xen: add support for PAT

Convert Linux PAT entries into Xen ones when constructing ptes.  Linux
doesn't use _PAGE_PAT for ptes, so the only difference in the first 4
entries is that Linux uses _PAGE_PWT for WC, whereas Xen (and default)
use it for WT.

xen_pte_val does the inverse conversion.

We hard-code assumptions about Linux's current PAT layout, but a
warning on the wrmsr to MSR_IA32_CR_PAT should point out any problems.
If necessary we could go to a more general table-based conversion between
Linux and Xen PAT entries.

hugetlbfs poses a problem at the moment, the x86 architecture uses the
same flag for _PAGE_PAT and _PAGE_PSE, which changes meaning depending
on which pagetable level we're using.  At the moment this should be OK
so long as nobody tries to do a pte_val on a hugetlbfs pte.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/xen/enlighten.c

@@ -829,6 +829,11 @@ static int xen_write_msr_safe(unsigned int msr, unsigned low, unsigned high)
 		   Xen console noise. */
 		break;
 
+	case MSR_IA32_CR_PAT:
+		if (smp_processor_id() == 0)
+			xen_set_pat(((u64)high << 32) | low);
+		break;
+
 	default:
 		ret = native_write_msr_safe(msr, low, high);
 	}