Re: [PATCH] KVM: x86: Fix emulated CPUID features being applied to wrong sub-leaf

[Xiaoyao Li]

On 6/9/2026 3:57 PM, Binbin Wu wrote:
> Pass the CPUID index into cpuid_func_emulated() and return no emulated
> features for indexed CPUID leaves with a non-zero index.
>
> KVM currently emulates CPUID features only for index 0, but
> kvm_vcpu_after_set_cpuid() looks up emulated features by function alone.
> As a result, reverse_cpuid[] entries that share a function but use a
> non-zero index, e.g. CPUID.7.1:ECX, can inherit emulated features that
> belong to index 0.  For example, RDPID, which is CPUID.7.0:ECX[22], can
> be incorrectly OR'd into CPUID.7.1:ECX.
>
> This is benign today because the affected bits do not correspond to
> features KVM cares about, but it can become a real bug as new CPUID
> features are defined.  Make the helper index-aware so emulated features
> are applied only to the CPUID entry they actually describe.
>
> Fixes: e592ec657d84 ("KVM: x86: Initialize guest cpu_caps based on KVM support")
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>

Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>