Re: [PATCH v2] KVM: x86: Track kvm_vcpu_arch.pending_ioapic_eoi as a u8 to drop dead BUG_ON()

On Thu, Aug 6, 2026 at 7:14 PM Sean Christopherson <[email protected]> wrote:
>
> Track the vector associated with KVM_EXIT_IOAPIC_EOI as a u8, as the vector
> is zero-initialized and is only ever set to a legal vector.  I.e. unlike
> many of the other vector variables in KVM, it can't be -1 (and can't be
> greater than 255, though that should hold true for all vectors in KVM).
>
> Drop the now fully dead BUG_ON(), which aside from being completely
> unnecessary, was also useless, as it wouldn't guard against a negative
> value, i.e. wouldn't detect KVM's magic -1 value.
>
> For all intents and purposes, no functional change intended.
>
> Suggested-by: Paolo Bonzini <[email protected]>

Reviewed-by: Paolo Bonzini <[email protected]>

Paolo

> Signed-off-by: Sean Christopherson <[email protected]>
> ---
>
> v2: Use a u8 to make it impossible for the test to be out-of-bounds. [Paolo]
>
> v1: https://lore.kernel.org/all/[email protected]
>
>  arch/x86/include/asm/kvm_host.h | 2 +-
>  arch/x86/kvm/lapic.c            | 4 ++--
>  arch/x86/kvm/lapic.h            | 2 +-
>  arch/x86/kvm/vmx/vmx.c          | 3 +--
>  arch/x86/kvm/x86.c              | 1 -
>  5 files changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 283847619ff8..9614314fd237 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -973,7 +973,7 @@ struct kvm_vcpu_arch {
>                 bool pv_unhalted;
>         } pv;
>
> -       int pending_ioapic_eoi;
> +       u8 pending_ioapic_eoi;
>         int pending_external_vector;
>         int highest_stale_pending_ioapic_eoi;
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 586a5cee29e1..87bb8e5f4ca1 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1552,7 +1552,7 @@ static bool kvm_ioapic_handles_vector(struct kvm_lapic *apic, int vector)
>         return test_bit(vector, apic->vcpu->arch.ioapic_handled_vectors);
>  }
>
> -static void kvm_ioapic_send_eoi(struct kvm_lapic *apic, int vector)
> +static void kvm_ioapic_send_eoi(struct kvm_lapic *apic, u8 vector)
>  {
>         int __maybe_unused trigger_mode;
>
> @@ -1622,7 +1622,7 @@ static int apic_set_eoi(struct kvm_lapic *apic)
>   * this interface assumes a trap-like exit, which has already finished
>   * desired side effect including vISR and vPPR update.
>   */
> -void kvm_apic_set_eoi_accelerated(struct kvm_vcpu *vcpu, int vector)
> +void kvm_apic_set_eoi_accelerated(struct kvm_vcpu *vcpu, u8 vector)
>  {
>         struct kvm_lapic *apic = vcpu->arch.apic;
>
> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> index bd1098c89d99..db071d0b172d 100644
> --- a/arch/x86/kvm/lapic.h
> +++ b/arch/x86/kvm/lapic.h
> @@ -177,7 +177,7 @@ u64 kvm_get_lapic_tscdeadline_msr(struct kvm_vcpu *vcpu);
>  void kvm_set_lapic_tscdeadline_msr(struct kvm_vcpu *vcpu, u64 data);
>
>  void kvm_apic_write_nodecode(struct kvm_vcpu *vcpu, u32 offset);
> -void kvm_apic_set_eoi_accelerated(struct kvm_vcpu *vcpu, int vector);
> +void kvm_apic_set_eoi_accelerated(struct kvm_vcpu *vcpu, u8 vector);
>
>  int kvm_lapic_set_vapic_addr(struct kvm_vcpu *vcpu, gpa_t vapic_addr);
>  void kvm_lapic_sync_from_vapic(struct kvm_vcpu *vcpu);
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index e3bfe6aca1a0..c1fdfccd20fa 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5886,10 +5886,9 @@ static int handle_apic_access(struct kvm_vcpu *vcpu)
>  static int handle_apic_eoi_induced(struct kvm_vcpu *vcpu)
>  {
>         unsigned long exit_qualification = vmx_get_exit_qual(vcpu);
> -       int vector = exit_qualification & 0xff;
>
>         /* EOI-induced VM exit is trap-like and thus no need to adjust IP */
> -       kvm_apic_set_eoi_accelerated(vcpu, vector);
> +       kvm_apic_set_eoi_accelerated(vcpu, exit_qualification & 0xff);
>         return 1;
>  }
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d94b59140c45..8b23f342a1fa 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8151,7 +8151,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>                 if (kvm_check_request(KVM_REQ_NMI, vcpu))
>                         process_nmi(vcpu);
>                 if (kvm_check_request(KVM_REQ_IOAPIC_EOI_EXIT, vcpu)) {
> -                       BUG_ON(vcpu->arch.pending_ioapic_eoi > 255);
>                         if (test_bit(vcpu->arch.pending_ioapic_eoi,
>                                      vcpu->arch.ioapic_handled_vectors)) {
>                                 vcpu->run->exit_reason = KVM_EXIT_IOAPIC_EOI;
>
> base-commit: a806d364ef288a6443a1337820ea8410a7ccc6b3
> --
> 2.55.0.679.g6767b8d81c-goog
>