[PATCH 1/1] scsi: storvsc: Enable scatterlist entry lengths > 4Kbytes

storvsc currently sets .dma_boundary to limit scatterlist entries
to 4 Kbytes, which is less efficient with huge pages that offer
large chunks of contiguous physical memory. Improve the algorithm
for creating the Hyper-V guest physical address PFN array so
that scatterlist entries with lengths > 4Kbytes are handled.
As a result, remove the .dma_boundary setting.

The improved algorithm also adds support for scatterlist
entries with offsets >= 4Kbytes, which is supported by many
other SCSI low-level drivers.  And it retains support for
architectures where possibly PAGE_SIZE != HV_HYP_PAGE_SIZE
(such as ARM64).

Signed-off-by: Michael Kelley <redacted>
---
 drivers/scsi/storvsc_drv.c | 63 ++++++++++++++++------------------------------
 1 file changed, 22 insertions(+), 41 deletions(-)

diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
index 2e4fa77..5d06061 100644
--- a/drivers/scsi/storvsc_drv.c
+++ b/drivers/scsi/storvsc_drv.c

@@ -1678,9 +1678,8 @@ static int storvsc_queuecommand(struct Scsi_Host *host, struct scsi_cmnd *scmnd)
 	struct storvsc_cmd_request *cmd_request = scsi_cmd_priv(scmnd);
 	int i;
 	struct scatterlist *sgl;
-	unsigned int sg_count = 0;
+	unsigned int sg_count;
 	struct vmscsi_request *vm_srb;
-	struct scatterlist *cur_sgl;
 	struct vmbus_packet_mpb_array  *payload;
 	u32 payload_sz;
 	u32 length;

@@ -1759,7 +1758,7 @@ static int storvsc_queuecommand(struct Scsi_Host *host, struct scsi_cmnd *scmnd)
 	payload_sz = sizeof(cmd_request->mpb);
 
 	if (sg_count) {
-		unsigned int hvpgoff = 0;
+		unsigned int hvpgoff, sgl_size;
 		unsigned long offset_in_hvpg = sgl->offset & ~HV_HYP_PAGE_MASK;
 		unsigned int hvpg_count = HVPFN_UP(offset_in_hvpg + length);
 		u64 hvpfn;

@@ -1773,51 +1772,35 @@ static int storvsc_queuecommand(struct Scsi_Host *host, struct scsi_cmnd *scmnd)
 				return SCSI_MLQUEUE_DEVICE_BUSY;
 		}
 
-		/*
-		 * sgl is a list of PAGEs, and payload->range.pfn_array
-		 * expects the page number in the unit of HV_HYP_PAGE_SIZE (the
-		 * page size that Hyper-V uses, so here we need to divide PAGEs
-		 * into HV_HYP_PAGE in case that PAGE_SIZE > HV_HYP_PAGE_SIZE.
-		 * Besides, payload->range.offset should be the offset in one
-		 * HV_HYP_PAGE.
-		 */
 		payload->range.len = length;
 		payload->range.offset = offset_in_hvpg;
-		hvpgoff = sgl->offset >> HV_HYP_PAGE_SHIFT;
 
-		cur_sgl = sgl;
-		for (i = 0; i < hvpg_count; i++) {
+
+		for (i = 0; sgl != NULL; sgl = sg_next(sgl)) {
 			/*
-			 * 'i' is the index of hv pages in the payload and
-			 * 'hvpgoff' is the offset (in hv pages) of the first
-			 * hv page in the the first page. The relationship
-			 * between the sum of 'i' and 'hvpgoff' and the offset
-			 * (in hv pages) in a payload page ('hvpgoff_in_page')
-			 * is as follow:
-			 *
-			 * |------------------ PAGE -------------------|
-			 * |   NR_HV_HYP_PAGES_IN_PAGE hvpgs in total  |
-			 * |hvpg|hvpg| ...              |hvpg|... |hvpg|
-			 * ^         ^                                 ^                 ^
-			 * +-hvpgoff-+                                 +-hvpgoff_in_page-+
-			 *           ^                                                   |
-			 *           +--------------------- i ---------------------------+
+			 * Init values for the current sgl entry. sgl_size
+			 * and hvpgoff are in units of Hyper-V size pages.
+			 * Handling the PAGE_SIZE != HV_HYP_PAGE_SIZE case
+			 * also handles values of sgl->offset that are
+			 * larger than PAGE_SIZE. Such offsets are handled
+			 * even on other than the first sgl entry, provided
+			 * they are a multiple of PAGE_SIZE.
 			 */
-			unsigned int hvpgoff_in_page =
-				(i + hvpgoff) % NR_HV_HYP_PAGES_IN_PAGE;
+			sgl_size = HVPFN_UP(sgl->offset + sgl->length);
+			hvpgoff = sgl->offset >> HV_HYP_PAGE_SHIFT;
+			hvpfn = page_to_hvpfn(sg_page(sgl));
 
