Introduce an interface that returns a specific MSR entry from a cpu
policy in xen_msr_entry_t format. Provide a helper to perform a binary
search against an array of MSR entries.

This is useful to callers can peek data from the opaque
xc_cpu_policy_t type.

No caller of the interface introduced on this patch.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Changes since v1:
 - Introduce a helper to perform a binary search of the MSR entries
   array.
---
 tools/include/xenctrl.h         |  2 ++
 tools/libs/guest/xg_cpuid_x86.c | 42 +++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/tools/include/xenctrl.h b/tools/include/xenctrl.h
index cbca7209e34..605c632cf30 100644
--- a/tools/include/xenctrl.h
+++ b/tools/include/xenctrl.h
@@ -2611,6 +2611,8 @@ int xc_cpu_policy_serialise(xc_interface *xch, const xc_cpu_policy_t policy,
 int xc_cpu_policy_get_cpuid(xc_interface *xch, const xc_cpu_policy_t policy,
                             uint32_t leaf, uint32_t subleaf,
                             xen_cpuid_leaf_t *out);
+int xc_cpu_policy_get_msr(xc_interface *xch, const xc_cpu_policy_t policy,
+                          uint32_t msr, xen_msr_entry_t *out);
 
 int xc_get_cpu_levelling_caps(xc_interface *xch, uint32_t *caps);
 int xc_get_cpu_featureset(xc_interface *xch, uint32_t index,
diff --git a/tools/libs/guest/xg_cpuid_x86.c b/tools/libs/guest/xg_cpuid_x86.c
index d146c5bbc99..a4307d50ddb 100644
--- a/tools/libs/guest/xg_cpuid_x86.c
+++ b/tools/libs/guest/xg_cpuid_x86.c
@@ -845,3 +845,45 @@ int xc_cpu_policy_get_cpuid(xc_interface *xch, const xc_cpu_policy_t policy,
     *out = *tmp;
     return 0;
 }
+
+static int compare_entries(const void *l, const void *r)
+{
+    const xen_msr_entry_t *lhs = l;
+    const xen_msr_entry_t *rhs = r;
+
+    if ( lhs->idx == rhs->idx )
+        return 0;
+    return lhs->idx < rhs->idx ? -1 : 1;
+}
+
+static xen_msr_entry_t *find_entry(xen_msr_entry_t *entries,
+                                   unsigned int nr_entries, unsigned int index)
+{
+    const xen_msr_entry_t key = { index };
+
+    return bsearch(&key, entries, nr_entries, sizeof(*entries), compare_entries);
+}
+
+int xc_cpu_policy_get_msr(xc_interface *xch, const xc_cpu_policy_t policy,
+                          uint32_t msr, xen_msr_entry_t *out)
+{
+    unsigned int nr_entries = ARRAY_SIZE(policy->entries);
+    xen_msr_entry_t *tmp;
+    int rc;
+
+    rc = xc_cpu_policy_serialise(xch, policy, NULL, 0,
+                                 policy->entries, &nr_entries);
+    if ( rc )
+        return rc;
+
+    tmp = find_entry(policy->entries, nr_entries, msr);
+    if ( !tmp )
+    {
+        /* Unable to find a matching MSR. */
+        errno = ENOENT;
+        return -1;
+    }
+
+    *out = *tmp;
+    return 0;
+}
-- 
2.30.1