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Date: Sat, 5 Sep 2020 15:37:47 +0000
From: Michael Kelley <mikelley@...rosoft.com>
To: Boqun Feng <boqun.feng@...il.com>
CC: "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
KY Srinivasan <kys@...rosoft.com>,
Haiyang Zhang <haiyangz@...rosoft.com>,
Stephen Hemminger <sthemmin@...rosoft.com>,
Wei Liu <wei.liu@...nel.org>, Jiri Kosina <jikos@...nel.org>,
Benjamin Tissoires <benjamin.tissoires@...hat.com>,
Dmitry Torokhov <dmitry.torokhov@...il.com>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
"James E.J. Bottomley" <jejb@...ux.ibm.com>,
"Martin K. Petersen" <martin.petersen@...cle.com>
Subject: RE: [RFC v2 11/11] scsi: storvsc: Support PAGE_SIZE larger than 4K
From: Boqun Feng <boqun.feng@...il.com> Sent: Saturday, September 5, 2020 7:15 AM
>
> On Sat, Sep 05, 2020 at 02:55:48AM +0000, Michael Kelley wrote:
> > From: Boqun Feng <boqun.feng@...il.com> Sent: Tuesday, September 1, 2020 8:01 PM
> > >
> > > Hyper-V always use 4k page size (HV_HYP_PAGE_SIZE), so when
> > > communicating with Hyper-V, a guest should always use HV_HYP_PAGE_SIZE
> > > as the unit for page related data. For storvsc, the data is
> > > vmbus_packet_mpb_array. And since in scsi_cmnd, sglist of pages (in unit
> > > of PAGE_SIZE) is used, we need convert pages in the sglist of scsi_cmnd
> > > into Hyper-V pages in vmbus_packet_mpb_array.
> > >
> > > This patch does the conversion by dividing pages in sglist into Hyper-V
> > > pages, offset and indexes in vmbus_packet_mpb_array are recalculated
> > > accordingly.
> > >
> > > Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> > > ---
> > > drivers/scsi/storvsc_drv.c | 60 ++++++++++++++++++++++++++++++++++----
> > > 1 file changed, 54 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
> > > index 8f5f5dc863a4..3f6610717d4e 100644
> > > --- a/drivers/scsi/storvsc_drv.c
> > > +++ b/drivers/scsi/storvsc_drv.c
> > > @@ -1739,23 +1739,71 @@ static int storvsc_queuecommand(struct Scsi_Host *host,
> struct
> > > scsi_cmnd *scmnd)
> > > payload_sz = sizeof(cmd_request->mpb);
> > >
> > > if (sg_count) {
> > > - if (sg_count > MAX_PAGE_BUFFER_COUNT) {
> > > + unsigned int hvpg_idx = 0;
> > > + unsigned int j = 0;
> > > + unsigned long hvpg_offset = sgl->offset & ~HV_HYP_PAGE_MASK;
> > > + unsigned int hvpg_count = HVPFN_UP(hvpg_offset + length);
> > >
> > > - payload_sz = (sg_count * sizeof(u64) +
> > > + if (hvpg_count > MAX_PAGE_BUFFER_COUNT) {
> > > +
> > > + payload_sz = (hvpg_count * sizeof(u64) +
> > > sizeof(struct vmbus_packet_mpb_array));
> > > payload = kzalloc(payload_sz, GFP_ATOMIC);
> > > if (!payload)
> > > 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.
> > > + */
> > > payload->range.len = length;
> > > - payload->range.offset = sgl[0].offset;
> > > + payload->range.offset = sgl[0].offset & ~HV_HYP_PAGE_MASK;
> > > + hvpg_idx = sgl[0].offset >> HV_HYP_PAGE_SHIFT;
> > >
> > > cur_sgl = sgl;
> > > - for (i = 0; i < sg_count; i++) {
> > > - payload->range.pfn_array[i] =
> > > - page_to_pfn(sg_page((cur_sgl)));
> > > + for (i = 0, j = 0; i < sg_count; i++) {
> > > + /*
> > > + * "PAGE_SIZE / HV_HYP_PAGE_SIZE - hvpg_idx" is the #
> > > + * of HV_HYP_PAGEs in the current PAGE.
> > > + *
> > > + * "hvpg_count - j" is the # of unhandled HV_HYP_PAGEs.
