to ask you to review a patch in this review request:
https://reviews.freebsd.org/D22312
The patch comes from Linux:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=40630f462824ee24bc00d692865c86c3828094e0

The change is quite simple, but I have a hard time understanding how it works.

 	ccb->csio.scsi_status = (vm_srb->scsi_status & 0xFF);
-	ccb->csio.resid = ccb->csio.dxfer_len - vm_srb->transfer_len;
+	if (srb_status == SRB_STATUS_SUCCESS ||
+	    srb_status == SRB_STATUS_DATA_OVERRUN)
+		ccb->csio.resid = ccb->csio.dxfer_len - vm_srb->transfer_len;
+	else
+		ccb->csio.resid = ccb->csio.dxfer_len;

My motivation for porting that change is a report from a user that they are
getting a lot of zpool checksum errors while concurrently running zpool scrub
and iozone -l 1 -s 100G on a pool created on top of storvsc devices.
The user reports that the change fixes the problem.
I have not been able to analyze the earlier errors in details, so I am not sure
if they were getting any SRB_STATUS_DATA_OVERRUN errors.  Also, I don't know
what kind of sense data was provided for errors.
I do know that they were getting a  lot of these messages duirng the test when
booverbose was enabled:
storvsc scsi_status = 2, srb_status = 4
scsi_status = 2 is SCSI_STATUS_CHECK_COND
srb_status = 4 is SRB_STATUS_ERROR
So, nothing unusual.

I would like to understand how the change can possibly make a difference.
As far as I can tell, csio.resid is completely ignored if a ccb fails.
Also, as far as I can see, it's ignored for ccb retries as well.
That is, we never modify dxfer_len.
And if the ccb succeeds then resid would be set the same with the old and new code.
The only possibility I could come up with is the ccb's sense data having
SSD_KEY_RECOVERED_ERROR.  In that case, the ccb would be considered successful.
But I am not sure even about that case.

Thanks!
-- 
Andriy Gapon