 			/*
-			 * Two cases that we need to fetch a page:
-			 * 1) i == 0, the first step or
-			 * 2) hvpgoff_in_page == 0, when we reach the boundary
-			 *    of a page.
+			 * Fill the next portion of the PFN array with
+			 * sequential Hyper-V PFNs for the continguous physical
+			 * memory described by the sgl entry. The end of the
+			 * last sgl should be reached at the same time that
+			 * the PFN array is filled.
 			 */
-			if (hvpgoff_in_page == 0 || i == 0) {
-				hvpfn = page_to_hvpfn(sg_page(cur_sgl));
-				cur_sgl = sg_next(cur_sgl);
+			while (hvpgoff != sgl_size) {
+				payload->range.pfn_array[i++] =
+							hvpfn + hvpgoff++;
 			}
-
-			payload->range.pfn_array[i] = hvpfn + hvpgoff_in_page;
 		}
 	}
 

@@ -1851,8 +1834,6 @@ static int storvsc_queuecommand(struct Scsi_Host *host, struct scsi_cmnd *scmnd)
 	.slave_configure =	storvsc_device_configure,
 	.cmd_per_lun =		2048,
 	.this_id =		-1,
-	/* Make sure we dont get a sg segment crosses a page boundary */
-	.dma_boundary =		PAGE_SIZE-1,
 	/* Ensure there are no gaps in presented sgls */
 	.virt_boundary_mask =	PAGE_SIZE-1,
 	.no_write_same =	1,

-- 
1.8.3.1

Michael Kelley [off-list ref] writes:

Minor nitpicking: while this seems to be correct I, personally, find it
a bit hard to read: 'hvpgoff' stands for "'sgl->offset' measured in
Hyper-V pages' but we immediately re-use it as a cycle counter.

If I'm not mistaken, we can count right away how many entries we're
going to add. Also, we could've introduced HVPFN_DOWN() to complement
HVPFN_UP():
...
#define HVPFN_DOWN(x)	((x) >> HV_HYP_PAGE_SHIFT)
...

hvpgoff = HVPFN_DOWN(sgl->offset);
hvpfn = page_to_hvpfn(sg_page(sgl)) + hvpgoff;
hvpfns_to_add = HVPFN_UP(sgl->offset + sgl->length) - hvpgoff;

and the cycle can look like:

while (hvpfns_to_add) {
	payload->range.pfn_array[i++] = hvpfn++;
	hvpfns_to_add--;
}

-
-			payload->range.pfn_array[i] = hvpfn + hvpgoff_in_page;
 		}

and then we can also make an explicit 

BUG_ON(i != hvpg_count) after the cycle to prove our math is correct :-)

-- 
Vitaly

From: Vitaly Kuznetsov <vkuznets@redhat.com> Sent: Tuesday, February 23, 2021 6:30 AM

Michael Kelley [off-list ref] writes:

quoted

storvsc currently sets .dma_boundary to limit scatterlist entries
to 4 Kbytes, which is less efficient with huge pages that offer
large chunks of contiguous physical memory. Improve the algorithm
for creating the Hyper-V guest physical address PFN array so
that scatterlist entries with lengths > 4Kbytes are handled.
As a result, remove the .dma_boundary setting.

