[PATCH] scsi: storvsc: Correct sysfs parameters as per Hyper-V storvsc requirement

[PATCH] scsi: storvsc: Correct sysfs parameters as per Hyper-V storvsc requirement

[Saurabh Sengar]

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

Re: [PATCH] scsi: storvsc: Correct sysfs parameters as per Hyper-V storvsc requirement

[Saurabh Singh Sengar]

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

RE: [PATCH] scsi: storvsc: Correct sysfs parameters as per Hyper-V storvsc requirement

[Michael Kelley (LINUX)]

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.

> >
> > 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.

Michael

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

Re: [PATCH] scsi: storvsc: Correct sysfs parameters as per Hyper-V storvsc requirement

[Saurabh Singh Sengar]

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

RE: [PATCH] scsi: storvsc: Correct sysfs parameters as per Hyper-V storvsc requirement

[Michael Kelley (LINUX)]

From: Saurabh Singh Sengar <ssengar@linux.microsoft.com> 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@linux.microsoft.com> 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.

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

Re: [PATCH] scsi: storvsc: Correct sysfs parameters as per Hyper-V storvsc requirement

[Saurabh Singh Sengar]

On Mon, Jun 13, 2022 at 01:55:14PM +0000, Michael Kelley (LINUX) wrote:
> From: Saurabh Singh Sengar <ssengar@linux.microsoft.com> 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@linux.microsoft.com> 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