Re: [PATCH v6] acpi/virt: suppress UART device & SPCR when guest has no serial hardware

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

On Thu, Dec 11, 2025 at 06:20:25PM +0800, Li Chen wrote:
> From: Li Chen <chenl311@chinatelecom.cn>
> 
> virt machines always instantiate a PL011/16550 UART at slot 0 and describe
> it in ACPI (DSDT and optional SPCR table). When the command line disables
> the serial backend (e.g. "-serial none"), the guest still sees the UART as
> a preferred console even though it is not usable.
> 
> Teach the virt ACPI code to omit the UART device and SPCR when there is no
> serial backend attached. This matches the hardware that the guest can
> actually use and avoids confusing firmware or OS code that relies on SPCR.
> 
> The bios-tables-test qtests rely on an ACPI UART node and SPCR entry for
> UEFI-based virt machines. To keep those tests working we create a UART
> with a "null" chardev backend instead. This preserves the ACPI tables
> while discarding the firmware's serial output so it does not corrupt the
> TAP stdout stream.
> 
> Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
> Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>

This bothers me:
won't this mean number of serial devices changes silently?  So for
example, if you install a guest, see that 1st serial device is broken,
configure it with 2nd one as a work around, now with your change there
is no second one so guest will be broken?


I seems safer to have compat machinery around and avoid
changing this for old machine types.