The improved algorithm also adds support for scatterlist
entries with offsets >= 4Kbytes, which is supported by many
other SCSI low-level drivers.  And it retains support for
architectures where possibly PAGE_SIZE != HV_HYP_PAGE_SIZE
(such as ARM64).

Signed-off-by: Michael Kelley <redacted>
---
 drivers/scsi/storvsc_drv.c | 63 ++++++++++++++++------------------------------
 1 file changed, 22 insertions(+), 41 deletions(-)

diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
index 2e4fa77..5d06061 100644
--- a/drivers/scsi/storvsc_drv.c
+++ b/drivers/scsi/storvsc_drv.c

@@ -1678,9 +1678,8 @@ static int storvsc_queuecommand(struct Scsi_Host *host, struct

scsi_cmnd *scmnd)

quoted

 	struct storvsc_cmd_request *cmd_request = scsi_cmd_priv(scmnd);
 	int i;
 	struct scatterlist *sgl;
-	unsigned int sg_count = 0;
+	unsigned int sg_count;
 	struct vmscsi_request *vm_srb;
-	struct scatterlist *cur_sgl;
 	struct vmbus_packet_mpb_array  *payload;
 	u32 payload_sz;
 	u32 length;

@@ -1759,7 +1758,7 @@ static int storvsc_queuecommand(struct Scsi_Host *host, struct

scsi_cmnd *scmnd)

quoted

 	payload_sz = sizeof(cmd_request->mpb);

 	if (sg_count) {
-		unsigned int hvpgoff = 0;
+		unsigned int hvpgoff, sgl_size;
 		unsigned long offset_in_hvpg = sgl->offset & ~HV_HYP_PAGE_MASK;
 		unsigned int hvpg_count = HVPFN_UP(offset_in_hvpg + length);
 		u64 hvpfn;

@@ -1773,51 +1772,35 @@ static int storvsc_queuecommand(struct Scsi_Host *host,

struct scsi_cmnd *scmnd)

quoted

 				return SCSI_MLQUEUE_DEVICE_BUSY;
 		}

-		/*
-		 * sgl is a list of PAGEs, and payload->range.pfn_array
-		 * expects the page number in the unit of HV_HYP_PAGE_SIZE (the
-		 * page size that Hyper-V uses, so here we need to divide PAGEs
-		 * into HV_HYP_PAGE in case that PAGE_SIZE > HV_HYP_PAGE_SIZE.
-		 * Besides, payload->range.offset should be the offset in one
-		 * HV_HYP_PAGE.
-		 */
 		payload->range.len = length;
 		payload->range.offset = offset_in_hvpg;
-		hvpgoff = sgl->offset >> HV_HYP_PAGE_SHIFT;

-		cur_sgl = sgl;
-		for (i = 0; i < hvpg_count; i++) {
+
+		for (i = 0; sgl != NULL; sgl = sg_next(sgl)) {
 			/*
-			 * 'i' is the index of hv pages in the payload and
-			 * 'hvpgoff' is the offset (in hv pages) of the first
-			 * hv page in the the first page. The relationship
-			 * between the sum of 'i' and 'hvpgoff' and the offset
-			 * (in hv pages) in a payload page ('hvpgoff_in_page')
-			 * is as follow:
-			 *
-			 * |------------------ PAGE -------------------|
-			 * |   NR_HV_HYP_PAGES_IN_PAGE hvpgs in total  |
-			 * |hvpg|hvpg| ...              |hvpg|... |hvpg|
-			 * ^         ^                                 ^                 ^
-			 * +-hvpgoff-+                                 +-hvpgoff_in_page-+
-			 *           ^                                                   |
-			 *           +--------------------- i ---------------------------+
+			 * Init values for the current sgl entry. sgl_size
+			 * and hvpgoff are in units of Hyper-V size pages.
+			 * Handling the PAGE_SIZE != HV_HYP_PAGE_SIZE case
+			 * also handles values of sgl->offset that are
+			 * larger than PAGE_SIZE. Such offsets are handled
+			 * even on other than the first sgl entry, provided
+			 * they are a multiple of PAGE_SIZE.
 			 */
-			unsigned int hvpgoff_in_page =
-				(i + hvpgoff) % NR_HV_HYP_PAGES_IN_PAGE;
+			sgl_size = HVPFN_UP(sgl->offset + sgl->length);
+			hvpgoff = sgl->offset >> HV_HYP_PAGE_SHIFT;
+			hvpfn = page_to_hvpfn(sg_page(sgl));

