[PATCH v2 16/21] libs/guest: make a cpu policy compatible with older Xen versions

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

Older Xen versions used to expose some CPUID bits which are no longer
exposed by default. In order to keep a compatible behavior with
guests migrated from versions of Xen that don't encode the CPUID data
on the migration stream introduce a function that sets the same bits
as older Xen versions.

This is pulled out from xc_cpuid_apply_policy which already has this
logic present.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - Move comments and explicitly mention pre-4.14 Xen.
---
 tools/include/xenctrl.h         |  4 +++
 tools/libs/guest/xg_cpuid_x86.c | 46 ++++++++++++++++++++++++++++-----
 2 files changed, 43 insertions(+), 7 deletions(-)

diff --git a/tools/include/xenctrl.h b/tools/include/xenctrl.h
index c41d794683c..89a73fd6823 100644
--- a/tools/include/xenctrl.h
+++ b/tools/include/xenctrl.h
@@ -2627,6 +2627,10 @@ int xc_cpu_policy_calc_compatible(xc_interface *xch,
                                   const xc_cpu_policy_t p2,
                                   xc_cpu_policy_t out);
 
+/* Make a policy compatible with pre-4.14 Xen versions. */
+int xc_cpu_policy_make_compatible(xc_interface *xch, xc_cpu_policy_t policy,
+                                  bool hvm);
+
 int xc_get_cpu_levelling_caps(xc_interface *xch, uint32_t *caps);
 int xc_get_cpu_featureset(xc_interface *xch, uint32_t index,
                           uint32_t *nr_features, uint32_t *featureset);
diff --git a/tools/libs/guest/xg_cpuid_x86.c b/tools/libs/guest/xg_cpuid_x86.c
index 6cfa4cb39d1..6486ac4c673 100644
--- a/tools/libs/guest/xg_cpuid_x86.c
+++ b/tools/libs/guest/xg_cpuid_x86.c
@@ -446,6 +446,7 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
domid, bool restore,
     unsigned int i, nr_leaves, nr_msrs;
     xen_cpuid_leaf_t *leaves = NULL;
     struct cpuid_policy *p = NULL;
+    struct xc_cpu_policy policy = { };
     uint32_t err_leaf = -1, err_subleaf = -1, err_msr = -1;
     uint32_t host_featureset[FEATURESET_NR_ENTRIES] = {};
     uint32_t len = ARRAY_SIZE(host_featureset);
@@ -509,17 +510,14 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
domid, bool restore,
     }
 
     /*
-     * Account for feature which have been disabled by default since Xen 4.13,
+     * Account for features which have been disabled by default since Xen 4.13,
      * so migrated-in VM's don't risk seeing features disappearing.
      */
     if ( restore )
     {
-        p->basic.rdrand = test_bit(X86_FEATURE_RDRAND, host_featureset);
-
-        if ( di.hvm )
-        {
-            p->feat.mpx = test_bit(X86_FEATURE_MPX, host_featureset);
-        }
+        policy.cpuid = *p;
+        xc_cpu_policy_make_compatible(xch, &policy, di.hvm);
+        *p = policy.cpuid;
     }
 
     if ( featureset )
@@ -1072,3 +1070,37 @@ int xc_cpu_policy_calc_compatible(xc_interface *xch,
 
     return rc;
 }
+
+int xc_cpu_policy_make_compatible(xc_interface *xch, xc_cpu_policy_t policy,
+                                  bool hvm)
+{
+    xc_cpu_policy_t host;
+    int rc;
+
+    host = xc_cpu_policy_init();
+    if ( !host )
+    {
+        errno = ENOMEM;
+        return -1;
+    }
+
+    rc = xc_cpu_policy_get_system(xch, XEN_SYSCTL_cpu_policy_host, host);
+    if ( rc )
+    {
+        ERROR("Failed to get host policy");
+        goto out;
+    }
+
+    /*
+     * Account for features which have been disabled by default since Xen 4.13,
+     * so migrated-in VM's don't risk seeing features disappearing.
+     */
+    policy->cpuid.basic.rdrand = host->cpuid.basic.rdrand;
+
+    if ( hvm )
+        policy->cpuid.feat.mpx = host->cpuid.feat.mpx;
+
+ out:
+    xc_cpu_policy_destroy(host);
+    return rc;
+}
-- 
2.30.1