acpi: Add machine option to disable SPCR table

The ACPI SPCR (Serial Port Console Redirection) table allows firmware
to specify a preferred serial console device to the operating system.
On ARM64 systems, Linux by default respects this table: even if the
kernel command line does not include a hardware serial console (e.g.,
"console=ttyAMA0"), the kernel still register the serial device
referenced by SPCR as a printk console.

While this behavior is standard-compliant, it can lead to situations
where guest console behavior is influenced by platform firmware rather
than user-specified configuration. To make guest console behavior more
predictable and under user control, this patch introduces a machine
option to explicitly disable SPCR table exposure:

    -machine spcr=off

By default, the option is enabled (spcr=on), preserving existing
behavior. When disabled, QEMU will omit the SPCR table from the guest's
ACPI namespace, ensuring that only consoles explicitly declared in the
kernel command line are registered.

Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>
Message-Id: <20250528105404.457729-2-me@linux.beauty>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/hw/core/machine.c b/hw/core/machine.c
index e869821b22..ceee058cad 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -577,6 +577,20 @@ static void machine_set_nvdimm(Object *obj, bool value, Error **errp)
     ms->nvdimms_state->is_enabled = value;
 }
 
+static bool machine_get_spcr(Object *obj, Error **errp)
+{
+    MachineState *ms = MACHINE(obj);
+
+    return ms->acpi_spcr_enabled;
+}
+
+static void machine_set_spcr(Object *obj, bool value, Error **errp)
+{
+    MachineState *ms = MACHINE(obj);
+
+    ms->acpi_spcr_enabled = value;
+}
+
 static bool machine_get_hmat(Object *obj, Error **errp)
 {
     MachineState *ms = MACHINE(obj);
@@ -1281,6 +1295,14 @@ static void machine_initfn(Object *obj)
                                         "Table (HMAT)");
     }
 
+    /* SPCR */
+    ms->acpi_spcr_enabled = true;
+    object_property_add_bool(obj, "spcr", machine_get_spcr, machine_set_spcr);
+    object_property_set_description(obj, "spcr",
+                                   "Set on/off to enable/disable "
+                                   "ACPI Serial Port Console Redirection "
+                                   "Table (spcr)");
+
     /* default to mc->default_cpus */
     ms->smp.cpus = mc->default_cpus;
     ms->smp.max_cpus = mc->default_cpus;