Re: [PATCH v9 30/32] samples: vfio-mdev/mbochs: fix common struct sg_table related issues

Robin Murphy
2020-09-01T20:16:21Z

On 2020-08-26 07:33, Marek Szyprowski wrote:
> 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.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

> 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);
> }
>