linux-kernel - [PATCH 1/2] KVM: x86: Only set APICV_INHIBIT_REASON

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Message-Id: <20240417200849.971433-2-alejandro.j.jimenez@oracle.com>
Date: Wed, 17 Apr 2024 20:08:48 +0000
From: Alejandro Jimenez <alejandro.j.jimenez@...cle.com>
To: kvm@...r.kernel.org
Cc: seanjc@...gle.com, pbonzini@...hat.com, linux-kernel@...r.kernel.org,
        joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
        suravee.suthikulpanit@....com, mlevitsk@...hat.com,
        alejandro.j.jimenez@...cle.com
Subject: [PATCH 1/2] KVM: x86: Only set APICV_INHIBIT_REASON_ABSENT if APICv is enabled

Use the APICv enablement status to determine if APICV_INHIBIT_REASON_ABSENT
needs to be set, instead of unconditionally setting the reason during
initialization.

Specifically, in cases where AVIC is disabled via module parameter or lack
of hardware support, unconditionally setting an inhibit reason due to the
absence of an in-kernel local APIC can lead to a scenario where the reason
incorrectly remains set after a local APIC has been created by either
KVM_CREATE_IRQCHIP or the enabling of KVM_CAP_IRQCHIP_SPLIT. This is
because the helpers in charge of removing the inhibit return early if
enable_apicv is not true, and therefore the bit remains set.

This leads to confusion as to the cause why APICv is not active, since an
incorrect reason will be reported by tracepoints and/or a debugging tool
that examines the currently set inhibit reasons.

Fixes: ef8b4b720368 ("KVM: ensure APICv is considered inactive if there is no APIC")
Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@...cle.com>
---
 arch/x86/kvm/x86.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 26288ca05364..eadd88fabadc 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9999,7 +9999,20 @@ static void kvm_apicv_init(struct kvm *kvm)

 	init_rwsem(&kvm->arch.apicv_update_lock);

-	set_or_clear_apicv_inhibit(inhibits, APICV_INHIBIT_REASON_ABSENT, true);
+	/*
+	 * Unconditionally inhibiting APICv due to the absence of in-kernel
+	 * local APIC can lead to a scenario where APICV_INHIBIT_REASON_ABSENT
+	 * remains set in the apicv_inhibit_reasons after a local APIC has been
+	 * created by either KVM_CREATE_IRQCHIP or the enabling of
+	 * KVM_CAP_IRQCHIP_SPLIT.
+	 * Hardware support and module parameters governing APICv enablement
+	 * have already been evaluated and the initial status is available in
+	 * enable_apicv, so it can be used here to determine if an inhibit needs
+	 * to be set.
+	 */
+	if (enable_apicv)
+		set_or_clear_apicv_inhibit(inhibits,
+					   APICV_INHIBIT_REASON_ABSENT, true);

 	if (!enable_apicv)
 		set_or_clear_apicv_inhibit(inhibits,
-- 
2.39.3