[PATCH v4 03/10] libx86: introduce helper to fetch msr entry

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Use such helper in order to replace the code in
x86_msr_copy_from_buffer. Note the introduced helper should not be
directly called and instead x86_msr_get_entry should be used that will
properly deal with const and non-const inputs.

Note this requires making the raw fields uint64_t so that it can
accommodate the maximum size of MSRs values, and in turn removing the
truncation tests.

Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v3:
 - New in this version.
---
 tools/tests/cpu-policy/test-cpu-policy.c | 48 +++++++++++++++++++-----
 xen/include/xen/lib/x86/msr.h            | 19 +++++++++-
 xen/lib/x86/msr.c                        | 41 ++++++++++----------
 3 files changed, 75 insertions(+), 33 deletions(-)

diff --git a/tools/tests/cpu-policy/test-cpu-policy.c 
b/tools/tests/cpu-policy/test-cpu-policy.c
index 81de9720c8d..854883fbb39 100644
--- a/tools/tests/cpu-policy/test-cpu-policy.c
+++ b/tools/tests/cpu-policy/test-cpu-policy.c
@@ -389,16 +389,6 @@ static void test_msr_deserialise_failure(void)
             .msr = { .idx = 0xce, .flags = 1 },
             .rc = -EINVAL,
         },
-        {
-            .name = "truncated val",
-            .msr = { .idx = 0xce, .val = ~0ull },
-            .rc = -EOVERFLOW,
-        },
-        {
-            .name = "truncated val",
-            .msr = { .idx = 0x10a, .val = ~0ull },
-            .rc = -EOVERFLOW,
-        },
     };
 
     printf("Testing MSR deserialise failure:\n");
@@ -744,6 +734,43 @@ static void test_cpuid_get_leaf_failure(void)
     }
 }
 
+static void test_msr_get_entry(void)
+{
+    static const struct test {
+        const char *name;
+        unsigned int idx;
+        bool success;
+    } tests[] = {
+        {
+            .name = "bad msr index",
+            .idx = -1,
+        },
+        {
+            .name = "good msr index",
+            .idx = 0xce,
+            .success = true,
+        },
+    };
+    const struct msr_policy pc;
+    const uint64_t *ec;
+    struct msr_policy p;
+    uint64_t *e;
+
+    /* Constness build test. */
+    ec = x86_msr_get_entry(&pc, 0);
+    e = x86_msr_get_entry(&p, 0);
+
+    printf("Testing MSR get leaf:\n");
+
+    for ( size_t i = 0; i < ARRAY_SIZE(tests); ++i )
+    {
+        const struct test *t = &tests[i];
+
+        if ( !!x86_msr_get_entry(&pc, t->idx) != t->success )
+            fail("  Test %s failed\n", t->name);
+    }
+}
+
 static void test_is_compatible_success(void)
 {
     static struct test {
@@ -864,6 +891,7 @@ int main(int argc, char **argv)
 
     test_msr_serialise_success();
     test_msr_deserialise_failure();
+    test_msr_get_entry();
 
     test_is_compatible_success();
     test_is_compatible_failure();
diff --git a/xen/include/xen/lib/x86/msr.h b/xen/include/xen/lib/x86/msr.h
index 48ba4a59c03..9d5bcfad886 100644
--- a/xen/include/xen/lib/x86/msr.h
+++ b/xen/include/xen/lib/x86/msr.h
@@ -17,7 +17,7 @@ struct msr_policy
      * is dependent on real hardware support.
      */
     union {
-        uint32_t raw;
+        uint64_t raw;
         struct {
             uint32_t :31;
             bool cpuid_faulting:1;
@@ -32,7 +32,7 @@ struct msr_policy
      * fixed in hardware.
      */
     union {
-        uint32_t raw;
+        uint64_t raw;
         struct {
             bool rdcl_no:1;
             bool ibrs_all:1;
@@ -91,6 +91,21 @@ int x86_msr_copy_from_buffer(struct msr_policy *policy,
                              const msr_entry_buffer_t msrs, uint32_t 
nr_entries,
                              uint32_t *err_msr);
 
+/**
+ * Get a MSR entry from a policy object.
+ *
+ * @param policy      The msr_policy object.
+ * @param idx         The index.
+ * @returns a pointer to the requested leaf or NULL in case of error.
+ *
+ * Do not call this function directly and instead use x86_msr_get_entry that
+ * will deal with both const and non-const policies returning a pointer with
+ * constness matching that of the input.
+ */
+const uint64_t *_x86_msr_get_entry(const struct msr_policy *policy,
+                                   uint32_t idx);
+#define x86_msr_get_entry(p, i) \
+    ((__typeof__(&(p)->platform_info.raw))_x86_msr_get_entry(p, i))
 #endif /* !XEN_LIB_X86_MSR_H */
 
 /*
diff --git a/xen/lib/x86/msr.c b/xen/lib/x86/msr.c
index 7d71e92a380..4b5e3553e34 100644
--- a/xen/lib/x86/msr.c
+++ b/xen/lib/x86/msr.c
@@ -74,6 +74,8 @@ int x86_msr_copy_from_buffer(struct msr_policy *p,
 
     for ( i = 0; i < nr_entries; i++ )
     {
+        uint64_t *val;
+
         if ( copy_from_buffer_offset(&data, msrs, i, 1) )
             return -EFAULT;
 
@@ -83,31 +85,13 @@ int x86_msr_copy_from_buffer(struct msr_policy *p,
             goto err;
         }
 
-        switch ( data.idx )
+        val = x86_msr_get_entry(p, data.idx);
+        if ( !val )
         {
-            /*
-             * Assign data.val to p->field, checking for truncation if the
-             * backing storage for field is smaller than uint64_t
-             */
-#define ASSIGN(field)                             \
-({                                                \
-    if ( (typeof(p->field))data.val != data.val ) \
-    {                                             \
-        rc = -EOVERFLOW;                          \
-        goto err;                                 \
-    }                                             \
-    p->field = data.val;                          \
-})
-
-        case MSR_INTEL_PLATFORM_INFO: ASSIGN(platform_info.raw); break;
-        case MSR_ARCH_CAPABILITIES:   ASSIGN(arch_caps.raw);     break;
-
-#undef ASSIGN
-
-        default:
             rc = -ERANGE;
             goto err;
         }
+        *val = data.val;
     }
 
     return 0;
@@ -119,6 +103,21 @@ int x86_msr_copy_from_buffer(struct msr_policy *p,
     return rc;
 }
 
+const uint64_t *_x86_msr_get_entry(const struct msr_policy *policy,
+                                   uint32_t idx)
+{
+    switch ( idx )
+    {
+    case MSR_INTEL_PLATFORM_INFO:
+        return &policy->platform_info.raw;
+
+    case MSR_ARCH_CAPABILITIES:
+        return &policy->arch_caps.raw;
+    }
+
+    return NULL;
+}
+
 /*
  * Local variables:
  * mode: C
-- 
2.31.1