[PATCH 4.4 312/342] xen/arm: correctly handle DMA mapping of compound pages

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ian Campbell <ian.campbell at citrix.com>

commit 52ba0746b3b44c86aee121babf3b2fd9b8f84090 upstream.

Currently xen_dma_map_page concludes that DMA to anything other than
the head page of a compound page must be foreign, since the PFN of the
page is that of the head.

Fix the check to instead consider the whole of a compound page to be
local if the PFN of the head passes the 1:1 check.

We can never see a compound page which is a mixture of foreign and
local sub-pages.

The comment already correctly described the intention, but fixup the
spelling and some grammar.

This fixes the various SSH protocol errors which we have been seeing
on the cubietrucks in our automated test infrastructure.

This has been broken since commit 3567258d281b ("xen/arm: use
hypercall to flush caches in map_page"), which was in v3.19-rc1.

NB arch/arm64/.../xen/page-coherent.h also includes this file.

Signed-off-by: Ian Campbell <ian.campbell at citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini at eu.citrix.com>
Cc: xen-devel at lists.xenproject.org
Cc: linux-arm-kernel at lists.infradead.org
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>

diff --git a/arch/arm/include/asm/xen/page-coherent.h b/arch/arm/include/asm/xen/page-coherent.h
index 0375c8caa061..9408a994cc91 100644
--- a/arch/arm/include/asm/xen/page-coherent.h
+++ b/arch/arm/include/asm/xen/page-coherent.h
@@ -35,14 +35,21 @@ static inline void xen_dma_map_page(struct device *hwdev, struct page *page,
 	     dma_addr_t dev_addr, unsigned long offset, size_t size,
 	     enum dma_data_direction dir, struct dma_attrs *attrs)
 {
-	bool local = XEN_PFN_DOWN(dev_addr) == page_to_xen_pfn(page);
+	unsigned long page_pfn = page_to_xen_pfn(page);
+	unsigned long dev_pfn = XEN_PFN_DOWN(dev_addr);
+	unsigned long compound_pages =
+		(1<<compound_order(page)) * XEN_PFN_PER_PAGE;
+	bool local = (page_pfn <= dev_pfn) &&
+		(dev_pfn - page_pfn < compound_pages);
+
 	/*
-	 * Dom0 is mapped 1:1, while the Linux page can be spanned accross
-	 * multiple Xen page, it's not possible to have a mix of local and
-	 * foreign Xen page. So if the first xen_pfn == mfn the page is local
-	 * otherwise it's a foreign page grant-mapped in dom0. If the page is
-	 * local we can safely call the native dma_ops function, otherwise we
-	 * call the xen specific function.
+	 * Dom0 is mapped 1:1, while the Linux page can span across
+	 * multiple Xen pages, it's not possible for it to contain a
+	 * mix of local and foreign Xen pages. So if the first xen_pfn
+	 * == mfn the page is local otherwise it's a foreign page
+	 * grant-mapped in dom0. If the page is local we can safely
+	 * call the native dma_ops function, otherwise we call the xen
+	 * specific function.
 	 */
 	if (local)
 		__generic_dma_ops(hwdev)->map_page(hwdev, page, offset, size, dir, attrs);