[PATCH 1/2] xen: mask unwanted pte bits in __supported_pte_mask

[PATCH 1/2] xen: mask unwanted pte bits in __supported_pte_mask

[Jeremy Fitzhardinge]

Rather than ad-hoc masking unwanted bits when constructing a pte, just
prevent them from ever being set by removing them from
__supported_pte_mask.

[ Stable: this isn't a bugfix in itself, but it's a pre-requiste
  for "xen: don't drop NX bit" ]

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stable Kernel <stable@kernel.org>
---
 arch/x86/xen/enlighten.c |    5 +++++
 arch/x86/xen/mmu.c       |    4 +---
 2 files changed, 6 insertions(+), 3 deletions(-)

===================================================================
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1294,6 +1294,11 @@
 	if (xen_feature(XENFEAT_supervisor_mode_kernel))
 		pv_info.kernel_rpl = 0;
 
+	/* Prevent unwanted bits from being set in PTEs. */
+	__supported_pte_mask &= ~_PAGE_GLOBAL;
+	if (!is_initial_xendomain())
+		__supported_pte_mask &= ~(_PAGE_PWT | _PAGE_PCD);
+
 	/* set the limit of our address space */
 	xen_reserve_top();
 
===================================================================
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -375,10 +375,8 @@
 
 pte_t xen_make_pte(pteval_t pte)
 {
-	if (pte & _PAGE_PRESENT) {
+	if (pte & _PAGE_PRESENT)
 		pte = phys_to_machine(XPADDR(pte)).maddr;
-		pte &= ~(_PAGE_PCD | _PAGE_PWT);
-	}
 
 	return (pte_t){ .pte = pte };
 }