On Thu, Dec 11, 2025 at 06:20:25PM +0800, Li Chen wrote:
> From: Li Chen <[email protected]>
>
> 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 <[email protected]>
> Signed-off-by: Li Chen <[email protected]>
> Reviewed-by: Sunil V L <[email protected]>
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
