xen: check EFER for NX before setting up GDT mapping

x86-64 assumes NX is available by default, so we need to
explicitly check for it before using NX.  Some first-generation
Intel x86-64 processors didn't support NX, and even recent systems
allow it to be disabled in BIOS.

[ Impact: prevent Xen crash on NX-less 64-bit machines ]

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stable Kernel <stable@kernel.org>

1 files changed, 5 insertions, 5 deletions

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 0dd0c2c6cae0..5d701bf66a23 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1083,6 +1083,11 @@ asmlinkage void __init xen_start_kernel(void)
 
         __supported_pte_mask |= _PAGE_IOMAP;
 
+#ifdef CONFIG_X86_64
+        /* Work out if we support NX */
+        check_efer();
+#endif
+
         xen_setup_features();
 
         /* Get mfn list */
@@ -1123,11 +1128,6 @@ asmlinkage void __init xen_start_kernel(void)
 
         pgd = (pgd_t *)xen_start_info->pt_base;
 
-#ifdef CONFIG_X86_64
-        /* Work out if we support NX */
-        check_efer();
-#endif
-
         /* Don't do the full vcpu_info placement stuff until we have a
            possible map and a non-dummy shared_info. */
         per_cpu(xen_vcpu, 0) = &HYPERVISOR_shared_info->vcpu_info[0];