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

[Thomas Huth <thuth@redhat.com>]

On 28/03/2024 16.12, Mark Cave-Ayland wrote:
> On 27/03/2024 16:54, Philippe Mathieu-Daudé wrote:
> 
>> 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);
> 
> The logic around checking CPUID_EXT2_LM looks good to me. Slightly curious 
> as to whether people feel updating PCMachineClass is necessary, or you can 
> simply do qdev_get_machine() and use object_dynamic_cast() to see if the 
> machine matches MACHINE_NAME("isapc") and warn that way?

Why don't you simply pass it as a parameter from pc_init_isa() instead? Or 
do the whole check in pc_init_isa() instead?

  Thomas