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

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

[Philippe Mathieu-Daudé]

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

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

[Marcin Juszkiewicz]

W dniu 27.03.2024 o 17:54, Philippe Mathieu-Daudé pisze:
> 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.

Can you s/Intel/x86-64/ here? Intel was not first with x86-64 (AMD 
invented it). Also "64-bit Intel" smells of Itanium too much.

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

[Philippe Mathieu-Daudé]

On 27/3/24 18:29, Marcin Juszkiewicz wrote:
> W dniu 27.03.2024 o 17:54, Philippe Mathieu-Daudé pisze:
>> 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.
> 
> Can you s/Intel/x86-64/ here? Intel was not first with x86-64 (AMD 
> invented it). Also "64-bit Intel" smells of Itanium too much.

OK ;)

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

[Mark Cave-Ayland]

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?

FWIW I'd be amazed if anyone were actually overriding the default and trying to do 
this, but I guess that's what the warn_report() is for.... anyhow:

Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>


ATB,

Mark.

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

[Daniel P. Berrangé]

On Wed, Mar 27, 2024 at 05:54:56PM +0100, 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

I've thought of a possible problem here..

   $ qemu-system-x86_64 -monitor stdio -S -M isapc -cpu max

is going to enable 'lm'  (and a bazillion other new features)
for 'isapc', which is a shame, as 'max' is something we want
to be usable in general. I'm not sure how to square that
circle.

I might suggest that 'isapc' instead only makes sense in the
context of qemu-system-i386, since that only has 32-bit CPUs.
We wanted to kill that binary in favour of qemu-system-x86_64
for both 32 & 64 bit though, so we can't block 'isapc' from
qemu-system-x86_64.

> 
> [*] 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
> 

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

[Thomas Huth]

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

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

[Philippe Mathieu-Daudé]

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 :/