linux-kernel - Re: [PATCH v4] KVM: x86: Add x2APIC "features" to control EOI broadcast suppression

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Message-ID: <9B3CAB86-B3E6-475E-8849-85CE63BD7E36@nutanix.com>
Date: Wed, 17 Dec 2025 12:06:43 +0000
From: Khushit Shah <khushit.shah@...anix.com>
To: Sean Christopherson <seanjc@...gle.com>,
        David Woodhouse
	<dwmw2@...radead.org>
CC: "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "kai.huang@...el.com"
	<kai.huang@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        Jon Kohler <jon@...anix.com>,
        Shaju Abraham <shaju.abraham@...anix.com>,
        "stable@...r.kernel.org"
	<stable@...r.kernel.org>
Subject: Re: [PATCH v4] KVM: x86: Add x2APIC "features" to control EOI
 broadcast suppression



> On 12 Dec 2025, at 12:38 PM, Khushit Shah <khushit.shah@...anix.com> wrote:
> 
> @@ -1515,6 +1552,17 @@ static void kvm_ioapic_send_eoi(struct kvm_lapic *apic, int vector)
>        if (apic->vcpu->arch.highest_stale_pending_ioapic_eoi == vector)
>                kvm_make_request(KVM_REQ_SCAN_IOAPIC, apic->vcpu);
> 
> +       /*
> +       * Don't send the EOI to the I/O APIC 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). Ignore the suppression if the guest has not
> +       * explicitly enabled Suppress EOI broadcast.
> +       */
> +       if ((kvm_lapic_get_reg(apic, APIC_SPIV) & APIC_SPIV_DIRECTED_EOI) &&
> +                 !kvm_lapic_ignore_suppress_eoi_broadcast(apic->vcpu->kvm))
> +               return;
> +
>        /* Request a KVM exit to inform the userspace IOAPIC. */
>        if (irqchip_split(apic->vcpu->kvm)) {
>                apic->vcpu->arch.pending_ioapic_eoi = vector;
> 
> 
> I am not entirely sure if returning from kvm_ioapic_send_eoi() early is correct
> for kernel IOAPIC. The original code (which is now redundant) does this very 
> late in kvm_ioapic_update_eoi_one().


Am I correct in assuming we still need a call to rtc_irq_eoi() even if the guest
has enabled SEOIB? 

We will call kvm_ioapic_update_eoi_one() on I/O APIC EOIR write. But, the
following condition in kvm_ioapic_update_eoi_one() blocks EOI processing:

if (trigger_mode != IOAPIC_LEVEL_TRIG || 
        kvm_lapic_get_reg(apic, APIC_SPIV) & APIC_SPIV_DIRECTED_EOI)
                return;

So, the condition needs to moved from kvm_ioapic_update_eoi_one(). It makes
sense to keep it in kvm_ioapic_send_eoi() even for kernel IRQCHIP. But if a call
to rtc_irq_eoi() is required (likely), then we need something similar to following: 

diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
index 2c2783296aed..76e511a36699 100644
--- a/arch/x86/kvm/ioapic.c
+++ b/arch/x86/kvm/ioapic.c
@@ -560,8 +560,7 @@ static void kvm_ioapic_update_eoi_one(struct kvm_vcpu *vcpu,
        kvm_notify_acked_irq(ioapic->kvm, KVM_IRQCHIP_IOAPIC, pin);
        spin_lock(&ioapic->lock);
 
-       if (trigger_mode != IOAPIC_LEVEL_TRIG ||
-           kvm_lapic_get_reg(apic, APIC_SPIV) & APIC_SPIV_DIRECTED_EOI)
+       if (trigger_mode != IOAPIC_LEVEL_TRIG)
                return;
 
        ASSERT(ent->fields.trig_mode == IOAPIC_LEVEL_TRIG);
@@ -595,6 +594,11 @@ void kvm_ioapic_update_eoi(struct kvm_vcpu *vcpu, int vector, int trigger_mode)
 
        spin_lock(&ioapic->lock);
        rtc_irq_eoi(ioapic, vcpu, vector);
+
+       if((kvm_lapic_get_reg(apic, APIC_SPIV) & APIC_SPIV_DIRECTED_EOI) &&
+           kvm_lapic_respect_suppress_eoi_broadcast(ioapic->kvm))
+               goto out;
+
        for (i = 0; i < IOAPIC_NUM_PINS; i++) {
                union kvm_ioapic_redirect_entry *ent = &ioapic->redirtbl[i];
 
@@ -602,6 +606,8 @@ void kvm_ioapic_update_eoi(struct kvm_vcpu *vcpu, int vector, int trigger_mode)
                        continue;
                kvm_ioapic_update_eoi_one(vcpu, ioapic, trigger_mode, i);
        }
+
+out:
        spin_unlock(&ioapic->lock);
 }
---
Finally, just to double-check, it is safe to not call kvm_notify_acked_irq()
on LAPIC EOI when guest has enabled Suppress EOI Broadcast, right? As it will 
anyway be called on Direct EOI.

Thanks, 
Khushit