[PATCH v4 1/3] x86/CPUID: adjust extended leaves out of range clearing

[Jan Beulich <jbeulich@xxxxxxxx>]

A maximum extended leaf input value with the high half different from
0x8000 should not be considered valid - all leaves should be cleared in
this case.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
TBD: Andrew suggested to drop this patch, but that sub-thread still has
     a loose end. Hence, until I'm convinced otherwise, I've retained
     this patch here. I don't think it conflicts with any of the
     subsequent ones.
---
v2: Integrate into series.

--- a/tools/tests/cpu-policy/test-cpu-policy.c
+++ b/tools/tests/cpu-policy/test-cpu-policy.c
@@ -528,11 +528,22 @@ static void test_cpuid_out_of_range_clea
             },
         },
         {
+            .name = "no extd",
+            .nr_markers = 0,
+            .p = {
+                /* Clears all markers. */
+                .extd.max_leaf = 0,
+
+                .extd.vendor_ebx = 0xc2,
+                .extd.raw_fms = 0xc2,
+            },
+        },
+        {
             .name = "extd",
             .nr_markers = 1,
             .p = {
                 /* Retains marker in leaf 0.  Clears others. */
-                .extd.max_leaf = 0,
+                .extd.max_leaf = 0x80000000,
                 .extd.vendor_ebx = 0xc2,
 
                 .extd.raw_fms = 0xc2,
--- a/xen/lib/x86/cpuid.c
+++ b/xen/lib/x86/cpuid.c
@@ -285,7 +285,9 @@ void x86_cpu_policy_clear_out_of_range_l
                     ARRAY_SIZE(p->xstate.raw) - 1);
     }
 
-    zero_leaves(p->extd.raw, (p->extd.max_leaf & 0xffff) + 1,
+    zero_leaves(p->extd.raw,
+                ((p->extd.max_leaf >> 16) == 0x8000
+                 ? (p->extd.max_leaf & 0xffff) + 1 : 0),
                 ARRAY_SIZE(p->extd.raw) - 1);
 }