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 <[email protected]>
---
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
CC : [email protected], [email protected], [email protected]
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 <[email protected]>
> ---
> 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
From: Saurabh Singh Sengar <[email protected]> Sent: Friday, June 10, 2022 9:37 AM
>
> CC : [email protected], [email protected], [email protected]
>
> 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 <[email protected]>
> > ---
> > 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
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 <[email protected]> Sent: Friday, June 10, 2022 9:37 AM
> >
> > CC : [email protected], [email protected], [email protected]
> >
> > 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 <[email protected]>
> > > ---
> > > 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
From: Saurabh Singh Sengar <[email protected]> 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 <[email protected]> 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
On Mon, Jun 13, 2022 at 01:55:14PM +0000, Michael Kelley (LINUX) wrote:
> From: Saurabh Singh Sengar <[email protected]> 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 <[email protected]> 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