> ---
> v4->v5:
> - Also suppress UART device & SPCR when guest has no serial hardware on loongarch
> - rename serial_exist to serial_exists
> - fix style issue
> v5->v6: 
> - Fix: tap parsingg error caused by changing serial to none; now set to null.
> 
>  hw/arm/virt-acpi-build.c       | 15 +++++++++------
>  hw/loongarch/virt-acpi-build.c |  8 +++++---
>  hw/riscv/virt-acpi-build.c     |  8 ++++++--
>  include/system/system.h        |  2 ++
>  system/vl.c                    |  5 +++++
>  tests/qtest/bios-tables-test.c |  6 ++++--
>  6 files changed, 31 insertions(+), 13 deletions(-)
> 
> diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
> index 200e2a1da7..16d3b63030 100644
> --- a/hw/arm/virt-acpi-build.c
> +++ b/hw/arm/virt-acpi-build.c
> @@ -64,6 +64,7 @@
>  #include "hw/virtio/virtio-acpi.h"
>  #include "target/arm/cpu.h"
>  #include "target/arm/multiprocessing.h"
> +#include "system/system.h"
>  
>  #define ARM_SPI_BASE 32
>  
> @@ -1031,11 +1032,13 @@ build_dsdt(GArray *table_data, BIOSLinker *linker, VirtMachineState *vms)
>       */
>      scope = aml_scope("\\_SB");
>      acpi_dsdt_add_cpus(scope, vms);
> -    acpi_dsdt_add_uart(scope, &memmap[VIRT_UART0],
> -                       (irqmap[VIRT_UART0] + ARM_SPI_BASE), 0);
> -    if (vms->second_ns_uart_present) {
> -        acpi_dsdt_add_uart(scope, &memmap[VIRT_UART1],
> -                           (irqmap[VIRT_UART1] + ARM_SPI_BASE), 1);
> +    if (serial_exists()) {
> +        acpi_dsdt_add_uart(scope, &memmap[VIRT_UART0],
> +                           (irqmap[VIRT_UART0] + ARM_SPI_BASE), 0);
> +        if (vms->second_ns_uart_present) {
> +            acpi_dsdt_add_uart(scope, &memmap[VIRT_UART1],
> +                               (irqmap[VIRT_UART1] + ARM_SPI_BASE), 1);
> +        }
>      }
>      if (vmc->acpi_expose_flash) {
>          acpi_dsdt_add_flash(scope, &memmap[VIRT_FLASH]);
> @@ -1184,7 +1187,7 @@ void virt_acpi_build(VirtMachineState *vms, AcpiBuildTables *tables)
>  
>      acpi_add_table(table_offsets, tables_blob);
>  
> -    if (ms->acpi_spcr_enabled) {
> +    if (ms->acpi_spcr_enabled && serial_exists()) {
>          spcr_setup(tables_blob, tables->linker, vms);
>      }
>  
> diff --git a/hw/loongarch/virt-acpi-build.c b/hw/loongarch/virt-acpi-build.c
> index 3694c9827f..6eab907087 100644
> --- a/hw/loongarch/virt-acpi-build.c
> +++ b/hw/loongarch/virt-acpi-build.c
> @@ -484,8 +484,10 @@ build_dsdt(GArray *table_data, BIOSLinker *linker, MachineState *machine)
>  
>      acpi_table_begin(&table, table_data);
>      dsdt = init_aml_allocator();
> -    for (i = 0; i < VIRT_UART_COUNT; i++) {
> -        build_uart_device_aml(dsdt, i);
> +    if (serial_exists()) {
> +        for (i = 0; i < VIRT_UART_COUNT; i++) {
> +            build_uart_device_aml(dsdt, i);
> +        }
>      }
>      build_pci_device_aml(dsdt, lvms);
>      build_la_ged_aml(dsdt, machine);
> @@ -557,7 +559,7 @@ static void acpi_build(AcpiBuildTables *tables, MachineState *machine)
>      build_srat(tables_blob, tables->linker, machine);
>      acpi_add_table(table_offsets, tables_blob);
>  
> -    if (machine->acpi_spcr_enabled)
> +    if (machine->acpi_spcr_enabled && serial_exists())
>          spcr_setup(tables_blob, tables->linker, machine);
>  
>      if (machine->numa_state->num_nodes) {
> diff --git a/hw/riscv/virt-acpi-build.c b/hw/riscv/virt-acpi-build.c
> index f1406cb683..e895bc4ddb 100644
> --- a/hw/riscv/virt-acpi-build.c
> +++ b/hw/riscv/virt-acpi-build.c
> @@ -39,6 +39,7 @@
>  #include "qapi/error.h"
>  #include "qemu/error-report.h"
>  #include "system/reset.h"
> +#include "system/system.h"
>  
>  #define ACPI_BUILD_TABLE_SIZE             0x20000
>  #define ACPI_BUILD_INTC_ID(socket, index) ((socket << 24) | (index))
> @@ -474,7 +475,10 @@ static void build_dsdt(GArray *table_data,
>                                   memmap[VIRT_APLIC_S].size, "RSCV0002");
>      }
>  
> -    acpi_dsdt_add_uart(scope, &memmap[VIRT_UART0], UART0_IRQ);
> +    if (serial_exists()) {
> +        acpi_dsdt_add_uart(scope, &memmap[VIRT_UART0], UART0_IRQ);
> +    }
> +
>      if (virt_is_iommu_sys_enabled(s)) {
>          acpi_dsdt_add_iommu_sys(scope, &memmap[VIRT_IOMMU_SYS], IOMMU_SYS_IRQ);
>      }
> @@ -890,7 +894,7 @@ static void virt_acpi_build(RISCVVirtState *s, AcpiBuildTables *tables)
>  
>      acpi_add_table(table_offsets, tables_blob);
>  
> -    if (ms->acpi_spcr_enabled) {
> +    if (ms->acpi_spcr_enabled && serial_exists()) {
>          spcr_setup(tables_blob, tables->linker, s);
>      }
>  
> diff --git a/include/system/system.h b/include/system/system.h
> index 03a2d0e900..4a03f17e91 100644
> --- a/include/system/system.h
> +++ b/include/system/system.h
> @@ -74,6 +74,8 @@ extern unsigned int nb_prom_envs;
>  /* Return the Chardev for serial port i, or NULL if none */
>  Chardev *serial_hd(int i);
>  
> +bool serial_exists(void);
> +
>  /* parallel ports */
>  
>  #define MAX_PARALLEL_PORTS 3
> diff --git a/system/vl.c b/system/vl.c
> index 5091fe52d9..3a2988cb47 100644
> --- a/system/vl.c
> +++ b/system/vl.c
> @@ -1487,6 +1487,11 @@ Chardev *serial_hd(int i)
>      return NULL;
>  }
>  
> +bool serial_exists(void)
> +{
> +    return serial_hd(0) ? true : false;
> +}
> +
>  static bool parallel_parse(const char *devname, Error **errp)
>  {
>      static int index = 0;
> diff --git a/tests/qtest/bios-tables-test.c b/tests/qtest/bios-tables-test.c
> index 6b892ef23e..6b0e4be752 100644
> --- a/tests/qtest/bios-tables-test.c
> +++ b/tests/qtest/bios-tables-test.c
> @@ -824,10 +824,12 @@ static char *test_acpi_create_args(test_data *data, const char *params)
>          /*
>           * TODO: convert '-drive if=pflash' to new syntax (see e33763be7cd3)
>           * when arm/virt boad starts to support it.
> +         * NOTE: Explicitly add "-serial null" to enable uart in DSDT
> +         *       without mixing guest output into TAP stdout.
>           */
>          if (data->cd) {
>              args = g_strdup_printf("-machine %s%s %s -accel tcg "
> -                "-nodefaults -nographic "
> +                "-nodefaults -serial null -nographic "
>                  "-drive if=pflash,format=raw,file=%s,readonly=on "
>                  "-drive if=pflash,format=raw,file=%s,snapshot=on -cdrom %s %s",
>                  data->machine, data->machine_param ?: "",
> @@ -835,7 +837,7 @@ static char *test_acpi_create_args(test_data *data, const char *params)
>                  data->uefi_fl1, data->uefi_fl2, data->cd, params ? params : "");
>          } else {
>              args = g_strdup_printf("-machine %s%s %s -accel tcg "
> -                "-nodefaults -nographic "
> +                "-nodefaults -serial null -nographic "
>                  "-drive if=pflash,format=raw,file=%s,readonly=on "
>                  "-drive if=pflash,format=raw,file=%s,snapshot=on %s",
>                  data->machine, data->machine_param ?: "",
> -- 
> 2.51.0