[PATCH-for-9.0 v2] hw/i386/pc: Deprecate 64-bit CPUs on ISA-only PC machine

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

Per Daniel suggestion [*]:

 > isapc could arguably be restricted to just 32-bit CPU models,
 > because we should not need it to support any feature that didn't
 > exist prior to circa 1995. eg refuse to start with isapc, if 'lm'
 > is present in the CPU model for example.

Display a warning when such CPU is used:

  $ qemu-system-x86_64 -monitor stdio -S -M isapc -cpu Westmere
  qemu-system-x86_64: warning: Use of 64-bit CPU 'Westmere' is deprecated on the ISA-only PC machine
  QEMU 8.2.91 monitor - type 'help' for more information
  (qemu) q

  $ qemu-system-x86_64 -monitor stdio -S -M isapc -cpu athlon
  QEMU 8.2.91 monitor - type 'help' for more information
  (qemu) q

[*] https://lore.kernel.org/qemu-devel/ZgQkS4RPmSt5Xa08@redhat.com/

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
---
 docs/about/deprecated.rst |  7 +++++++
 include/hw/i386/pc.h      |  1 +
 hw/i386/pc_piix.c         | 14 ++++++++++++++
 3 files changed, 22 insertions(+)

diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
index 7b548519b5..345c35507f 100644
--- a/docs/about/deprecated.rst
+++ b/docs/about/deprecated.rst
@@ -208,6 +208,13 @@ is no longer packaged in any distro making it harder to run the
 ``check-tcg`` tests. Unless we can improve the testing situation there
 is a chance the code will bitrot without anyone noticing.
 
+64-bit (x86_64) CPUs on the ``isapc`` machine (since 9.0)
+'''''''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+The ``isapc`` machine aims to emulate old PC machine without PCI was
+generalized, so hardware available around 1995, before 64-bit intel
+CPUs were produced.
+
 System emulator machines
 ------------------------
 
diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
index 27a68071d7..2d202b9549 100644
--- a/include/hw/i386/pc.h
+++ b/include/hw/i386/pc.h
@@ -96,6 +96,7 @@ struct PCMachineClass {
     const char *default_south_bridge;
 
     /* Compat options: */
+    bool deprecate_64bit_cpu; /* Specific to the 'isapc' machine */
 
     /* Default CPU model version.  See x86_cpu_set_default_version(). */
     int default_cpu_version;
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 18ba076609..2e5b2efc33 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -182,7 +182,20 @@ static void pc_init1(MachineState *machine, const char *pci_type)
     }
 
     pc_machine_init_sgx_epc(pcms);
+
     x86_cpus_init(x86ms, pcmc->default_cpu_version);
+    if (pcmc->deprecate_64bit_cpu) {
+        X86CPU *cpu = X86_CPU(first_cpu);
+
+        if (cpu->env.features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM) {
+            const char *cpu_type = object_get_typename(OBJECT(first_cpu));
+            int cpu_len = strlen(cpu_type) - strlen(X86_CPU_TYPE_SUFFIX);
+
+            warn_report("Use of 64-bit CPU '%.*s' is deprecated"
+                        " on the ISA-only PC machine",
+                        cpu_len, cpu_type);
+        }
+    }
 
     if (kvm_enabled()) {
         kvmclock_create(pcmc->kvmclock_create_always);
@@ -918,6 +931,7 @@ static void isapc_machine_options(MachineClass *m)
     pcmc->gigabyte_align = false;
     pcmc->smbios_legacy_mode = true;
     pcmc->has_reserved_memory = false;
+    pcmc->deprecate_64bit_cpu = true;
     m->default_nic = "ne2k_isa";
     m->default_cpu_type = X86_CPU_TYPE_NAME("486");
     m->no_parallel = !module_object_class_by_name(TYPE_ISA_PARALLEL);
-- 
2.41.0