diff --git a/hw/acpi/core.c b/hw/acpi/core.c
index 7170bff657..03a6e949e8 100644
--- a/hw/acpi/core.c
+++ b/hw/acpi/core.c
@@ -579,6 +579,10 @@ void acpi_pm1_cnt_update(ACPIREGS *ar,
                          bool sci_enable, bool sci_disable)
 {
     /* ACPI specs 3.0, 4.7.2.5 */
+    if (ar->pm1.cnt.acpi_only) {
+        return;
+    }
+
     if (sci_enable) {
         ar->pm1.cnt.cnt |= ACPI_BITMASK_SCI_ENABLE;
     } else if (sci_disable) {
@@ -608,11 +612,22 @@ static const MemoryRegionOps acpi_pm_cnt_ops = {
 };
 
 void acpi_pm1_cnt_init(ACPIREGS *ar, MemoryRegion *parent,
-                       bool disable_s3, bool disable_s4, uint8_t s4_val)
+                       bool disable_s3, bool disable_s4, uint8_t s4_val,
+                       bool acpi_only)
 {
     FWCfgState *fw_cfg;
 
+    /*
+     * Until v5, pm1 cnt allows to change mode (legacy vs acpi)
+     * even SMM isn't enabled.
+     * Keep the old behavior for compatibility.
+     */
+    if (object_property_get_bool(qdev_get_machine(), "x-smm-compat-5", NULL)) {
+        acpi_only = false;
+    }
+
     ar->pm1.cnt.s4_val = s4_val;
+    ar->pm1.cnt.acpi_only = acpi_only;
     ar->wakeup.notify = acpi_notify_wakeup;
     qemu_register_wakeup_notifier(&ar->wakeup);
 
@@ -638,6 +653,9 @@ void acpi_pm1_cnt_init(ACPIREGS *ar, MemoryRegion *parent,
 void acpi_pm1_cnt_reset(ACPIREGS *ar)
 {
     ar->pm1.cnt.cnt = 0;
+    if (ar->pm1.cnt.acpi_only) {
+        ar->pm1.cnt.cnt |= ACPI_BITMASK_SCI_ENABLE;
+    }
 }
 
 /* ACPI GPE */
diff --git a/hw/acpi/ich9.c b/hw/acpi/ich9.c
index 5ff4e01c36..1a34d7f621 100644
--- a/hw/acpi/ich9.c
+++ b/hw/acpi/ich9.c
@@ -282,7 +282,7 @@ void ich9_pm_init(PCIDevice *lpc_pci, ICH9LPCPMRegs *pm,
     acpi_pm_tmr_init(&pm->acpi_regs, ich9_pm_update_sci_fn, &pm->io);
     acpi_pm1_evt_init(&pm->acpi_regs, ich9_pm_update_sci_fn, &pm->io);
     acpi_pm1_cnt_init(&pm->acpi_regs, &pm->io, pm->disable_s3, pm->disable_s4,
-                      pm->s4_val);
+                      pm->s4_val, !smm_enabled);
 
     acpi_gpe_init(&pm->acpi_regs, ICH9_PMIO_GPE0_LEN);
     memory_region_init_io(&pm->io_gpe, OBJECT(lpc_pci), &ich9_gpe_ops, pm,
diff --git a/hw/acpi/piix4.c b/hw/acpi/piix4.c
index 669be5bbf6..0cddf91de5 100644
--- a/hw/acpi/piix4.c
+++ b/hw/acpi/piix4.c
@@ -496,7 +496,8 @@ static void piix4_pm_realize(PCIDevice *dev, Error **errp)
 
     acpi_pm_tmr_init(&s->ar, pm_tmr_timer, &s->io);
     acpi_pm1_evt_init(&s->ar, pm_tmr_timer, &s->io);
-    acpi_pm1_cnt_init(&s->ar, &s->io, s->disable_s3, s->disable_s4, s->s4_val);
+    acpi_pm1_cnt_init(&s->ar, &s->io, s->disable_s3, s->disable_s4, s->s4_val,
+                      !s->smm_enabled);
     acpi_gpe_init(&s->ar, GPE_LEN);
 
     s->powerdown_notifier.notify = piix4_pm_powerdown_req;
diff --git a/hw/isa/vt82c686.c b/hw/isa/vt82c686.c
index a6f5a0843d..071b64b497 100644
--- a/hw/isa/vt82c686.c
+++ b/hw/isa/vt82c686.c
@@ -240,7 +240,7 @@ static void vt82c686b_pm_realize(PCIDevice *dev, Error **errp)
 
     acpi_pm_tmr_init(&s->ar, pm_tmr_timer, &s->io);
     acpi_pm1_evt_init(&s->ar, pm_tmr_timer, &s->io);
-    acpi_pm1_cnt_init(&s->ar, &s->io, false, false, 2);
+    acpi_pm1_cnt_init(&s->ar, &s->io, false, false, 2, false);
 }
 
 static Property via_pm_properties[] = {
diff --git a/include/hw/acpi/acpi.h b/include/hw/acpi/acpi.h
index 22b0b65bb2..9e8a76f2e2 100644
--- a/include/hw/acpi/acpi.h
+++ b/include/hw/acpi/acpi.h
@@ -128,6 +128,7 @@ struct ACPIPM1CNT {
     MemoryRegion io;
     uint16_t cnt;
     uint8_t s4_val;
+    bool acpi_only;
 };
 
 struct ACPIGPE {
@@ -163,7 +164,8 @@ void acpi_pm1_evt_init(ACPIREGS *ar, acpi_update_sci_fn update_sci,
 
 /* PM1a_CNT: piix and ich9 don't implement PM1b CNT. */
 void acpi_pm1_cnt_init(ACPIREGS *ar, MemoryRegion *parent,
-                       bool disable_s3, bool disable_s4, uint8_t s4_val);
+                       bool disable_s3, bool disable_s4, uint8_t s4_val,
+                       bool acpi_only);
 void acpi_pm1_cnt_update(ACPIREGS *ar,
                          bool sci_enable, bool sci_disable);
 void acpi_pm1_cnt_reset(ACPIREGS *ar);
