linux-kernel - [PATCH] KVM: x86: Synchronize APIC State with QEMU when irqchip=split

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Message-ID: <20250806081051.3533470-1-hugoolli@tencent.com>
Date: Wed,  6 Aug 2025 16:10:51 +0800
From: Yuguo Li <cs.hugolee@...il.com>
To: seanjc@...gle.com,
	pbonzini@...hat.com,
	tglx@...utronix.de,
	mingo@...hat.com,
	bp@...en8.de,
	dave.hansen@...ux.intel.com,
	hpa@...or.com
Cc: x86@...nel.org,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Yuguo Li <hugoolli@...cent.com>
Subject: [PATCH] KVM: x86: Synchronize APIC State with QEMU when irqchip=split

When using split irqchip mode, IOAPIC is handled by QEMU while the LAPIC is emulated by KVM.
When guest disables LINT0, KVM doesn't exit to QEMU for synchronization, leaving IOAPIC unaware of this change.
This may cause vCPU to be kicked when external devices(e.g. PIT)keep sending interrupts.
This patch ensure that KVM exits to QEMU for synchronization when the guest disables LINT0.

Signed-off-by: Yuguo Li <hugoolli@...cent.com>
---
 arch/x86/include/asm/kvm_host.h | 1 +
 arch/x86/kvm/lapic.c            | 4 ++++
 arch/x86/kvm/x86.c              | 5 +++++
 include/uapi/linux/kvm.h        | 1 +
 4 files changed, 11 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index f19a76d3ca0e..f69ce111bbe0 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -129,6 +129,7 @@
 	KVM_ARCH_REQ_FLAGS(32, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
 #define KVM_REQ_UPDATE_PROTECTED_GUEST_STATE \
 	KVM_ARCH_REQ_FLAGS(34, KVM_REQUEST_WAIT)
+#define KVM_REQ_LAPIC_UPDATE              KVM_ARCH_REQ(35)
 
 #define CR0_RESERVED_BITS                                               \
 	(~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 8172c2042dd6..65ffa89bf8a6 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2329,6 +2329,10 @@ static int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
 			val |= APIC_LVT_MASKED;
 		val &= apic_lvt_mask[index];
 		kvm_lapic_set_reg(apic, reg, val);
+		if (irqchip_split(apic->vcpu->kvm) && (val & APIC_LVT_MASKED)) {
+			kvm_make_request(KVM_REQ_LAPIC_UPDATE, apic->vcpu);
+			kvm_vcpu_kick(apic->vcpu);
+		}
 		break;
 	}
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a1c49bc681c4..0d6d29488ee9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10779,6 +10779,11 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 			r = 0;
 			goto out;
 		}
+		if (kvm_check_request(KVM_REQ_LAPIC_UPDATE, vcpu)) {
+			vcpu->run->exit_reason = KVM_EXIT_APIC_SYNC;
+			r = 0;
+			goto out;
+		}
 
 		/*
 		 * KVM_REQ_HV_STIMER has to be processed after
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index f0f0d49d2544..3425076d5c7b 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -179,6 +179,7 @@ struct kvm_xen_exit {
 #define KVM_EXIT_LOONGARCH_IOCSR  38
 #define KVM_EXIT_MEMORY_FAULT     39
 #define KVM_EXIT_TDX              40
+#define KVM_EXIT_APIC_SYNC        41
 
 /* For KVM_EXIT_INTERNAL_ERROR */
 /* Emulate instruction failed. */
-- 
2.43.5