Re: [PATCH] acpi: Add machine option to disable SPCR table

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

On Tue, Apr 22, 2025 at 03:47:18PM +0800, Li Chen wrote:
> 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>


The patches look ok to me

Acked-by: Michael S. Tsirkin <mst@redhat.com>


> ---
>  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 ++++-
>  include/hw/boards.h            |  1 +
>  qemu-options.hx                |  5 +++++
>  6 files changed, 39 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
> index 3ac8f8e178..f25c3b26ce 100644
> --- a/hw/arm/virt-acpi-build.c
> +++ b/hw/arm/virt-acpi-build.c
> @@ -940,7 +940,10 @@ void virt_acpi_build(VirtMachineState *vms, AcpiBuildTables *tables)
>      }
>  
>      acpi_add_table(table_offsets, tables_blob);
> -    spcr_setup(tables_blob, tables->linker, vms);
> +
> +    if (ms->enable_spcr) {
> +        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 63c6ef93d2..d56f44f4e8 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -590,6 +590,20 @@ 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->enable_spcr;
> +}
> +
> +static void machine_set_spcr(Object *obj, bool value, Error **errp)
> +{
> +    MachineState *ms = MACHINE(obj);
> +
> +    ms->enable_spcr = value;
> +}
> +
>  static bool machine_get_hmat(Object *obj, Error **errp)
>  {
>      MachineState *ms = MACHINE(obj);
> @@ -1294,6 +1308,14 @@ static void machine_initfn(Object *obj)
>                                          "Table (HMAT)");
>      }
>  
> +    /* SPCR */
> +    ms->enable_spcr = 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 fced6c445a..0e437bcf25 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->enable_spcr)
> +        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 1ad6800508..7f6d221c63 100644
> --- a/hw/riscv/virt-acpi-build.c
> +++ b/hw/riscv/virt-acpi-build.c
> @@ -680,7 +680,10 @@ static void virt_acpi_build(RISCVVirtState *s, AcpiBuildTables *tables)
>      build_rhct(tables_blob, tables->linker, s);
>  
>      acpi_add_table(table_offsets, tables_blob);
> -    spcr_setup(tables_blob, tables->linker, s);
> +
> +    if (ms->enable_spcr) {
> +        spcr_setup(tables_blob, tables->linker, s);
> +    }
>  
>      acpi_add_table(table_offsets, tables_blob);
>      {
> diff --git a/include/hw/boards.h b/include/hw/boards.h
> index f22b2e7fc7..cdf2791a50 100644
> --- a/include/hw/boards.h
> +++ b/include/hw/boards.h
> @@ -444,6 +444,7 @@ struct MachineState {
>      SmpCache smp_cache;
>      struct NVDIMMState *nvdimms_state;
>      struct NumaState *numa_state;
> +    bool enable_spcr;
>  };
>  
>  /*
> diff --git a/qemu-options.hx b/qemu-options.hx
> index dc694a99a3..953680595f 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
> -- 
> 2.49.0