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    SubjectRe: [PATCH] scsi: storvsc: Correct sysfs parameters as per Hyper-V storvsc requirement
    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@linux.microsoft.com> Sent: Friday, June 10, 2022 9:37 AM
    > >
    > > CC : linux-scsi@vger.kernel.org, jejb@linux.ibm.com, martin.petersen@oracle.com
    > >
    > > On Fri, Jun 10, 2022 at 09:33:44AM -0700, Saurabh Sengar wrote:
    > > > 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.
    >
    > I think a richer explanation in the commit message is warranted.
    > Something like:
    >
    > Current code is based on the idea that the max number of SGL entries also
    > determines the max size of an I/O request. While this idea was true in older
    > versions of the storvsc driver when SGL entry length was limited to 4 Kbytes,
    > commit 3d9c3dcc58e9 ("scsi: storvsc: Enable scatter list entry lengths > 4Kbytes")
    > removed that limitation. It's now theoretically possible for the block
    > layer to send requests that exceed the maximum size supported by Hyper-V.
    > This problem doesn't currently happen in practice because the block layer
    > defaults to a 512 Kbyte maximum, while Hyper-V in Azure supports 2 Mbyte
    > I/O sizes. But some future configuration of Hyper-V could have a smaller
    > max I/O size, and the block layer could exceed that max.
    >
    > Fix this by correctly setting max_sectors as well as sg_tablesize to reflect
    > the maximum I/O size that Hyper-V reports. While allowing larger I/O sizes
    > larger than the block layer default of 512 Kbytes doesn't provide any
    > noticeable performance benefit in the tests we ran, it's still appropriate
    > to report the correct underlying Hyper-V capabilities to the Linux block layer.
    >
    > Also tweak the virt_boundary_mask to reflect that the needed alignment
    > derives from Hyper-V communication using a 4 Kbyte page size, and not
    > on the guest page size, which might be bigger (on ARM64, for example).
    >
    > I don't think the title of the commit should focus on sysfs. This
    > commit is about correctly reporting Hyper-V I/O size limits; the sysfs
    > entries just provide visibility into the values.
    >
    > And given that the problem was introduced by the above mentioned
    > commit, it would be appropriate to add a "Fixes:" tag.

    [SS] : Thanks, will fix this
    >
    > > >
    > > > 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;
    >
    > 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.

    - Saurabh

    >
    > Michael
    >
    > > > /*
    > > > * For non-IDE disks, the host supports multiple channels.
    > > > * Set the number of HW queues we are supporting.
    > > > --
    > > > 2.25.1

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