[PULL 25/97] acpi: Add machine option to disable SPCR table

["Michael S. Tsirkin" <mst@redhat.com>]

From: Li Chen <chenl311@chinatelecom.cn>

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>
---
 include/hw/boards.h            |  1 +
 hw/arm/virt-acpi-build.c       |  5 ++++-
 hw/core/machine.c              | 22 ++++++++++++++++++++++
 hw/loongarch/virt-acpi-build.c |  4 +++-
 hw/riscv/virt-acpi-build.c     |  5 ++++-
 qemu-options.hx                |  5 +++++
 6 files changed, 39 insertions(+), 3 deletions(-)

diff --git a/include/hw/boards.h b/include/hw/boards.h
index f424b2b505..f94713e6e2 100644
--- a/include/hw/boards.h
+++ b/include/hw/boards.h
@@ -443,6 +443,7 @@ struct MachineState {
     SmpCache smp_cache;
     struct NVDIMMState *nvdimms_state;
     struct NumaState *numa_state;
+    bool acpi_spcr_enabled;
 };
 
 /*
diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
index 0dfb8ec2c3..782b17b966 100644
--- a/hw/arm/virt-acpi-build.c
+++ b/hw/arm/virt-acpi-build.c
@@ -1023,7 +1023,10 @@ void virt_acpi_build(VirtMachineState *vms, AcpiBuildTables *tables)
     }
 
     acpi_add_table(table_offsets, tables_blob);
-    spcr_setup(tables_blob, tables->linker, vms);
+
+    if (ms->acpi_spcr_enabled) {
+        spcr_setup(tables_blob, tables->linker, vms);
+    }
 
     acpi_add_table(table_offsets, tables_blob);
     build_dbg2(tables_blob, tables->linker, vms);
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;
diff --git a/hw/loongarch/virt-acpi-build.c b/hw/loongarch/virt-acpi-build.c
index 2cd2d9d842..8c2228a772 100644
--- a/hw/loongarch/virt-acpi-build.c
+++ b/hw/loongarch/virt-acpi-build.c
@@ -557,7 +557,9 @@ static void acpi_build(AcpiBuildTables *tables, MachineState *machine)
     acpi_add_table(table_offsets, tables_blob);
     build_srat(tables_blob, tables->linker, machine);
     acpi_add_table(table_offsets, tables_blob);
-    spcr_setup(tables_blob, tables->linker, machine);
+
+    if (machine->acpi_spcr_enabled)
+        spcr_setup(tables_blob, tables->linker, machine);
 
     if (machine->numa_state->num_nodes) {
         if (machine->numa_state->have_numa_distance) {
diff --git a/hw/riscv/virt-acpi-build.c b/hw/riscv/virt-acpi-build.c
index 8b5683dbde..ee1416d264 100644
--- a/hw/riscv/virt-acpi-build.c
+++ b/hw/riscv/virt-acpi-build.c
@@ -894,7 +894,10 @@ static void virt_acpi_build(RISCVVirtState *s, AcpiBuildTables *tables)
     }
 
     acpi_add_table(table_offsets, tables_blob);
-    spcr_setup(tables_blob, tables->linker, s);
+
+    if (ms->acpi_spcr_enabled) {
+        spcr_setup(tables_blob, tables->linker, s);
+    }
 
     acpi_add_table(table_offsets, tables_blob);
     {
diff --git a/qemu-options.hx b/qemu-options.hx
index 1f862b19a6..9b2d5f4b7a 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -38,6 +38,7 @@ DEF("machine", HAS_ARG, QEMU_OPTION_machine, \
     "                nvdimm=on|off controls NVDIMM support (default=off)\n"
     "                memory-encryption=@var{} memory encryption object to use (default=none)\n"
     "                hmat=on|off controls ACPI HMAT support (default=off)\n"
+    "                spcr=on|off controls ACPI SPCR support (default=on)\n"
 #ifdef CONFIG_POSIX
     "                aux-ram-share=on|off allocate auxiliary guest RAM as shared (default: off)\n"
 #endif
@@ -105,6 +106,10 @@ SRST
         Enables or disables ACPI Heterogeneous Memory Attribute Table
         (HMAT) support. The default is off.
 
+    ``spcr=on|off``
+        Enables or disables ACPI Serial Port Console Redirection Table
+        (SPCR) support. The default is on.
+
     ``aux-ram-share=on|off``
         Allocate auxiliary guest RAM as an anonymous file that is
         shareable with an external process.  This option applies to
-- 
MST