[PATCH RFC] x86/CPUID: bump max leaf values for guest exposure

[Jan Beulich <jbeulich@xxxxxxxx>]

Generalize what was done for LFENCE_DISPATCH: To make certain features
that we artificially enable recognizable by guests, respective maximum
leaf values may need to be bumped as well.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
RFC: The call to the function from recalculate_cpuid_policy() is
     somewhat problematic: It's not clear whether a tool stack request
     with a low maximum leaf value should be honored despite our desire
     to expose certain features.

This omits checking basic/extended leaf 1 features, some of which we
also force. I think requests to run guests with max basic/extended leaf
set to zero are sufficiently insane.

Among the large set of changes I don't think I can identify the commit
to validly name in a possible Fixes: tag here.

The cpu-policy.c change would imo look quite a bit better on top of
"x86/CPUID: move bounding of max_{,sub}leaf fields to library code",
which has been pending for a long time.

--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -360,7 +360,6 @@ void calculate_raw_cpu_policy(void)
 static void __init calculate_host_policy(void)
 {
     struct cpu_policy *p = &host_cpu_policy;
-    unsigned int max_extd_leaf;
 
     *p = raw_cpu_policy;
 
@@ -368,20 +367,8 @@ static void __init calculate_host_policy
         min_t(uint32_t, p->basic.max_leaf,   ARRAY_SIZE(p->basic.raw) - 1);
     p->feat.max_subleaf =
         min_t(uint32_t, p->feat.max_subleaf, ARRAY_SIZE(p->feat.raw) - 1);
-
-    max_extd_leaf = p->extd.max_leaf;
-
-    /*
-     * For AMD/Hygon hardware before Zen3, we unilaterally modify LFENCE to be
-     * dispatch serialising for Spectre mitigations.  Extend max_extd_leaf
-     * beyond what hardware supports, to include the feature leaf containing
-     * this information.
-     */
-    if ( cpu_has_lfence_dispatch )
-        max_extd_leaf = max(max_extd_leaf, 0x80000021);
-
-    p->extd.max_leaf = 0x80000000 | min_t(uint32_t, max_extd_leaf & 0xffff,
-                                          ARRAY_SIZE(p->extd.raw) - 1);
+    p->extd.max_leaf = 0x80000000 | min(p->extd.max_leaf & 0xffffUL,
+                                        ARRAY_SIZE(p->extd.raw) - 1);
 
     x86_cpu_featureset_to_policy(boot_cpu_data.x86_capability, p);
     recalculate_xstate(p);
--- a/xen/lib/x86/cpuid.c
+++ b/xen/lib/x86/cpuid.c
@@ -104,6 +104,22 @@ void x86_cpu_featureset_to_policy(
     p->feat._7d1             = fs[FEATURESET_7d1];
     p->arch_caps.lo          = fs[FEATURESET_m10Al];
     p->arch_caps.hi          = fs[FEATURESET_m10Ah];
+
+    /*
+     * We may force-enable certain features, which then needs reflecting in
+     * respective max leaf / subleaf values.
+     *
+     * ARCH_CAPS lives in 7d0.
+     */
+    if ( p->feat._7d0 && p->basic.max_leaf < 7 )
+        p->basic.max_leaf = 7;
+
+    /*
+     * AMD's speculation related features (e.g. LFENCE_DISPATCH) live in
+     * leaf e21a.
+     */
+    if ( p->extd.e21a && p->extd.max_leaf < 0x80000021 )
+        p->extd.max_leaf = 0x80000021;
 }
 
 void x86_cpu_policy_recalc_synth(struct cpu_policy *p)