Re: [PATCH v4 01/18] hw/i386/pc_piix.c: restrict isapc machine to 32-bit CPUs

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 10/7/25 10:52, 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>
> ---
>   hw/i386/pc_piix.c | 25 +++++++++++++++++++++++++
>   1 file changed, 25 insertions(+)
> 
> diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
> index ea7572e783..67c52d79b2 100644
> --- a/hw/i386/pc_piix.c
> +++ b/hw/i386/pc_piix.c
> @@ -418,6 +418,18 @@ 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");

Please warn() the user, otherwise:
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

> +    }
> +
>       pc_init1(machine, NULL);
>   }