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

[Xiaoyao Li]

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.caveayl...@nutanix.com>
Reviewed-by: Philippe Mathieu-Daudé <phi...@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);
  }