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

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

On 28/3/24 16:39, Thomas Huth wrote:
> 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?

Because the CPU isn't instantiated so we can't check the CPUID_EXT2_LM
feature :/