linux-kernel - RE: [PATCH v3] scsi: storvsc: Correct reporting of Hyper-V I/O size limits

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Date:   Tue, 14 Jun 2022 14:09:01 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Saurabh Sengar <ssengar@...ux.microsoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Saurabh Singh Sengar <ssengar@...rosoft.com>
Subject: RE: [PATCH v3] scsi: storvsc: Correct reporting of Hyper-V I/O size
 limits

From: Saurabh Sengar <ssengar@...ux.microsoft.com> Sent: Tuesday, June 14, 2022 12:06 AM
> 
> 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).
> 
> Fixes: 3d9c3dcc58e9 ("scsi: storvsc: Enable scatter list entry lengths > 4Kbytes")
> Signed-off-by: Saurabh Sengar <ssengar@...ux.microsoft.com>
> ---
> V3
>   - Remove single quotes around the 'Fixes' tag
>   - max_tx_bytes -> max_xfer_bytes
>   - Added empty line at start of comment
> 
>  drivers/scsi/storvsc_drv.c | 27 ++++++++++++++++++++++-----
>  1 file changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
> index ca35309..fe000da 100644
> --- a/drivers/scsi/storvsc_drv.c
> +++ b/drivers/scsi/storvsc_drv.c
> @@ -1844,7 +1844,7 @@ static int storvsc_queuecommand(struct Scsi_Host *host,
> struct scsi_cmnd *scmnd)
>  	.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.
> --
> 1.8.3.1

Reviewed-by: Michael Kelley <mikelley@...rosoft.com>