Re: [PATCH 2/5] i386: Add CPU property x-force-cpuid-0x80000026

[Zhao Liu <zhao1.liu@intel.com>]

> On 07-01-2026 13:17, Zhao Liu wrote:
> > On Fri, Nov 21, 2025 at 08:34:49AM +0000, Shivansh Dhiman wrote:
> >> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> >> index b7827e448aa5..01c4da7cf134 100644
> >> --- a/target/i386/cpu.c
> >> +++ b/target/i386/cpu.c
> >> @@ -9158,6 +9158,12 @@ void x86_cpu_expand_features(X86CPU *cpu, Error **errp)
> >>          if (env->features[FEAT_7_0_EBX] & CPUID_7_0_EBX_SGX) {
> >>              x86_cpu_adjust_level(cpu, &env->cpuid_min_level, 0x12);
> >>          }
> >> +
> >> +        /* Enable CPUID[0x80000026] for AMD Genoa models and above */
> >> +        if (cpu->force_cpuid_0x80000026 ||
> >> +            (!xcc->model && x86_is_amd_zen4_or_above(cpu))) {
> > 
> > I understand you want to address max/host CPU case here, but it's still
> > may not guarentee the compatibility with old QEMU PC mahinces, e.g.,
> > boot a old PC machine on v11.0 QEMU, it can still have this leaf.
> 
> Wouldn't initializing x-force-cpuid-0x80000026 default to false prevent this?
> Oh, but, this CPUID can still be enabled on an older machine-type with latest
> QEMU with the existing checks. And probably this could also affect live migration.

Yes, on a zen4 host, booting an older machine with latest QEMU will have
this CPUID leaf.
 
> > So it would be better to add a compat option to disable 0x80000026 for
> > old PC machines by default.
> 
> Does this look fine?
> 
> GlobalProperty pc_compat_10_2[] = {
>     { TYPE_X86_CPU, "x-force-cpuid-0x80000026", "false" },
> };
> const size_t pc_compat_10_2_len = G_N_ELEMENTS(pc_compat_10_2);

It looks fine if we only check "if (cpu->force_cpuid_0x80000026)".

> > If needed, to avoid unnecessarily enabling extended CPU topology, I think
> > it's possible to implement a check similar to x86_has_cpuid_0x1f().
> 
> Do you mean something like this? I avoided it initially because it is
> functionally same as current one, and a bit lengthy.

Sorry for confusion. Could we get rid of model check
(x86_is_amd_zen4_or_above)? and could we do something like 0x1f leaf,

static inline bool x86_has_cpuid_0x1f(X86CPU *cpu)
{
    return cpu->force_cpuid_0x1f ||
           x86_has_extended_topo(cpu->env.avail_cpu_topo);
	   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
}

similarly, apply x86_has_extended_topo() for AMD CPU as well?

x86_has_extended_topo() also checks "module" level, but I think we could
return error in encode_topo_cpuid80000026() for unsupported "moduel"
level?

Thus, when users explicitly set these levels, the 0x80000026 leaf will be
enabled.

Furthermore, I think it's better that different x86 vendors could adopt
similar behavior for these extended topology levels, especially since
they are all all configured through a unified "-smp" interface.

Thanks,
Zhao