cortex-a8: fix check_export_pmu

- read PMUSERENR_ENABLE first before updating. On the cortex-a8
(specifically omap3) not doing this would result in the kernel aborting.
- do not read DBGDSCR_ext on cortex-a8, read DBGDSCR_int. This is only
implemented in armv7.1, not armv7, and also causes the kernel to abort.

include/arch/arm/armv/armv7-a/armv/debug.h

@@ -25,8 +25,11 @@
 static inline word_t readDscrCp(void)
 {
     word_t v;
-
+#ifdef CONFIG_ARM_CORTEX_A8
+    MRC(DBGDSCR_int, v);
+#else
     MRC(DBGDSCR_ext, v);
+#endif
     return v;
 }
 

src/arch/arm/armv/armv7-a/user_access.c

@@ -31,7 +31,8 @@ static void check_export_pmu(void)
 #if defined CONFIG_EXPORT_PMU_USER || defined CONFIG_ENABLE_BENCHMARKS
     /* Export performance counters */
     uint32_t v;
-    v = PMUSERENR_ENABLE;
+    MRC(PMUSERENR, v);
+    v |= PMUSERENR_ENABLE;
     MCR(PMUSERENR, v);
 
     /* enable user-level pmu event counter if we're in secure mode */