Index: xen/xen-unstable.hg/linux-2.6-xen-sparse/drivers/xen/tpmback/tpmback.c
===================================================================
--- xen.orig/xen-unstable.hg/linux-2.6-xen-sparse/drivers/xen/tpmback/tpmback.c
+++ xen/xen-unstable.hg/linux-2.6-xen-sparse/drivers/xen/tpmback/tpmback.c
@@ -297,9 +297,8 @@ int _packet_write(struct packet *pak,
 
 		pfn = __pa(MMAP_VADDR(tpmif, i)) >> PAGE_SHIFT;
 		mfn = FOREIGN_FRAME(map_op.dev_bus_addr >> PAGE_SHIFT);
-		mfn_orig = phys_to_machine_mapping[pfn];
 
-		set_phys_to_machine(pfn, mfn);
+		mfn_orig = set_phys_to_machine(pfn, mfn);
 
 		tocopy = MIN(size - offset, PAGE_SIZE);
 
Index: xen/xen-unstable.hg/linux-2.6-xen-sparse/include/asm-i386/mach-xen/asm/page.h
===================================================================
--- xen.orig/xen-unstable.hg/linux-2.6-xen-sparse/include/asm-i386/mach-xen/asm/page.h
+++ xen/xen-unstable.hg/linux-2.6-xen-sparse/include/asm-i386/mach-xen/asm/page.h
@@ -137,13 +137,17 @@ static inline unsigned long mfn_to_local
 	return pfn;
 }
 
-static inline void set_phys_to_machine(unsigned long pfn, unsigned long mfn)
+static inline
+unsigned long set_phys_to_machine(unsigned long pfn, unsigned long mfn)
 {
+	unsigned long mfn_orig;
 	if (xen_feature(XENFEAT_auto_translated_physmap)) {
 		BUG_ON(pfn != mfn && mfn != INVALID_P2M_ENTRY);
-		return;
+		return phys_to_machine_mapping[pfn];
 	}
+	mfn_orig = phys_to_machine_mapping[pfn];
 	phys_to_machine_mapping[pfn] = mfn;
+	return mfn_orig;
 }
 
 /* Definitions for machine and pseudophysical addresses. */