diff --git a/tools/tests/cpu-policy/test-cpu-policy.c b/tools/tests/cpu-policy/test-cpu-policy.c
index 9216010b1c5d..870f7ecee0e5 100644
--- a/tools/tests/cpu-policy/test-cpu-policy.c
+++ b/tools/tests/cpu-policy/test-cpu-policy.c
@@ -654,6 +654,137 @@ static void test_is_compatible_failure(void)
     }
 }
 
+static void test_topo_from_parts(void)
+{
+    static const struct test {
+        unsigned int threads_per_core;
+        unsigned int cores_per_pkg;
+        struct cpu_policy policy;
+    } tests[] = {
+        {
+            .threads_per_core = 3, .cores_per_pkg = 1,
+            .policy = {
+                .x86_vendor = X86_VENDOR_AMD,
+                .topo.subleaf = {
+                    { .nr_logical = 3, .level = 0, .type = 1, .id_shift = 2, },
+                    { .nr_logical = 1, .level = 1, .type = 2, .id_shift = 2, },
+                },
+            },
+        },
+        {
+            .threads_per_core = 1, .cores_per_pkg = 3,
+            .policy = {
+                .x86_vendor = X86_VENDOR_AMD,
+                .topo.subleaf = {
+                    { .nr_logical = 1, .level = 0, .type = 1, .id_shift = 0, },
+                    { .nr_logical = 3, .level = 1, .type = 2, .id_shift = 2, },
+                },
+            },
+        },
+        {
+            .threads_per_core = 7, .cores_per_pkg = 5,
+            .policy = {
+                .x86_vendor = X86_VENDOR_AMD,
+                .topo.subleaf = {
+                    { .nr_logical = 7, .level = 0, .type = 1, .id_shift = 3, },
+                    { .nr_logical = 5, .level = 1, .type = 2, .id_shift = 6, },
+                },
+            },
+        },
+        {
+            .threads_per_core = 2, .cores_per_pkg = 128,
+            .policy = {
+                .x86_vendor = X86_VENDOR_AMD,
+                .topo.subleaf = {
+                    { .nr_logical = 2, .level = 0, .type = 1, .id_shift = 1, },
+                    { .nr_logical = 128, .level = 1, .type = 2,
+                      .id_shift = 8, },
+                },
+            },
+        },
+        {
+            .threads_per_core = 3, .cores_per_pkg = 1,
+            .policy = {
+                .x86_vendor = X86_VENDOR_INTEL,
+                .topo.subleaf = {
+                    { .nr_logical = 3, .level = 0, .type = 1, .id_shift = 2, },
+                    { .nr_logical = 3, .level = 1, .type = 2, .id_shift = 2, },
+                },
+            },
+        },
+        {
+            .threads_per_core = 1, .cores_per_pkg = 3,
+            .policy = {
+                .x86_vendor = X86_VENDOR_INTEL,
+                .topo.subleaf = {
+                    { .nr_logical = 1, .level = 0, .type = 1, .id_shift = 0, },
+                    { .nr_logical = 3, .level = 1, .type = 2, .id_shift = 2, },
+                },
+            },
+        },
+        {
+            .threads_per_core = 7, .cores_per_pkg = 5,
+            .policy = {
+                .x86_vendor = X86_VENDOR_INTEL,
+                .topo.subleaf = {
+                    { .nr_logical = 7, .level = 0, .type = 1, .id_shift = 3, },
+                    { .nr_logical = 35, .level = 1, .type = 2, .id_shift = 6, },
+                },
+            },
+        },
+        {
+            .threads_per_core = 2, .cores_per_pkg = 128,
+            .policy = {
+                .x86_vendor = X86_VENDOR_INTEL,
+                .topo.subleaf = {
+                    { .nr_logical = 2, .level = 0, .type = 1, .id_shift = 1, },
+                    { .nr_logical = 256, .level = 1, .type = 2,
+                      .id_shift = 8, },
+                },
+            },
+        },
+    };
+
+    printf("Testing topology synthesis from parts:\n");
+
+    for ( size_t i = 0; i < ARRAY_SIZE(tests); ++i )
+    {
+        const struct test *t = &tests[i];
+        struct cpu_policy actual = { .x86_vendor = t->policy.x86_vendor };
+        int rc = x86_topo_from_parts(&actual, t->threads_per_core,
+                                     t->cores_per_pkg);
+
+        if ( rc || memcmp(&actual.topo, &t->policy.topo, sizeof(actual.topo)) )
+        {
+#define TOPO(n, f)  t->policy.topo.subleaf[(n)].f, actual.topo.subleaf[(n)].f
+            fail("FAIL[%d] - '%s %u t/c, %u c/p'\n",
+                 rc,
+                 x86_cpuid_vendor_to_str(t->policy.x86_vendor),
+                 t->threads_per_core, t->cores_per_pkg);
+            printf("  subleaf=%u  expected_n=%u actual_n=%u\n"
+                   "             expected_lvl=%u actual_lvl=%u\n"
+                   "             expected_type=%u actual_type=%u\n"
+                   "             expected_shift=%u actual_shift=%u\n",
+                   0,
+                   TOPO(0, nr_logical),
+                   TOPO(0, level),
+                   TOPO(0, type),
+                   TOPO(0, id_shift));
+
+            printf("  subleaf=%u  expected_n=%u actual_n=%u\n"
+                   "             expected_lvl=%u actual_lvl=%u\n"
+                   "             expected_type=%u actual_type=%u\n"
+                   "             expected_shift=%u actual_shift=%u\n",
+                   1,
+                   TOPO(1, nr_logical),
+                   TOPO(1, level),
+                   TOPO(1, type),
+                   TOPO(1, id_shift));
+#undef TOPO
+        }
+    }
+}
+
 int main(int argc, char **argv)
 {
     printf("CPU Policy unit tests\n");
@@ -671,6 +802,8 @@ int main(int argc, char **argv)
     test_is_compatible_success();
     test_is_compatible_failure();
 
