xen: mask _PAGE_PCD from ptes

_PAGE_PCD maps a page with caching disabled, which is typically used for
mapping harware registers.  Xen never allows it to be set on a mapping, and
unprivileged guests never need it since they can't see the real underlying
hardware.  However, some uncached mappings are made early when probing the
(non-existent) APIC, and its OK to mask off the PCD flag in these cases.

This became necessary because Xen started checking for this bit, rather
than silently masking it off.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/xen/mmu.c

@@ -244,6 +244,8 @@ pte_t xen_make_pte(unsigned long long pte)
 	if (pte & 1)
 		pte = phys_to_machine(XPADDR(pte)).maddr;
 
+	pte &= ~_PAGE_PCD;
+
 	return (pte_t){ pte, pte >> 32 };
 }
 
@@ -291,6 +293,8 @@ pte_t xen_make_pte(unsigned long pte)
 	if (pte & _PAGE_PRESENT)
 		pte = phys_to_machine(XPADDR(pte)).maddr;
 
+	pte &= ~_PAGE_PCD;
+
 	return (pte_t){ pte };
 }