 			/*
-			 * Two cases that we need to fetch a page:
-			 * 1) i == 0, the first step or
-			 * 2) hvpgoff_in_page == 0, when we reach the boundary
-			 *    of a page.
+			 * Fill the next portion of the PFN array with
+			 * sequential Hyper-V PFNs for the continguous physical
+			 * memory described by the sgl entry. The end of the
+			 * last sgl should be reached at the same time that
+			 * the PFN array is filled.
 			 */
-			if (hvpgoff_in_page == 0 || i == 0) {
-				hvpfn = page_to_hvpfn(sg_page(cur_sgl));
-				cur_sgl = sg_next(cur_sgl);
+			while (hvpgoff != sgl_size) {
+				payload->range.pfn_array[i++] =
+							hvpfn + hvpgoff++;
 			}

Minor nitpicking: while this seems to be correct I, personally, find it
a bit hard to read: 'hvpgoff' stands for "'sgl->offset' measured in
Hyper-V pages' but we immediately re-use it as a cycle counter.

If I'm not mistaken, we can count right away how many entries we're
going to add. Also, we could've introduced HVPFN_DOWN() to complement
HVPFN_UP():
...
#define HVPFN_DOWN(x)	((x) >> HV_HYP_PAGE_SHIFT)
...

hvpgoff = HVPFN_DOWN(sgl->offset);
hvpfn = page_to_hvpfn(sg_page(sgl)) + hvpgoff;
hvpfns_to_add = HVPFN_UP(sgl->offset + sgl->length) - hvpgoff;

and the cycle can look like:

while (hvpfns_to_add) {
	payload->range.pfn_array[i++] = hvpfn++;
	hvpfns_to_add--;
}

quoted

-
-			payload->range.pfn_array[i] = hvpfn + hvpgoff_in_page;
 		}

and then we can also make an explicit

BUG_ON(i != hvpg_count) after the cycle to prove our math is correct :-)

Your proposal works for me, and it doesn't actually change the number of
arithmetic operations that need to be done.

But I don't think I'll actually add the BUG_ON().  The math is either right
or it isn't, and we have it right.

I'll spin a v2.

Michael

quoted

 	}

@@ -1851,8 +1834,6 @@ static int storvsc_queuecommand(struct Scsi_Host *host, struct

scsi_cmnd *scmnd)

quoted

 	.slave_configure =	storvsc_device_configure,
 	.cmd_per_lun =		2048,
 	.this_id =		-1,
-	/* Make sure we dont get a sg segment crosses a page boundary */
-	.dma_boundary =		PAGE_SIZE-1,
 	/* Ensure there are no gaps in presented sgls */
 	.virt_boundary_mask =	PAGE_SIZE-1,
 	.no_write_same =	1,

--
Vitaly

Shouldn't storvsc just use blk_queue_virt_boundary instead of all this
mess?

From: Christoph Hellwig <hch@infradead.org>  Sent: Wednesday, February 24, 2021 7:53 AM

Shouldn't storvsc just use blk_queue_virt_boundary instead of all this
mess?

The storvsc driver does set the virt boundary to PAGE_SIZE - 1.  But
the driver still has to translate the scatterlist into a list of guest
physical frame numbers (each representing 4K bytes) that the
Hyper-V host understands so it can do the I/O.

This patch improves that translation so it can handle a single
scatterlist entry that represents more than PAGE_SIZE bytes of
data.  Then the SCSI dma_boundary (which turns into the blk level
segment_boundary) no longer needs to be set to restrict scatterlist
entries to just PAGE_SIZE bytes.

We also have to preserve the ability to run guests on ARM64 with
PAGE_SIZE of 16K or 64K, while Hyper-V still expects each PFN to
represent only 4K bytes.

Michael