libx86: Proactively initialise error pointers

This results in better behaviour for the caller.

Suggested-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>

tools/tests/cpu-policy/test-cpu-policy.c

@@ -283,7 +283,7 @@ static void test_cpuid_deserialise_failure(void)
     for ( size_t i = 0; i < ARRAY_SIZE(tests); ++i )
     {
         const struct test *t = &tests[i];
-        uint32_t err_leaf = ~0u, err_subleaf = ~0u;
+        uint32_t err_leaf, err_subleaf;
         int rc;
 
         /* No writes should occur.  Use NULL to catch errors. */
@@ -336,7 +336,7 @@ static void test_msr_deserialise_failure(void)
     for ( size_t i = 0; i < ARRAY_SIZE(tests); ++i )
     {
         const struct test *t = &tests[i];
-        uint32_t err_msr = ~0u;
+        uint32_t err_msr;
         int rc;
 
         /* No writes should occur.  Use NULL to catch errors. */

xen/include/xen/lib/x86/cpuid.h

@@ -376,13 +376,13 @@ int x86_cpuid_copy_to_buffer(const struct cpuid_policy *policy,
  * @param policy      The cpuid_policy to unserialise into.
  * @param leaves      The array of leaves to unserialise from.
  * @param nr_entries  The number of entries in 'leaves'.
- * @param err_leaf    Optional hint filled on error.
- * @param err_subleaf Optional hint filled on error.
+ * @param err_leaf    Optional hint for error diagnostics.
+ * @param err_subleaf Optional hint for error diagnostics.
  * @returns -errno
  *
  * Reads at most CPUID_MAX_SERIALISED_LEAVES.  May return -ERANGE if an
  * incoming leaf is out of range of cpuid_policy, in which case the optional
- * err_* pointers are filled to aid diagnostics.
+ * err_* pointers will identify the out-of-range indicies.
  *
  * No content validation of in-range leaves is performed.  Synthesised data is
  * recalculated.

xen/include/xen/lib/x86/msr.h

@@ -54,14 +54,14 @@ int x86_msr_copy_to_buffer(const struct msr_policy *policy,
  * @param policy     The msr_policy object to unserialise into.
  * @param msrs       The array of msrs to unserialise from.
  * @param nr_entries The number of entries in 'msrs'.
- * @param err_msr    Optional hint filled on error.
+ * @param err_msr    Optional hint for error diagnostics.
  * @returns -errno
  *
  * Reads at most MSR_MAX_SERIALISED_ENTRIES.  May fail for a number of reasons
  * based on the content in an individual 'msrs' entry, including the MSR index
  * not being valid in the policy, the flags field being nonzero, or if the
  * value provided would truncate when stored in the policy.  In such cases,
- * the optional err_* pointer is filled in to aid diagnostics.
+ * the optional err_* pointer will identify the problematic MSR.
  *
  * No content validation is performed on the data stored in the policy object.
  */

xen/lib/x86/cpuid.c

@@ -381,6 +381,11 @@ int x86_cpuid_copy_from_buffer(struct cpuid_policy *p,
     unsigned int i;
     xen_cpuid_leaf_t data;
 
+    if ( err_leaf )
+        *err_leaf = -1;
+    if ( err_subleaf )
+        *err_subleaf = -1;
+
     /*
      * A well formed caller is expected to pass an array with leaves in order,
      * and without any repetitions.  However, due to per-vendor differences,

xen/lib/x86/msr.c

@@ -55,6 +55,9 @@ int x86_msr_copy_from_buffer(struct msr_policy *p,
     xen_msr_entry_t data;
     int rc;
 
+    if ( err_msr )
+        *err_msr = -1;
+
     /*
      * A well formed caller is expected to pass an array with entries in
      * order, and without any repetitions.  However, due to per-vendor