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    Subject[PATCH v6 33/36] samples: vfio-mdev/mbochs: fix common struct sg_table related issues
    Date
    The Documentation/DMA-API-HOWTO.txt states that the dma_map_sg() function
    returns the number of the created entries in the DMA address space.
    However the subsequent calls to the dma_sync_sg_for_{device,cpu}() and
    dma_unmap_sg must be called with the original number of the entries
    passed to the dma_map_sg().

    struct sg_table is a common structure used for describing a non-contiguous
    memory buffer, used commonly in the DRM and graphics subsystems. It
    consists of a scatterlist with memory pages and DMA addresses (sgl entry),
    as well as the number of scatterlist entries: CPU pages (orig_nents entry)
    and DMA mapped pages (nents entry).

    It turned out that it was a common mistake to misuse nents and orig_nents
    entries, calling DMA-mapping functions with a wrong number of entries or
    ignoring the number of mapped entries returned by the dma_map_sg()
    function.

    To avoid such issues, lets use a common dma-mapping wrappers operating
    directly on the struct sg_table objects and use scatterlist page
    iterators where possible. This, almost always, hides references to the
    nents and orig_nents entries, making the code robust, easier to follow
    and copy/paste safe.

    While touching this code, also add missing call to dma_unmap_sgtable.

    Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
    ---
    samples/vfio-mdev/mbochs.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    diff --git a/samples/vfio-mdev/mbochs.c b/samples/vfio-mdev/mbochs.c
    index 3cc5e5921682..e03068917273 100644
    --- a/samples/vfio-mdev/mbochs.c
    +++ b/samples/vfio-mdev/mbochs.c
    @@ -846,7 +846,7 @@ static struct sg_table *mbochs_map_dmabuf(struct dma_buf_attachment *at,
    if (sg_alloc_table_from_pages(sg, dmabuf->pages, dmabuf->pagecount,
    0, dmabuf->mode.size, GFP_KERNEL) < 0)
    goto err2;
    - if (!dma_map_sg(at->dev, sg->sgl, sg->nents, direction))
    + if (dma_map_sgtable(at->dev, sg, direction, 0))
    goto err3;

    return sg;
    @@ -868,6 +868,7 @@ static void mbochs_unmap_dmabuf(struct dma_buf_attachment *at,

    dev_dbg(dev, "%s: %d\n", __func__, dmabuf->id);

    + dma_unmap_sgtable(at->dev, sg, direction, 0);
    sg_free_table(sg);
    kfree(sg);
    }
    --
    2.17.1
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