From: Mark Cave-Ayland <mark.caveayland@nutanix.com>

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 either -cpu max or
-cpu host 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>
Link: https://lore.kernel.org/r/20250828111057.468712-2-mark.caveayland@nutanix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/i386/pc_piix.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index d165ac72ed7..8f5fb3cf90b 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -436,6 +436,31 @@ 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");
+    }
+
+    /*
+     * Similarly if someone unintentionally passes "-cpu host" for the isapc
+     * machine then display a warning and also switch to the "best" 32-bit
+     * cpu possible which we consider to be the pentium3. This is because any
+     * host CPU will already be modern than this, but it also ensures any
+     * newer CPU flags/features are filtered out for older guests.
+     */
+    if (!strcmp(machine->cpu_type, X86_CPU_TYPE_NAME("host"))) {
+        machine->cpu_type = X86_CPU_TYPE_NAME("pentium3");
+        warn_report("-cpu host is invalid for isapc machine, using pentium3");
+    }
+
     pc_init1(machine, NULL);
 }
 #endif
@@ -815,7 +840,20 @@ 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"),
+        X86_CPU_TYPE_NAME("host"),
+        NULL
+    };
     PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
+
     m->desc = "ISA-only PC";
     m->max_cpus = 1;
     m->option_rom_has_mr = true;
@@ -828,6 +866,7 @@ 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);
 }
-- 
2.51.0