Re: [PATCH v5 1/3] KVM: x86: Refactor suppress EOI broadcast logic

[Sean Christopherson <seanjc@google.com>]

On Mon, Dec 29, 2025, Khushit Shah wrote:
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 0ae7f913d782..2c24fd8d815f 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -105,6 +105,39 @@ bool kvm_apic_pending_eoi(struct kvm_vcpu *vcpu, int vector)
>  		apic_test_vector(vector, apic->regs + APIC_IRR);
>  }
>  
> +bool kvm_lapic_advertise_suppress_eoi_broadcast(struct kvm *kvm)

This can be static, its only caller is kvm_apic_set_version().

> +{
> +	/*
> +	 * The default in-kernel I/O APIC emulates the 82093AA and does not
> +	 * implement an EOI register. Some guests (e.g. Windows with the
> +	 * Hyper-V role enabled) disable LAPIC EOI broadcast without checking
> +	 * the I/O APIC version, which can cause level-triggered interrupts to
> +	 * never be EOI'd.
> +	 *
> +	 * To avoid this, KVM must not advertise Suppress EOI Broadcast support
> +	 * when using the default in-kernel I/O APIC.
> +	 *
> +	 * Historically, in split IRQCHIP mode, KVM always advertised Suppress
> +	 * EOI Broadcast support but did not actually suppress EOIs, resulting
> +	 * in quirky behavior.
> +	 */
> +	return !ioapic_in_kernel(kvm);
> +}
> +
> +bool kvm_lapic_respect_suppress_eoi_broadcast(struct kvm *kvm)

I don't see any point in forcing every caller to check SPIV *and* this helper.
Just do:

bool kvm_lapic_suppress_eoi_broadcast(struct kvm_lapic *apic)
{
	struct kvm *kvm = apic->vcpu->kvm;

	if (!(kvm_lapic_get_reg(apic, APIC_SPIV) & APIC_SPIV_DIRECTED_EOI))
		return false;

	switch (kvm->arch.suppress_eoi_broadcast_mode) {

	...
	}
}

And then callers are much more readable, e.g. (spoiler alert if you haven't read
my other mail, which I haven't sent yet):

	if (trigger_mode != IOAPIC_LEVEL_TRIG ||
	    kvm_lapic_suppress_eoi_broadcast(apic))
		return;

and

	/* Request a KVM exit to inform the userspace IOAPIC. */
	if (irqchip_split(apic->vcpu->kvm)) {
		/*
		 * Don't exit to userspace if the guest has enabled Directed
		 * EOI, a.k.a. Suppress EOI Broadcasts, in which case the local
		 * APIC doesn't broadcast EOIs (the guest must EOI the target
		 * I/O APIC(s) directly).
		 */
		if (kvm_lapic_suppress_eoi_broadcast(apic))
			return;

		apic->vcpu->arch.pending_ioapic_eoi = vector;
		kvm_make_request(KVM_REQ_IOAPIC_EOI_EXIT, apic->vcpu);
		return;
	}