From be0704beb229431f206fee3ddd65fa2c5eebdce3 Mon Sep 17 00:00:00 2001 From: Marek Szyprowski Date: Thu, 30 Apr 2020 13:57:22 +0200 Subject: [PATCH] samples: vfio-mdev/mbochs: fix common struct sg_table related issues 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 Reviewed-by: Robin Murphy --- 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); }