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SubjectRE: [PATCH 1/1] scsi: storvsc: Enable scatterlist entry lengths > 4Kbytes
Date
From: Vitaly Kuznetsov <vkuznets@redhat.com> Sent: Tuesday, February 23, 2021 6:30 AM
>
> 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 :-)
>

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

> > }
> >
> > @@ -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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