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

[Michael Kelley (LINUX)]

From: Saurabh Singh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx> Sent: Sunday, June 12, 2022 9:06 PM
> 
> Thanks Michael for review, please find my comments inline
> 
> On Mon, Jun 13, 2022 at 02:49:09AM +0000, Michael Kelley (LINUX) wrote:
> > From: Saurabh Singh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx> Sent: Friday, June 10,
> 2022 9:37 AM
> > >

[snip]

> > > >  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;
> >
> > This calculation covers all possible I/O request sizes up to and including
> > the value of max_transfer_bytes, even if max_transfer_bytes is some
> > weird number that's not a multiple of 512.   So I think it works as
> > intended.
> >
> > But setting host->max_sectors means that storvsc won't see an I/O request
> > with a weird size, and some of the cases handled by the calculation don't
> > actually occur.  You could use a simpler calculation that's a bit easier to
> > understand:
> >
> > host->sg_tablesize = (stor_device->max_transfer_bytes >> HV_HYP_PAGE_SIZE) + 1;
> >
> > The "+1" handles the unaligned case you mention above.
> 
> [SS] : As per my understanding this may give incorrect result for unaligned cases. Lets
> take an
> example of 6KB, "stor_device->max_transfer_bytes >> HV_HYP_PAGE_SIZE" will give
> only 1, and then
> host->sq_tablesize will get final value as 2. Where as there is a possibility of 3 segments
> 1. 1KB
> 2. 4KB
> 3. 1KB
> 
> Please correct me if this scenario is not possible.

Ah yes, you are right.

But consider the case where max_transfer_bytes is something like 8292
 (i.e., 8K + 100).  sg_tablesize will be calculated as 4, but it really only needs to
be 3 because max_sectors is the equivalent of 8K.

Here's an alternate approach that might be simpler.  Since any reasonable
Hyper-V configuration will provide a max_transfer_bytes value that is a
multiple of HV_HYP_PAGE_SIZE, this approach doesn't change anything
but still protects against Hyper-V providing a weird value:

	u32 maxbytes;

	maxbytes = round_down(stor_device->max_transfer_bytes, HV_HYP_PAGE_SIZE);
	host->max_sectors = maxbytes >> 9;
	host->sg_tablesize = (maxbytes >> HV_HYP_PAGE_SHIFT) + 1;

Michael