Expose sensible topologies in leaf 0xb. At the moment it synthesises
non-HT systems, in line with the previous code intent.

Leaf 0xb in the host policy is no longer zapped and the guest {max,def}
policies have their topology leaves zapped instead. The intent is for
toolstack to populate them. There's no current use for the topology
information in the host policy, but it makes no harm.

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
---
v5:
  * No change
---
 tools/libs/guest/xg_cpuid_x86.c | 24 +++++++++++++++++++++++-
 xen/arch/x86/cpu-policy.c       |  9 ++++++---
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/tools/libs/guest/xg_cpuid_x86.c b/tools/libs/guest/xg_cpuid_x86.c
index 4453178100ad..6062dcab01ce 100644
--- a/tools/libs/guest/xg_cpuid_x86.c
+++ b/tools/libs/guest/xg_cpuid_x86.c
@@ -725,8 +725,16 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid, bool restore,
         p->policy.basic.htt       = test_bit(X86_FEATURE_HTT, host_featureset);
         p->policy.extd.cmp_legacy = test_bit(X86_FEATURE_CMP_LEGACY, host_featureset);
     }
-    else
+    else if ( restore )
     {
+        /*
+         * Reconstruct the topology exposed on Xen <= 4.13. It makes very little
+         * sense, but it's what those guests saw so it's set in stone now.
+         *
+         * Guests from Xen 4.14 onwards carry their own CPUID leaves in the
+         * migration stream so they don't need special treatment.
+         */
+
         /*
          * Topology for HVM guests is entirely controlled by Xen.  For now, we
          * hardcode APIC_ID = vcpu_id * 2 to give the illusion of no SMT.
@@ -782,6 +790,20 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid, bool restore,
             break;
         }
     }
+    else
+    {
+        /* TODO: Expose the ability to choose a custom topology for HVM/PVH */
+        unsigned int threads_per_core = 1;
+        unsigned int cores_per_pkg = di.max_vcpu_id + 1;
+
+        rc = x86_topo_from_parts(&p->policy, threads_per_core, cores_per_pkg);
+        if ( rc )
+        {
+            ERROR("Failed to generate topology: rc=%d t/c=%u c/p=%u",
+                  rc, threads_per_core, cores_per_pkg);
+            goto out;
+        }
+    }
 
     nr_leaves = ARRAY_SIZE(p->leaves);
     rc = x86_cpuid_copy_to_buffer(&p->policy, p->leaves, &nr_leaves);
diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c
index 304dc20cfab8..55a95f6e164c 100644
--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -263,9 +263,6 @@ static void recalculate_misc(struct cpu_policy *p)
 
     p->basic.raw[0x8] = EMPTY_LEAF;
 
-    /* TODO: Rework topology logic. */
-    memset(p->topo.raw, 0, sizeof(p->topo.raw));
-
     p->basic.raw[0xc] = EMPTY_LEAF;
 
     p->extd.e1d &= ~CPUID_COMMON_1D_FEATURES;
@@ -613,6 +610,9 @@ static void __init calculate_pv_max_policy(void)
     recalculate_xstate(p);
 
     p->extd.raw[0xa] = EMPTY_LEAF; /* No SVM for PV guests. */
+
+    /* Wipe host topology. Populated by toolstack */
+    memset(p->topo.raw, 0, sizeof(p->topo.raw));
 }
 
 static void __init calculate_pv_def_policy(void)
@@ -776,6 +776,9 @@ static void __init calculate_hvm_max_policy(void)
 
     /* It's always possible to emulate CPUID faulting for HVM guests */
     p->platform_info.cpuid_faulting = true;
+
+    /* Wipe host topology. Populated by toolstack */
+    memset(p->topo.raw, 0, sizeof(p->topo.raw));
 }
 
 static void __init calculate_hvm_def_policy(void)
-- 
2.45.2