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

["Daniel P. Berrangé" <berrange@redhat.com>]

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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