Re: [PATCH] KVM: x86: skip userspace IOAPIC EOI exit when Directed EOI is enabled

["Huang, Kai" <kai.huang@intel.com>]

> From: Khushit Shah <khushit.shah@nutanix.com>
> Date: Thu, 18 Sep 2025 09:25:28 -0700
> Subject: [PATCH] KVM: x86: Suppress EOI broadcasts with split IRQCHIP if
>  Directed EOI is enabled
> 
> Do not generate a KVM_EXIT_IOAPIC_EOI exit to userspace when handling EOIs
> for a split IRQCHIP and the vCPU has enabled Directed EOIs in its local
> APIC, i.e. if the guest has set "Suppress EOI Broadcasts" in Intel
> parlance.
> 
> Incorrectly broadcasting EOIs can lead to a potentially fatal interrupt
> storm if the IRQ line is still asserted and userspace reacts to the EOI by
> re-injecting the IRQ.  E.g. Windows with Hyper-V enabled gets stuck during
> boot when running under QEMU with a split IRQCHIP.
> 
> Note, Suppress EOI Broadcasts is defined only in Intel's SDM, not in AMD's
> APM.  But the bit is writable on some AMD CPUs, e.g. Turin, and KVM's ABI
> is to support Directed EOI (KVM's name) irrespective of guest CPU vendor.
> 
> Note #2, KVM doesn't support Directed EOIs for its in-kernel I/O APIC.
> See commit 0bcc3fb95b97 ("KVM: lapic: stop advertising DIRECTED_EOI when
> in-kernel IOAPIC is in use").
> 
> Fixes: 7543a635aa09 ("KVM: x86: Add KVM exit for IOAPIC EOIs")
> Cc: stable@vger.kernel.org
> Closes: https://lore.kernel.org/kvm/7D497EF1-607D-4D37-98E7-DAF95F099342@nutanix.com
> Signed-off-by: Khushit Shah <khushit.shah@nutanix.com>
> Link: https://lore.kernel.org/r/20250918162529.640943-1-jon@nutanix.com
> [sean: rewrite changelog and comment]
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Kai Huang <kai.huang@intel.com>

> ---
>  arch/x86/kvm/lapic.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 5fc437341e03..4d77112b887d 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1429,6 +1429,15 @@ static void kvm_ioapic_send_eoi(struct kvm_lapic *apic, int vector)
>  
>  	/* 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 the local APIC
> +		 * doesn't broadcast EOIs (the the guest must EOI the target
> +		 * I/O APIC(s) directly).
> +		 */
> +		if (kvm_lapic_get_reg(apic, APIC_SPIV) & APIC_SPIV_DIRECTED_EOI)
> +			return;
> +
>  		apic->vcpu->arch.pending_ioapic_eoi = vector;
>  		kvm_make_request(KVM_REQ_IOAPIC_EOI_EXIT, apic->vcpu);
>  		return;
> 
> base-commit: 07e27ad16399afcd693be20211b0dfae63e0615f
> --