Re: [PATCH v7 1/1] acpi/virt: suppress SPCR when UART0 has no serial backend

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

On Thu, Feb 19, 2026 at 09:04:02PM +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"), SPCR still points at UART0 and
> the guest may pick it as the preferred console even though it is not
> usable.
> 
> Only generate SPCR when UART0 has a chardev backend attached. Keep the
> UART device description in DSDT unchanged so the guest-visible UART
> topology does not change.
> 
> The bios-tables-test qtests use "-serial null" to keep SPCR enabled while
> discarding firmware serial output, avoiding TAP stdout corruption.
> 
> Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
> Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>
> ---

So -serial null gives a device -serial none hides it from guest.
Sensible. And yet, out of abundance of caution, maybe keep
the legacy behaviour for existing machine types. No?


> v6 -> v7:
>   Do not gate DSDT UART devices on serial_hd(0); only gate SPCR.
>   Drop the serial_exists() helper added by v6.
>   Keep bios-tables-test using "-serial null" (comment wording update).
> 
> v5 -> v6:
>   Fix TAP parsing errors by using "-serial null" in bios-tables-test.
> 
> v4 -> v5:
>   Also suppress UART device & SPCR when guest has no serial hardware on
>   loongarch.
>   Rename serial_exist to serial_exists.
>   Fix style issues.
> 
>  hw/arm/virt-acpi-build.c       | 3 ++-
>  hw/loongarch/virt-acpi-build.c | 2 +-
>  hw/riscv/virt-acpi-build.c     | 3 ++-
>  tests/qtest/bios-tables-test.c | 7 +++++--
>  4 files changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
> index c145678185..1411f98ea1 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
>  
> @@ -1291,7 +1292,7 @@ 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_hd(0)) {
>          spcr_setup(tables_blob, tables->linker, vms);
>      }
>  
> diff --git a/hw/loongarch/virt-acpi-build.c b/hw/loongarch/virt-acpi-build.c
> index 54ac94e17d..dc8bbb9fdc 100644
> --- a/hw/loongarch/virt-acpi-build.c
> +++ b/hw/loongarch/virt-acpi-build.c
> @@ -546,7 +546,7 @@ 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_hd(0))
>          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..29080d1125 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))
> @@ -890,7 +891,7 @@ 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_hd(0)) {
>          spcr_setup(tables_blob, tables->linker, s);
>      }
>  
> diff --git a/tests/qtest/bios-tables-test.c b/tests/qtest/bios-tables-test.c
> index e489d94331..37f2b72d83 100644
> --- a/tests/qtest/bios-tables-test.c
> +++ b/tests/qtest/bios-tables-test.c
> @@ -825,10 +825,13 @@ 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 keep SPCR enabled while
> +         *       discarding firmware serial output (avoid corrupting 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 ?: "",
> @@ -836,7 +839,7 @@ 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 ?: "",
> -- 
> 2.52.0