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SubjectRe: [PATCH 1/1] scsi: storvsc: Enable scatterlist entry lengths > 4Kbytes
Date
Michael Kelley <mikelley@microsoft.com> writes:

> 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 <mikelley@microsoft.com>
> ---
> 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++;
> }

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 :-)

> }
>
> @@ -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,

--
Vitaly

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