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pc, xen: make PAT support optional (for xen)

--- a/sys/src/9/pc/devarch.c

+++ b/sys/src/9/pc/devarch.c

@@ -882,6 +882,7 @@

 			rdmsr(0x01, &mct);

 	}

+#ifdef PATWC

 	/* IA32_PAT write combining */

 	if((m->cpuiddx & Pat) != 0 && rdmsr(0x277, &pat) != -1){

 		pat &= ~(255LL<<(PATWC*8));

@@ -888,6 +889,7 @@

 		pat |= 1LL<<(PATWC*8);	/* WC */

 		wrmsr(0x277, pat);

 	}

+#endif

 	if(m->cpuiddx & Mtrr)

 		mtrrsync();