Re: [PATCH v2 1/3] KVM: x86: Explicitly configure supported XSS from {svm,vmx}_set_cpu_caps()

[Xiaoyao Li]

On 1/29/2026 3:34 PM, Binbin Wu wrote:
> On 1/28/2026 9:43 AM, Sean Christopherson wrote:
> > Explicitly configure KVM's supported XSS as part of each vendor's setup
> > flow to fix a bug where clearing SHSTK and IBT in kvm_cpu_caps, e.g. due
> > to lack of CET XFEATURE support, makes kvm-intel.ko unloadable when nested
> > VMX is enabled, i.e. when nested=1.  The late clearing results in
> > nested_vmx_setup_{entry,exit}_ctls() clearing VM_{ENTRY,EXIT}_LOAD_CET_STATE
> > when nested_vmx_setup_ctls_msrs() runs during the CPU compatibility checks,
> > ultimately leading to a mismatched VMCS config due to the reference config
> > having the CET bits set, but every CPU's "local" config having the bits
> > cleared.
>
> A bit confuse about the description.
>
> Before this patch:
>
> kvm_x86_vendor_init
> | vmx_hardware_setup
> |   nested_vmx_hardware_setup
> |     nested_vmx_setup_ctls_msrs
> | ...
> | for_each_online_cpu(cpu)
> |   smp_call_function_single(cpu, kvm_x86_check_cpu_compat, &r, 1)
> |                                 | kvm_x86_check_processor_compatibility
> |                                 |   kvm_x86_call(check_processor_compatibility)()
> |                                 |     vmx_check_processor_compatibility
> |                                 |       setup_vmcs_config
> |                                 |         nested_vmx_setup_ctls_msrs
> | ...
> | //late clearing of SHSTK and IBT
>
> If we don't consider CPU hotplug case, both the setup of reference VMCS and the
> local config are before the late clearing of SHSTK and IBT. They should be
> consistent.
>
> So you are referring the mismatch situation during CPU hotplug?

I guess it's triggered the path

  kvm_init()
    kvm_init_virtualization()
      kvm_enable_virtualization()
        cpuhp_setup_state()
          kvm_online_cpu()
            ...

  (note, it requires enable_virt_at_load to be true)

which is after
  vmx_init()
    kvm_x86_vendor_init()