+    test_topo_from_parts();
+
     if ( nr_failures )
         printf("Done: %u failures\n", nr_failures);
     else
diff --git a/xen/include/xen/lib/x86/cpu-policy.h b/xen/include/xen/lib/x86/cpu-policy.h
index f43e1a3b21e9..116b305a1d7f 100644
--- a/xen/include/xen/lib/x86/cpu-policy.h
+++ b/xen/include/xen/lib/x86/cpu-policy.h
@@ -542,6 +542,22 @@ int x86_cpu_policies_are_compatible(const struct cpu_policy *host,
                                     const struct cpu_policy *guest,
                                     struct cpu_policy_errors *err);
 
+/**
+ * Synthesise topology information in `p` given high-level constraints
+ *
+ * Topology is given in various fields accross several leaves, some of
+ * which are vendor-specific. This function uses the policy itself to
+ * derive such leaves from threads/core and cores/package.
+ *
+ * @param p                   CPU policy of the domain.
+ * @param threads_per_core    threads/core. Doesn't need to be a power of 2.
+ * @param cores_per_package   cores/package. Doesn't need to be a power of 2.
+ * @return                    0 on success; -errno on failure
+ */
+int x86_topo_from_parts(struct cpu_policy *p,
+                        unsigned int threads_per_core,
+                        unsigned int cores_per_pkg);
+
 #endif /* !XEN_LIB_X86_POLICIES_H */
 
 /*
diff --git a/xen/lib/x86/policy.c b/xen/lib/x86/policy.c
index 63bc96451d2c..16b09a427841 100644
--- a/xen/lib/x86/policy.c
+++ b/xen/lib/x86/policy.c
@@ -2,6 +2,94 @@
 
