linux-kernel - Re: [RFC PATCH v2 15/17] KVM: SVM: Check injected timers for Secure AVIC guests

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Message-ID: <d2f7ae16-6326-3f62-ac0b-c83b68decc1a@amd.com>
Date: Tue, 23 Sep 2025 11:32:30 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Neeraj Upadhyay <Neeraj.Upadhyay@....com>, kvm@...r.kernel.org,
 seanjc@...gle.com, pbonzini@...hat.com
Cc: linux-kernel@...r.kernel.org, nikunj@....com, Santosh.Shukla@....com,
 Vasant.Hegde@....com, Suravee.Suthikulpanit@....com, bp@...en8.de,
 David.Kaplan@....com, huibo.wang@....com, naveen.rao@....com,
 tiala@...rosoft.com
Subject: Re: [RFC PATCH v2 15/17] KVM: SVM: Check injected timers for Secure
 AVIC guests

On 9/23/25 00:03, Neeraj Upadhyay wrote:
> The kvm_wait_lapic_expire() function is a pre-VMRUN optimization that
> allows a vCPU to wait for an imminent LAPIC timer interrupt. However,
> this function is not fully compatible with protected APIC models like
> Secure AVIC because it relies on inspecting KVM's software vAPIC state.
> For Secure AVIC, the true timer state is hardware-managed and opaque
> to KVM. For this reason, kvm_wait_lapic_expire() does not check whether
> timer interrupt is injected for the guests which have protected APIC
> state.
> 
> For the protected APIC guests, the check for injected timer need to be
> done by the callers of kvm_wait_lapic_expire(). So, for Secure AVIC
> guests, check to be injected vectors in the requested_IRR for injected
> timer interrupt before doing a kvm_wait_lapic_expire().
> 
> Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
> ---
>  arch/x86/kvm/svm/sev.c | 8 ++++++++
>  arch/x86/kvm/svm/svm.c | 3 ++-
>  arch/x86/kvm/svm/svm.h | 2 ++
>  3 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 5be2956fb812..3f6cf8d5068a 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -5405,3 +5405,11 @@ bool sev_savic_has_pending_interrupt(struct kvm_vcpu *vcpu)
>  	return READ_ONCE(to_svm(vcpu)->sev_savic_has_pending_ipi) ||
>  		kvm_apic_has_interrupt(vcpu) != -1;
>  }
> +
> +bool sev_savic_timer_int_injected(struct kvm_vcpu *vcpu)
> +{
> +	u32 reg  = kvm_lapic_get_reg(vcpu->arch.apic, APIC_LVTT);

Extra space before the "="

> +	int vec = reg & APIC_VECTOR_MASK;
> +
> +	return to_svm(vcpu)->vmcb->control.requested_irr[vec / 32] & BIT(vec % 32);
> +}
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index a945bc094c1a..d0d972731ea7 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4335,7 +4335,8 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
>  	    vcpu->arch.host_debugctl != svm->vmcb->save.dbgctl)
>  		update_debugctlmsr(svm->vmcb->save.dbgctl);
>  
> -	kvm_wait_lapic_expire(vcpu);
> +	if (!sev_savic_active(vcpu->kvm) || sev_savic_timer_int_injected(vcpu))
> +		kvm_wait_lapic_expire(vcpu);
>  
>  	/*
>  	 * If this vCPU has touched SPEC_CTRL, restore the guest's value if
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 8043833a1a8c..ecc4ea11822d 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -878,6 +878,7 @@ static inline bool sev_savic_active(struct kvm *kvm)
>  }
>  void sev_savic_set_requested_irr(struct vcpu_svm *svm, bool reinjected);
>  bool sev_savic_has_pending_interrupt(struct kvm_vcpu *vcpu);
> +bool sev_savic_timer_int_injected(struct kvm_vcpu *vcpu);
>  #else
>  static inline struct page *snp_safe_alloc_page_node(int node, gfp_t gfp)
>  {
> @@ -917,6 +918,7 @@ static inline struct vmcb_save_area *sev_decrypt_vmsa(struct kvm_vcpu *vcpu)
>  static inline void sev_free_decrypted_vmsa(struct kvm_vcpu *vcpu, struct vmcb_save_area *vmsa) {}
>  static inline void sev_savic_set_requested_irr(struct vcpu_svm *svm, bool reinjected) {}
>  static inline bool sev_savic_has_pending_interrupt(struct kvm_vcpu *vcpu) { return false; }
> +static inline bool sev_savic_timer_int_injected(struct kvm_vcpu *vcpu) { return true; }

Shouldn't this return false? If CONFIG_KVM_AMD_SEV isn't defined, then
sev_savic_active() will always be false and this won't be called anyway.

Thanks,
Tom

>  #endif
>  
>  /* vmenter.S */