Re: [PATCH v2 1/3] KVM: SVM: Configure ALLOW_LARGER_RAP in svm_vcpu_after_set_cpuid()

[Sean Christopherson <seanjc@google.com>]

On Fri, Jul 17, 2026, Jim Mattson wrote:
> On initial vCPU creation, init_vmcb() is called before userspace configures
> guest CPUID via KVM_SET_CPUID2. Consequently, ERAP_CONTROL_ALLOW_LARGER_RAP
> is never set during initial vCPU creation. If an AP later receives an INIT
> signal after CPUID has been set, init_vmcb() would set the bit for that AP,
> but the BSP would be left with ALLOW_LARGER_RAP clear.
> 
> Configure ALLOW_LARGER_RAP in svm_vcpu_after_set_cpuid() to handle the BSP.
> 
> Keep the code in init_vmcb() to restore ALLOW_LARGER_RAP after the VMCB has
> been cleared on the VCPU triple-fault reset path.
> 
> Fixes: db5e82496492 ("KVM: SVM: Virtualize and advertise support for ERAPS")
> Assisted-by: Gemini:Gemini-Next
> Signed-off-by: Jim Mattson <jmattson@google.com>
> ---
>  arch/x86/kvm/svm/svm.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 4d2bacd00ec4..bf868da14cd2 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4736,6 +4736,11 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  	if (guest_cpuid_is_intel_compatible(vcpu))
>  		guest_cpu_cap_clear(vcpu, X86_FEATURE_V_VMSAVE_VMLOAD);
>  
> +	if (guest_cpu_cap_has(vcpu, X86_FEATURE_ERAPS))
> +		svm->vmcb01.ptr->control.erap_ctl |= ERAP_CONTROL_ALLOW_LARGER_RAP;
> +	else
> +		svm->vmcb01.ptr->control.erap_ctl &= ~ERAP_CONTROL_ALLOW_LARGER_RAP;

If we handle ERAPS here, can't we drop the code in init_vmcb()?  In the context
of PML enablement, AMD is updating the APM to clarify that the control area remains
valid after shutdown, i.e. KVM doesn't need to re-initialize erap_ctl when
synthesizing INIT from shutdown_interception().

  "After an intercepted shutdown, the VMCB control area is valid (with the
   exception of offsets 60h, 61h, and 68h) and the VMCB state save area is
   undefined."

https://lore.kernel.org/all/6688fe2e-c723-404a-80e1-3f4c5f5cc4c6@amd.com


> +
>  	if (is_sev_guest(vcpu))
>  		sev_vcpu_after_set_cpuid(svm);
>  }
> -- 
> 2.55.0.229.g6434b31f56-goog
>