v4.19.13 snapshot.
diff --git a/drivers/xen/biomerge.c b/drivers/xen/biomerge.c
new file mode 100644
index 0000000..55ed80c
--- /dev/null
+++ b/drivers/xen/biomerge.c
@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bio.h>
+#include <linux/io.h>
+#include <linux/export.h>
+#include <xen/page.h>
+
+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 bfn1 + PFN_DOWN(vec1->bv_offset + vec1->bv_len) == bfn2;
+#else
+ /*
+ * XXX: Add support for merging bio_vec when using different page
+ * size in Xen and Linux.
+ */
+ return false;
+#endif
+}
+EXPORT_SYMBOL(xen_biovec_phys_mergeable);