Re: [PATCH v5 2/2] KVM: nSVM: Always intercept VMMCALL when L2 is active

[Yosry Ahmed]

On Tue, Mar 03, 2026 at 04:22:23PM -0800, Sean Christopherson wrote:
> Always intercept VMMCALL now that KVM properly synthesizes a #UD as
> appropriate, i.e. when L1 doesn't want to intercept VMMCALL, to avoid
> putting L2 into an infinite #UD loop if KVM_X86_QUIRK_FIX_HYPERCALL_INSN
> is enabled.
> 
> By letting L2 execute VMMCALL natively and thus #UD, for all intents and
> purposes KVM morphs the VMMCALL intercept into a #UD intercept (KVM always
> intercepts #UD).  When the hypercall quirk is enabled, KVM "emulates"
> VMMCALL in response to the #UD by trying to fixup the opcode to the "right"
> vendor, then restarts the guest, without skipping the VMMCALL.  As a
> result, the guest sees an endless stream of #UDs since it's already
> executing the correct vendor hypercall instruction, i.e. the emulator
> doesn't anticipate that the #UD could be due to lack of interception, as
> opposed to a truly undefined opcode.
> 
> Fixes: 0d945bd93511 ("KVM: SVM: Don't allow nested guest to VMMCALL into host")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/hyperv.h | 4 ----
>  arch/x86/kvm/svm/nested.c | 7 -------
>  2 files changed, 11 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/hyperv.h b/arch/x86/kvm/svm/hyperv.h
> index 9af03970d40c..f70d076911a6 100644
> --- a/arch/x86/kvm/svm/hyperv.h
> +++ b/arch/x86/kvm/svm/hyperv.h
> @@ -51,10 +51,6 @@ static inline bool nested_svm_is_l2_tlb_flush_hcall(struct kvm_vcpu *vcpu)
>  void svm_hv_inject_synthetic_vmexit_post_tlb_flush(struct kvm_vcpu *vcpu);
>  #else /* CONFIG_KVM_HYPERV */
>  static inline void nested_svm_hv_update_vm_vp_ids(struct kvm_vcpu *vcpu) {}
> -static inline bool nested_svm_l2_tlb_flush_enabled(struct kvm_vcpu *vcpu)
> -{
> -	return false;
> -}

Why is this dropped? We still need it for vmmcall_interception under
!CONFIG_KVM_HYPERV, right?