xen/biomerge: Don't allow biovec's to be merged when Linux is not using 4KB pages

On ARM all dma-capable devices on a same platform may not be protected
by an IOMMU. The DMA requests have to use the BFN (i.e MFN on ARM) in
order to use correctly the device.

While the DOM0 memory is allocated in a 1:1 fashion (PFN == MFN), grant
mapping will screw this contiguous mapping.

When Linux is using 64KB page granularitary, the page may be split
accross multiple non-contiguous MFN (Xen is using 4KB page
granularity). Therefore a DMA request will likely fail.

Checking that a 64KB page is using contiguous MFN is tedious. For
now, always says that biovec are not mergeable.
Signed-off-by: Julien Grall <julien.grall@citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/xen/biomerge.c

@@ -6,10 +6,18 @@
 bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
 			       const struct bio_vec *vec2)
 {
+#if XEN_PAGE_SIZE == PAGE_SIZE
 	unsigned long bfn1 = pfn_to_bfn(page_to_pfn(vec1->bv_page));
 	unsigned long bfn2 = pfn_to_bfn(page_to_pfn(vec2->bv_page));
 
 	return __BIOVEC_PHYS_MERGEABLE(vec1, vec2) &&
 		((bfn1 == bfn2) || ((bfn1+1) == bfn2));
+#else
+	/*
+	 * XXX: Add support for merging bio_vec when using different page
+	 * size in Xen and Linux.
+	 */
+	return 0;
+#endif
 }
 EXPORT_SYMBOL(xen_biovec_phys_mergeable);