 #include <xen/lib/x86/cpu-policy.h>
 
+static unsigned int order(unsigned int n)
+{
+    ASSERT(n); /* clz(0) is UB */
+
+    return 8 * sizeof(n) - __builtin_clz(n);
+}
+
+int x86_topo_from_parts(struct cpu_policy *p,
+                        unsigned int threads_per_core,
+                        unsigned int cores_per_pkg)
+{
+    unsigned int threads_per_pkg = threads_per_core * cores_per_pkg;
+    unsigned int apic_id_size;
+
+    if ( !p || !threads_per_core || !cores_per_pkg )
+        return -EINVAL;
+
+    p->basic.max_leaf = MAX(0xb, p->basic.max_leaf);
+
+    memset(p->topo.raw, 0, sizeof(p->topo.raw));
+
+    /* thread level */
+    p->topo.subleaf[0].nr_logical = threads_per_core;
+    p->topo.subleaf[0].id_shift = 0;
+    p->topo.subleaf[0].level = 0;
+    p->topo.subleaf[0].type = 1;
+    if ( threads_per_core > 1 )
+        p->topo.subleaf[0].id_shift = order(threads_per_core - 1);
+
+    /* core level */
+    p->topo.subleaf[1].nr_logical = cores_per_pkg;
+    if ( p->x86_vendor == X86_VENDOR_INTEL )
+        p->topo.subleaf[1].nr_logical = threads_per_pkg;
+    p->topo.subleaf[1].id_shift = p->topo.subleaf[0].id_shift;
+    p->topo.subleaf[1].level = 1;
+    p->topo.subleaf[1].type = 2;
+    if ( cores_per_pkg > 1 )
+        p->topo.subleaf[1].id_shift += order(cores_per_pkg - 1);
+
+    apic_id_size = p->topo.subleaf[1].id_shift;
+
+    /*
+     * Contrary to what the name might seem to imply. HTT is an enabler for
+     * SMP and there's no harm in setting it even with a single vCPU.
+     */
+    p->basic.htt = true;
+    p->basic.lppp = MIN(0xff, threads_per_pkg);
+
+    switch ( p->x86_vendor )
+    {
+    case X86_VENDOR_INTEL: {
+        struct cpuid_cache_leaf *sl = p->cache.subleaf;
+
+        for ( size_t i = 0; sl->type &&
+                            i < ARRAY_SIZE(p->cache.raw); i++, sl++ )
+        {
+            sl->cores_per_package = cores_per_pkg - 1;
+            sl->threads_per_cache = threads_per_core - 1;
+            if ( sl->type == 3 /* unified cache */ )
+                sl->threads_per_cache = threads_per_pkg - 1;
+        }
+        break;
+    }
+
+    case X86_VENDOR_AMD:
+    case X86_VENDOR_HYGON:
+        /* Expose p->basic.lppp */
+        p->extd.cmp_legacy = true;
+
+        /* Clip NC to the maximum value it can hold */
+        p->extd.nc = MIN(0xff, threads_per_pkg - 1);
+
+        /* TODO: Expose leaf e1E */
+        p->extd.topoext = false;
+
+        /*
+         * Clip APIC ID to 8 bits, as that's what high core-count machines do.
+         *
+         * That's what AMD EPYC 9654 does with >256 CPUs.
+         */
+        p->extd.apic_id_size = MIN(8, apic_id_size);
+
+        break;
+    }
+
+    return 0;
+}
+
 int x86_cpu_policies_are_compatible(const struct cpu_policy *host,
                                     const struct cpu_policy *guest,
                                     struct cpu_policy_errors *err)
diff --git a/xen/lib/x86/private.h b/xen/lib/x86/private.h
index 60bb82a400b7..2ec9dbee33c2 100644
--- a/xen/lib/x86/private.h
+++ b/xen/lib/x86/private.h
@@ -4,6 +4,7 @@
 #ifdef __XEN__
 
 #include <xen/bitops.h>
+#include <xen/bug.h>
 #include <xen/guest_access.h>
 #include <xen/kernel.h>
 #include <xen/lib.h>
@@ -17,6 +18,7 @@
 
 #else
 
+#include <assert.h>
 #include <errno.h>
 #include <inttypes.h>
 #include <stdbool.h>
@@ -28,6 +30,8 @@
 
 #include <xen-tools/common-macros.h>
 
+#define ASSERT(x) assert(x)
+
 static inline bool test_bit(unsigned int bit, const void *vaddr)
 {
     const char *addr = vaddr;
