[PATCH v3 24/25] samples: vfio-mdev/mbochs: fix common struct sg_table related issues

From: Marek Szyprowski
Date: Tue May 05 2020 - 04:47:07 EST


The Documentation/DMA-API-HOWTO.txt states that dma_map_sg returns the
numer of the created entries in the DMA address space. However the
subsequent calls to dma_sync_sg_for_{device,cpu} and dma_unmap_sg must be
called with the original number of the entries passed to dma_map_sg. The
sg_table->nents in turn holds the result of the dma_map_sg call as stated
in include/linux/scatterlist.h. A common mistake was to ignore a result
of the dma_map_sg function and don't use the sg_table->orig_nents at all.

To avoid such issues, lets use common dma-mapping wrappers operating
directly on the struct sg_table objects and adjust references to the
nents and orig_nents respectively.

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

Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
---
For more information, see '[PATCH v3 00/25] DRM: fix struct sg_table nents
vs. orig_nents misuse' thread: https://lkml.org/lkml/2020/5/5/187
---
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 3cc5e59..f2c62e0 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))
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);
sg_free_table(sg);
kfree(sg);
}
--
1.9.1