linux-kernel - Re: DMA error when sg->offset value is greater than PAGE

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Date:   Tue, 26 Sep 2017 20:50:02 +0000
From:   Casey Leedom <leedom@...lsio.com>
To:     "Raj, Ashok" <ashok.raj@...el.com>,
        Robin Murphy <robin.murphy@....com>
CC:     "Harsh  Jain" <Harsh@...lsio.com>,
        Herbert Xuy <herbert@...dor.apana.org.au>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>
Subject: Re: DMA error when sg->offset value is greater than PAGE_SIZE in
 Intel IOMMU

  So just to be 100% sure I understand the patch you're proposing, you got
the first use of VTD_PAGE_SHIFT wrong; it should have been VTD_PAGE_MASK?  I.e.

diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index 6784a05..d43b566 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/drivers/iommu/intel-iommu.c
@@ -2254,10 +2254,13 @@ static int __domain_mapping(struct dmar_domain *domain, unsigned long iov_pfn,
                uint64_t tmp;

                if (!sg_res) {
+                       size_t off = sg->offset & ~VTD_PAGE_MASK;
                        sg_res = aligned_nrpages(sg->offset, sg->length);
-                       sg->dma_address = ((dma_addr_t)iov_pfn << VTD_PAGE_SHIFT) + sg->offset;
+                       sg->dma_address = ((dma_addr_t)
+                               (iov_pfn + sg->offset) << VTD_PAGE_SHIFT) + off;
                        sg->dma_length = sg->length;
-                       pteval = page_to_phys(sg_page(sg)) | prot;
+                       pteval = (page_to_phys(sg_page(sg)) +
+                               (sg->offset << VTD_PAGE_SHIFT)) | prot;
                        phys_pfn = pteval >> VTD_PAGE_SHIFT;
                }

???  And I'm still confused about this portion:

-                       sg->dma_address = ((dma_addr_t)iov_pfn << VTD_PAGE_SHIFT) + sg->offset;
+                       sg->dma_address = ((dma_addr_t)
+                               (iov_pfn + sg->offset) << VTD_PAGE_SHIFT) + off;
 
Isn't iov_pfn a Physical Page Number and we're adding a Byte Offset
to that?  I would have though that it would be more like:

    size_t page_off = sg->offset & ~VTD_PAGE_MASK;
    unsigned long pfn_off = sg->offset >> VTD_PAGE_MASK;
    ...
    sg->dma_address = ((dma_addr_t)
                       (iov_pfn + pfn_off) << VTD_PAGE_SHIFT) + page_off;

  I want to be sure that Harsh has a concrete patch to work with when he
wakes up.

  How about it Robin, Dan, David, Herbert, what do you guys think of Raj's
proposed patch?

Casey