iommu: Decouple iommu_map_sg from CPU page size

If the IOMMU supports pages smaller than the CPU page size, segments
which lie at offsets within the CPU page may be mapped based on the
finer-grained IOMMU page boundaries. This minimises the amount of
non-buffer memory between the CPU page boundary and the start of the
segment which must be mapped and therefore exposed to the device, and
brings the default iommu_map_sg implementation in line with
iommu_map/unmap with respect to alignment.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/iommu.c

@@ -1143,14 +1143,24 @@ size_t default_iommu_map_sg(struct iommu_domain *domain, unsigned long iova,
 {
 	struct scatterlist *s;
 	size_t mapped = 0;
-	unsigned int i;
+	unsigned int i, min_pagesz;
 	int ret;
 
-	for_each_sg(sg, s, nents, i) {
-		phys_addr_t phys = page_to_phys(sg_page(s));
+	if (unlikely(domain->ops->pgsize_bitmap == 0UL))
+		return 0;
 
-		/* We are mapping on page boundarys, so offset must be 0 */
-		if (s->offset)
+	min_pagesz = 1 << __ffs(domain->ops->pgsize_bitmap);
+
+	for_each_sg(sg, s, nents, i) {
+		phys_addr_t phys = page_to_phys(sg_page(s)) + s->offset;
+
+		/*
+		 * We are mapping on IOMMU page boundaries, so offset within
+		 * the page must be 0. However, the IOMMU may support pages
+		 * smaller than PAGE_SIZE, so s->offset may still represent
+		 * an offset of that boundary within the CPU page.
+		 */
+		if (!IS_ALIGNED(s->offset, min_pagesz))
 			goto out_err;
 
 		ret = iommu_map(domain, iova + mapped, phys, s->length, prot);