scsi: storvsc: Correct reporting of Hyper-V I/O size limits

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
scatterlist 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
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 required
alignment derives from Hyper-V communication using a 4 Kbyte page size,
and not on the guest page size, which might be bigger (eg. ARM64).

Link: https://lore.kernel.org/r/1655190355-28722-1-git-send-email-ssengar@linux.microsoft.com
Fixes: 3d9c3dcc58e9 ("scsi: storvsc: Enable scatter list entry lengths > 4Kbytes")
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
index ca3530982e52c8f7f219dd51877bc76bc771cc2a..fe000da113327021c93f6c255acda683e86673af 100644 (file)
--- 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,
@@ -1895,6 +1895,7 @@ static int storvsc_probe(struct hv_device *device,
        int target = 0;
        struct storvsc_device *stor_device;
        int max_sub_channels = 0;
+       u32 max_xfer_bytes;
 
        /*
         * We support sub-channels for storage on SCSI and FC controllers.
@@ -1968,12 +1969,28 @@ static int storvsc_probe(struct hv_device *device,
        }
        /* max cmd length */
        host->max_cmd_len = STORVSC_MAX_CMD_LEN;
-
        /*
-        * set the table size based on the info we got
-        * from the host.
+        * Any reasonable Hyper-V configuration should provide
+        * max_transfer_bytes value aligning to HV_HYP_PAGE_SIZE,
+        * protecting it from any weird value.
+        */
+       max_xfer_bytes = round_down(stor_device->max_transfer_bytes, HV_HYP_PAGE_SIZE);
+       /* max_hw_sectors_kb */
+       host->max_sectors = max_xfer_bytes >> 9;
+       /*
+        * 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.
         */
-       host->sg_tablesize = (stor_device->max_transfer_bytes >> PAGE_SHIFT);
+       host->sg_tablesize = (max_xfer_bytes >> HV_HYP_PAGE_SHIFT) + 1;
        /*
         * For non-IDE disks, the host supports multiple channels.
         * Set the number of HW queues we are supporting.