> > > + *
> > > + * As shown in the following, the minimal of both is
> > > + * the # of HV_HYP_PAGEs, we need to handle in this
> > > + * PAGE.
> > > + *
> > > + * |------------------ PAGE ----------------------|
> > > + * | PAGE_SIZE / HV_HYP_PAGE_SIZE in total |
> > > + * |hvpg|hvpg| ... |hvpg|... |hvpg|
> > > + * ^ ^
> > > + * hvpg_idx |
> > > + * ^ |
> > > + * +---(hvpg_count - j)--+
> > > + *
> > > + * or
> > > + *
> > > + * |------------------ PAGE ----------------------|
> > > + * | PAGE_SIZE / HV_HYP_PAGE_SIZE in total |
> > > + * |hvpg|hvpg| ... |hvpg|... |hvpg|
> > > + * ^ ^
> > > + * hvpg_idx |
> > > + * ^ |
> > > + * +---(hvpg_count - j)------------------------+
> > > + */
> > > + unsigned int nr_hvpg = min((unsigned int)(PAGE_SIZE /
> HV_HYP_PAGE_SIZE) - hvpg_idx,
> > > + hvpg_count - j);
> > > + unsigned int k;
> > > +
> > > + for (k = 0; k < nr_hvpg; k++) {
> > > + payload->range.pfn_array[j] =
> > > + page_to_hvpfn(sg_page((cur_sgl))) + hvpg_idx + k;
> > > + j++;
> > > + }
> > > cur_sgl = sg_next(cur_sgl);
> > > + hvpg_idx = 0;
> > > }
> >
> > This code works; I don't see any errors. But I think it can be made simpler based
> > on doing two things:
> > 1) Rather than iterating over the sg_count, and having to calculate nr_hvpg on
> > each iteration, base the exit decision on having filled up the pfn_array[]. You've
> > already calculated the exact size of the array that is needed given the data
> > length, so it's easy to exit when the array is full.
> > 2) In the inner loop, iterate from hvpg_idx to PAGE_SIZE/HV_HYP_PAGE_SIZE
> > rather than from 0 to a calculated value.
> >
> > Also, as an optimization, pull page_to_hvpfn(sg_page((cur_sgl)) out of the
> > inner loop.
> >
> > I think this code does it (though I haven't tested it):
> >
> > for (j = 0; ; sgl = sg_next(sgl)) {
> > unsigned int k;
> > unsigned long pfn;
> >
> > pfn = page_to_hvpfn(sg_page(sgl));
> > for (k = hvpg_idx; k < (unsigned int)(PAGE_SIZE /HV_HYP_PAGE_SIZE); k++) {
> > payload->range.pfn_array[j] = pfn + k;
> > if (++j == hvpg_count)
> > goto done;
> > }
> > hvpg_idx = 0;
> > }
> > done:
> >
> > This approach also makes the limit of the inner loop a constant, and that
> > constant will be 1 when page size is 4K. So the compiler should be able to
> > optimize away the loop in that case.
> >
>
> Good point! I like your suggestion, and after thinking a bit harder
> based on your approach, I come up with the following:
>
> #define HV_HYP_PAGES_IN_PAGE ((unsigned int)(PAGE_SIZE / HV_HYP_PAGE_SIZE))
>
> for (j = 0; j < hvpg_count; j++) {
> unsigned int k = (j + hvpg_idx) % HV_HYP_PAGES_IN_PAGE;
>
> /*
> * Two cases that we need to fetch a page:
> * a) j == 0: the first step or
> * b) k == 0: when we reach the boundary of a
> * page.
> *
> if (k == 0 || j == 0) {
> pfn = page_to_hvpfn(sg_page(cur_sgl));
> cur_sgl = sg_next(cur_sgl);
> }
>
> payload->range.pfn_arrary[j] = pfn + k;
> }
>
> , given the HV_HYP_PAGES_IN_PAGE is always a power of 2, so I think
> compilers could easily optimize the "%" into bit masking operation. And
> when HV_HYP_PAGES_IN_PAGE is 1, I think compilers can easily figure out
> k is always zero, then the if-statement can be optimized as always
> taken. And that gives us the same code as before ;-)
>
> Thoughts? I will try with a test to see if I'm missing something subtle.
>
> Thanks for looking into this!
>
Your newest version looks right to me -- very clever! I like it even better
than my version.
Michael
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