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

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

On Fri, Nov 21, 2025 at 08:34:49AM +0000, Shivansh Dhiman wrote:
> Date: Fri, 21 Nov 2025 08:34:49 +0000
> From: Shivansh Dhiman <shivansh.dhiman@amd.com>
> Subject: [PATCH 2/5] i386: Add CPU property x-force-cpuid-0x80000026
> X-Mailer: git-send-email 2.43.0
> 
> Introduce new CPU property x-force-cpuid-0x80000026 using which the CPUID
> 0x80000026 is enabled. It defaults to false.
> 
> If a vCPU's model is host, then CPUID is enabled based on CPU family/model.
> Implement x86_is_amd_zen4_or_above() helper to detect Zen4+ CPUs using
> family/model.
> 
> Signed-off-by: Shivansh Dhiman <shivansh.dhiman@amd.com>
> ---
>  target/i386/cpu.c |  8 ++++++++
>  target/i386/cpu.h | 18 ++++++++++++++++++
>  2 files changed, 26 insertions(+)
> 
> 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.

So it would be better to add a compat option to disable 0x80000026 for
old PC machines by default.

If needed, to avoid unnecessarily enabling extended CPU topology, I think
it's possible to implement a check similar to x86_has_cpuid_0x1f().

> +            x86_cpu_adjust_level(cpu, &env->cpuid_min_xlevel, 0x80000026);
> +        }
>      }

Thanks,
Zhao