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Date:   Fri, 10 Jun 2022 09:37:14 -0700
From:   Saurabh Singh Sengar <ssengar@linux.microsoft.com>
To:     kys@microsoft.com, haiyangz@microsoft.com, sthemmin@microsoft.com,
        wei.liu@kernel.org, decui@microsoft.com,
        linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org,
        ssengar@microsoft.com, mikelley@microsoft.com,
        linux-scsi@vger.kernel.org, jejb@linux.ibm.com,
        martin.petersen@oracle.com
Subject: Re: [PATCH] scsi: storvsc: Correct sysfs parameters as per Hyper-V
 storvsc requirement
Message-ID: <20220610163714.GA25982@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
References: <1654878824-25691-1-git-send-email-ssengar@linux.microsoft.com>
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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.
> 
> 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;
>  	/*
>  	 * For non-IDE disks, the host supports multiple channels.
>  	 * Set the number of HW queues we are supporting.
> -- 
> 2.25.1