diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 3dedb41d48..f11422fc41 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -1840,8 +1840,6 @@ static bool spapr_ov5_cas_needed(void *opaque)
 {
     SpaprMachineState *spapr = opaque;
     SpaprOptionVector *ov5_mask = spapr_ovec_new();
-    SpaprOptionVector *ov5_legacy = spapr_ovec_new();
-    SpaprOptionVector *ov5_removed = spapr_ovec_new();
     bool cas_needed;
 
     /* Prior to the introduction of SpaprOptionVector, we had two option
@@ -1873,17 +1871,11 @@ static bool spapr_ov5_cas_needed(void *opaque)
     spapr_ovec_set(ov5_mask, OV5_DRCONF_MEMORY);
     spapr_ovec_set(ov5_mask, OV5_DRMEM_V2);
 
-    /* spapr_ovec_diff returns true if bits were removed. we avoid using
-     * the mask itself since in the future it's possible "legacy" bits may be
-     * removed via machine options, which could generate a false positive
-     * that breaks migration.
-     */
-    spapr_ovec_intersect(ov5_legacy, spapr->ov5, ov5_mask);
-    cas_needed = spapr_ovec_diff(ov5_removed, spapr->ov5, ov5_legacy);
+    /* We need extra information if we have any bits outside the mask
+     * defined above */
+    cas_needed = !spapr_ovec_subset(spapr->ov5, ov5_mask);
 
     spapr_ovec_cleanup(ov5_mask);
-    spapr_ovec_cleanup(ov5_legacy);
-    spapr_ovec_cleanup(ov5_removed);
 
     return cas_needed;
 }
diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c
index 0f19be794c..f1799b1b70 100644
--- a/hw/ppc/spapr_hcall.c
+++ b/hw/ppc/spapr_hcall.c
@@ -1671,7 +1671,7 @@ static target_ulong h_client_architecture_support(PowerPCCPU *cpu,
     target_ulong fdt_bufsize = args[2];
     target_ulong ov_table;
     uint32_t cas_pvr;
-    SpaprOptionVector *ov1_guest, *ov5_guest, *ov5_cas_old, *ov5_updates;
+    SpaprOptionVector *ov1_guest, *ov5_guest, *ov5_cas_old;
     bool guest_radix;
     Error *local_err = NULL;
     bool raw_mode_supported = false;
@@ -1770,9 +1770,7 @@ static target_ulong h_client_architecture_support(PowerPCCPU *cpu,
     /* capabilities that have been added since CAS-generated guest reset.
      * if capabilities have since been removed, generate another reset
      */
-    ov5_updates = spapr_ovec_new();
-    spapr->cas_reboot = spapr_ovec_diff(ov5_updates,
-                                        ov5_cas_old, spapr->ov5_cas);
+    spapr->cas_reboot = !spapr_ovec_subset(ov5_cas_old, spapr->ov5_cas);
     spapr_ovec_cleanup(ov5_cas_old);
     /* Now that processing is finished, set the radix/hash bit for the
      * guest if it requested a valid mode; otherwise terminate the boot. */
@@ -1849,8 +1847,6 @@ static target_ulong h_client_architecture_support(PowerPCCPU *cpu,
         spapr->fdt_blob = fdt;
     }
 
-    spapr_ovec_cleanup(ov5_updates);
-
     if (spapr->cas_reboot) {
         qemu_system_reset_request(SHUTDOWN_CAUSE_SUBSYSTEM_RESET);
     }
diff --git a/hw/ppc/spapr_ovec.c b/hw/ppc/spapr_ovec.c
index 811fadf143..0ff6d1aeae 100644
--- a/hw/ppc/spapr_ovec.c
+++ b/hw/ppc/spapr_ovec.c
@@ -76,31 +76,21 @@ void spapr_ovec_intersect(SpaprOptionVector *ov,
     bitmap_and(ov->bitmap, ov1->bitmap, ov2->bitmap, OV_MAXBITS);
 }
 
-/* returns true if options bits were removed, false otherwise */
-bool spapr_ovec_diff(SpaprOptionVector *ov,
-                     SpaprOptionVector *ov_old,
-                     SpaprOptionVector *ov_new)
+/* returns true if ov1 has a subset of bits in ov2 */
+bool spapr_ovec_subset(SpaprOptionVector *ov1, SpaprOptionVector *ov2)
 {
-    unsigned long *change_mask = bitmap_new(OV_MAXBITS);
-    unsigned long *removed_bits = bitmap_new(OV_MAXBITS);
-    bool bits_were_removed = false;
+    unsigned long *tmp = bitmap_new(OV_MAXBITS);
+    bool result;
 
-    g_assert(ov);
-    g_assert(ov_old);
-    g_assert(ov_new);
-
-    bitmap_xor(change_mask, ov_old->bitmap, ov_new->bitmap, OV_MAXBITS);
-    bitmap_and(ov->bitmap, ov_new->bitmap, change_mask, OV_MAXBITS);
-    bitmap_and(removed_bits, ov_old->bitmap, change_mask, OV_MAXBITS);
+    g_assert(ov1);
+    g_assert(ov2);
 
-    if (!bitmap_empty(removed_bits, OV_MAXBITS)) {
-        bits_were_removed = true;
-    }
+    bitmap_andnot(tmp, ov1->bitmap, ov2->bitmap, OV_MAXBITS);
+    result = bitmap_empty(tmp, OV_MAXBITS);
 
-    g_free(change_mask);
-    g_free(removed_bits);
+    g_free(tmp);
 
-    return bits_were_removed;
+    return result;
 }
 
 void spapr_ovec_cleanup(SpaprOptionVector *ov)
diff --git a/include/hw/ppc/spapr_ovec.h b/include/hw/ppc/spapr_ovec.h
index 7891e9caac..2bed517a2b 100644
--- a/include/hw/ppc/spapr_ovec.h
+++ b/include/hw/ppc/spapr_ovec.h
@@ -66,9 +66,7 @@ SpaprOptionVector *spapr_ovec_clone(SpaprOptionVector *ov_orig);
 void spapr_ovec_intersect(SpaprOptionVector *ov,
                           SpaprOptionVector *ov1,
                           SpaprOptionVector *ov2);
-bool spapr_ovec_diff(SpaprOptionVector *ov,
-                     SpaprOptionVector *ov_old,
-                     SpaprOptionVector *ov_new);
+bool spapr_ovec_subset(SpaprOptionVector *ov1, SpaprOptionVector *ov2);
 void spapr_ovec_cleanup(SpaprOptionVector *ov);
 void spapr_ovec_set(SpaprOptionVector *ov, long bitnr);
 void spapr_ovec_clear(SpaprOptionVector *ov, long bitnr);
