Re: [RFC PATCH 08/27] KVM: x86: Thread @kvm to KVM CPU capability helpers

[Binbin Wu <binbin.wu@linux.intel.com>]

Below are responses to issues reported by sashiko. [1]

[1] https://sashiko.dev/#/patchset/20260417073610.3246316-1-binbin.wu%40linux.intel.com?part=8

>  
> diff --git a/arch/x86/kvm/vmx/hyperv.c b/arch/x86/kvm/vmx/hyperv.c
> index fa41d036acd4..302f7953b939 100644
> --- a/arch/x86/kvm/vmx/hyperv.c
> +++ b/arch/x86/kvm/vmx/hyperv.c
> @@ -38,7 +38,7 @@ uint16_t nested_get_evmcs_version(struct kvm_vcpu *vcpu)
>  	 * Note, do not check the Hyper-V is fully enabled in guest CPUID, this
>  	 * helper is used to _get_ the vCPU's supported CPUID.
>  	 */
> -	if (kvm_cpu_cap_get(X86_FEATURE_VMX) &&
> +	if (kvm_cpu_cap_get(NULL, X86_FEATURE_VMX) &&
>  	    (!vcpu || to_vmx(vcpu)->nested.enlightened_vmcs_enabled))
>  		return (KVM_EVMCS_VERSION << 8) | 1;
>  

"
Will this hardcoded NULL bypass the VM-specific capability overlay when the
vcpu pointer is available?
Since the patch prepares KVM to select CPUID overlays based on the VM type,
passing NULL here will cause it to fall back to the host's global capability.
If a specific VM type's overlay disables VMX, could this result in KVM
improperly advertising Enlightened VMCS support in the vCPU's Hyper-V CPUID?
Would it be better to respect the VM-specific capability overlay while
preserving support for the global ioctl where vcpu is NULL, for example
by calling kvm_cpu_cap_get(vcpu ? vcpu->kvm : NULL, X86_FEATURE_VMX)?
"

VMX overlay is used here since this function assumes the vcpu is VMX vcpu if
it's not NULL. However, currently, KVM doesn't check whether the vcpu is VMX
in the code paths for EVMCS, e.g. nested_enable_evmcs(). If userspace
requests KVM_CAP_HYPERV_ENLIGHTENED_VMCS on a TDX vCPU, it could corrupt the
TDX vcpu (it's benign for now since the offset of nested.enlightened_vmcs_enabled
is not defined in struct vcpu_tdx, but it could cause problem in the future if
struct vcpu_tdx is extended).

So I am wondering KVM should reject the code paths for EVMCS if the vcpu is TDX.