diff --git a/xen/arch/x86/e820.c b/xen/arch/x86/e820.c
index 082f9928a1..3892c9cfb7 100644
--- a/xen/arch/x86/e820.c
+++ b/xen/arch/x86/e820.c
@@ -420,7 +420,7 @@ static uint64_t __init mtrr_top_of_ram(void)
 {
     uint32_t eax, ebx, ecx, edx;
     uint64_t mtrr_cap, mtrr_def, addr_mask, base, mask, top;
-    unsigned int i, phys_bits = 36;
+    unsigned int i;
 
     /* By default we check only Intel systems. */
     if ( e820_mtrr_clip == -1 )
@@ -445,15 +445,9 @@ static uint64_t __init mtrr_top_of_ram(void)
     if ( !test_bit(X86_FEATURE_MTRR & 31, &edx) )
          return 0;
 
-    /* Find the physical address size for this CPU. */
-    eax = cpuid_eax(0x80000000);
-    if ( (eax >> 16) == 0x8000 && eax >= 0x80000008 )
-    {
-        phys_bits = (uint8_t)cpuid_eax(0x80000008);
-        if ( phys_bits > PADDR_BITS )
-            phys_bits = PADDR_BITS;
-    }
-    addr_mask = ((1ull << phys_bits) - 1) & ~((1ull << 12) - 1);
+    /* paddr_bits must have been set at this point */
+    ASSERT(paddr_bits);
+    addr_mask = ((1ull << paddr_bits) - 1) & PAGE_MASK;
 
     rdmsrl(MSR_MTRRcap, mtrr_cap);
     rdmsrl(MSR_MTRRdefType, mtrr_def);
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index d858883404..0223967b24 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -954,6 +954,9 @@ void __init noreturn __start_xen(unsigned long mbi_p)
     else
         panic("Bootloader provided no memory information\n");
 
+    /* This must come before e820 code because it sets paddr_bits. */
+    early_cpu_init();
+
     /* Sanitise the raw E820 map to produce a final clean version. */
     max_page = raw_max_page = init_e820(memmap_type, &e820_raw);
 
@@ -1532,8 +1535,6 @@ void __init noreturn __start_xen(unsigned long mbi_p)
     softirq_init();
     tasklet_subsys_init();
 
-    early_cpu_init();
-
     paging_init();
 
     tboot_probe();