On Mon, Jun 13, 2022 at 01:55:14PM +0000, Michael Kelley (LINUX) wrote:
> 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.
>
[SS]: I agree, ultimately max transfer bytes will be calculated based on max_sectors,
and that value will always be aligned to 512b.
> 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
[SS] : Rounding down to HV_HYP_PAGE_SIZE, make perfect sense, thanks !!
- Saurabh
