rdmsr and wrmsr _safe with exception tables

Add new rdmsr_safe and wrmsr_safe. These two functions
use exception tables to try performing their operations
and returning safely if the operation could not be performed.

Signed-off-by: Daniele Ahmed <[email protected]>

include/lib.h

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2020 Amazon.com, Inc. or its affiliates.
+ * Copyright © 2021 Amazon.com, Inc. or its affiliates.
  * All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -27,6 +27,7 @@
 
 #include <asm-macros.h>
 #include <console.h>
+#include <extables.h>
 #include <ktf.h>
 #include <processor.h>
 #include <segment.h>
@@ -144,11 +145,42 @@ static inline uint64_t rdmsr(uint32_t msr_idx) {
     return (((uint64_t) high) << 32) | low;
 }
 
+/*
+ * read the msr_idx msr; the result is valid iff *read is true
+ */
+static inline uint64_t rdmsr_safe(uint32_t msr_idx, bool *read) {
+    volatile bool _read = false;
+    uint32_t low, high;
+
+    asm volatile("1: rdmsr; movq $1, %[read];"
+                 "2:" ASM_EXTABLE_HANDLER(1b, 2b, 0)
+                 : "=a"(low), "=d"(high), [ read ] "=m"(_read)
+                 : "c"(msr_idx)
+                 : "memory");
+    *read = _read;
+
+    return (((uint64_t) high) << 32) | low;
+}
+
 static inline void wrmsr(uint32_t msr_idx, uint64_t value) {
     asm volatile("wrmsr" ::"c"(msr_idx), "a"((uint32_t) value),
                  "d"((uint32_t)(value >> 32)));
 }
 
+/*
+ * write to the msr_idx msr; the result is valid iff *read is true
+ */
+static inline void wrmsr_safe(uint32_t msr_idx, uint64_t value, bool *read) {
+    volatile bool _read = false;
+
+    asm volatile("1: wrmsr; movq $1, %[read];"
+                 "2:" ASM_EXTABLE_HANDLER(1b, 2b, 0)
+                 : [ read ] "=m"(_read)
+                 : "c"(msr_idx), "a"((uint32_t) value), "d"((uint32_t)(value >> 32))
+                 : "memory");
+    *read = _read;
+}
+
 static inline void sti(void) { asm volatile("sti"); }
 
 static inline void cli(void) { asm volatile("cli"); }