2 files changed, 21 insertions, 2 deletions

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index c1f1a7eee953..24307dc85d08 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1885,6 +1885,7 @@ config XEN
         depends on !GENERIC_ATOMIC64
         select ARM_PSCI
         select SWIOTLB_XEN
+        select ARCH_DMA_ADDR_T_64BIT
         help
           Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
 
diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
index 1eac0731c349..ebd8f218a788 100644
--- a/drivers/xen/swiotlb-xen.c
+++ b/drivers/xen/swiotlb-xen.c
@@ -75,14 +75,32 @@ static unsigned long xen_io_tlb_nslabs;
 
 static u64 start_dma_addr;
 
+/*
+ * Both of these functions should avoid PFN_PHYS because phys_addr_t
+ * can be 32bit when dma_addr_t is 64bit leading to a loss in
+ * information if the shift is done before casting to 64bit.
+ */
 static inline dma_addr_t xen_phys_to_bus(phys_addr_t paddr)
 {
-        return phys_to_machine(XPADDR(paddr)).maddr;
+        unsigned long mfn = pfn_to_mfn(PFN_DOWN(paddr));
+        dma_addr_t dma = (dma_addr_t)mfn << PAGE_SHIFT;
+
+        dma |= paddr & ~PAGE_MASK;
+
+        return dma;
 }
 
 static inline phys_addr_t xen_bus_to_phys(dma_addr_t baddr)
 {
-        return machine_to_phys(XMADDR(baddr)).paddr;
+        unsigned long pfn = mfn_to_pfn(PFN_DOWN(baddr));
+        dma_addr_t dma = (dma_addr_t)pfn << PAGE_SHIFT;
+        phys_addr_t paddr = dma;
+
+        BUG_ON(paddr != dma); /* truncation has occurred, should never happen */
+
+        paddr |= baddr & ~PAGE_MASK;
+
+        return paddr;
 }
 
 static inline dma_addr_t xen_virt_to_bus(void *address)