Re: [PATCH v6 01/19] hw/i386/pc_piix.c: restrict isapc machine to 32-bit CPUs

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 8/22/2025 8:11 PM, Mark Cave-Ayland wrote:
> The isapc machine represents a legacy ISA PC with a 486 CPU. Whilst it is
> possible to specify any CPU via -cpu on the command line, it makes no
> sense to allow modern 64-bit CPUs to be used.
> 
> Restrict the isapc machine to the available 32-bit CPUs, taking care to
> handle the case where if a user inadvertently uses -cpu max then the "best"
> 32-bit CPU is used (in this case the pentium3).
> 
> Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>   hw/i386/pc_piix.c | 26 ++++++++++++++++++++++++++
>   1 file changed, 26 insertions(+)
> 
> diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
> index c03324281b..5720b6b556 100644
> --- a/hw/i386/pc_piix.c
> +++ b/hw/i386/pc_piix.c
> @@ -436,6 +436,19 @@ static void pc_set_south_bridge(Object *obj, int value, Error **errp)
>   #ifdef CONFIG_ISAPC
>   static void pc_init_isa(MachineState *machine)
>   {
> +    /*
> +     * There is a small chance that someone unintentionally passes "-cpu max"
> +     * for the isapc machine, which will provide a much more modern 32-bit
> +     * CPU than would be expected for an ISA-era PC. If the "max" cpu type has
> +     * been specified, choose the "best" 32-bit cpu possible which we consider
> +     * be the pentium3 (deliberately choosing an Intel CPU given that the
> +     * default 486 CPU for the isapc machine is also an Intel CPU).
> +     */
> +    if (!strcmp(machine->cpu_type, X86_CPU_TYPE_NAME("max"))) {
> +        machine->cpu_type = X86_CPU_TYPE_NAME("pentium3");
> +        warn_report("-cpu max is invalid for isapc machine, using pentium3");
> +    }

Do we need to handle the case of "-cpu host"?

>       pc_init1(machine, NULL);
>   }
>   #endif
> @@ -806,7 +819,19 @@ DEFINE_I440FX_MACHINE(2, 6);
>   #ifdef CONFIG_ISAPC
>   static void isapc_machine_options(MachineClass *m)
>   {
> +    static const char * const valid_cpu_types[] = {
> +        X86_CPU_TYPE_NAME("486"),
> +        X86_CPU_TYPE_NAME("athlon"),
> +        X86_CPU_TYPE_NAME("kvm32"),
> +        X86_CPU_TYPE_NAME("pentium"),
> +        X86_CPU_TYPE_NAME("pentium2"),
> +        X86_CPU_TYPE_NAME("pentium3"),
> +        X86_CPU_TYPE_NAME("qemu32"),
> +        X86_CPU_TYPE_NAME("max"),
> +        NULL
> +    };
>       PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
> +
>       m->desc = "ISA-only PC";
>       m->max_cpus = 1;
>       m->option_rom_has_mr = true;
> @@ -819,6 +844,7 @@ static void isapc_machine_options(MachineClass *m)
>       pcmc->has_reserved_memory = false;
>       m->default_nic = "ne2k_isa";
>       m->default_cpu_type = X86_CPU_TYPE_NAME("486");
> +    m->valid_cpu_types = valid_cpu_types;
>       m->no_floppy = !module_object_class_by_name(TYPE_ISA_FDC);
>       m->no_parallel = !module_object_class_by_name(TYPE_ISA_PARALLEL);
>   }