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Reply-To: Sean Christopherson <seanjc@google.com>
Date:   Fri,  8 Oct 2021 19:12:13 -0700
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Subject: [PATCH v2 20/43] KVM: VMX: Skip Posted Interrupt updates if APICv is
 hard disabled
From:   Sean Christopherson <seanjc@google.com>
To:     Marc Zyngier <maz@kernel.org>, Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Anup Patel <anup.patel@wdc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Cc:     James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Atish Patra <atish.patra@wdc.com>,
        David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-mips@vger.kernel.org, kvm@vger.kernel.org,
        kvm-ppc@vger.kernel.org, kvm-riscv@lists.infradead.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        David Matlack <dmatlack@google.com>,
        Oliver Upton <oupton@google.com>,
        Jing Zhang <jingzhangos@google.com>
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Precedence: bulk

Explicitly skip posted interrupt updates if APICv is disabled in all of
KVM, or if the guest doesn't have an in-kernel APIC.  The PI descriptor
is kept up-to-date if APICv is inhibited, e.g. so that re-enabling APICv
doesn't require a bunch of updates, but neither the module param nor the
APIC type can be changed on-the-fly.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/vmx/posted_intr.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
index 3263056784f5..351666c41bbc 100644
--- a/arch/x86/kvm/vmx/posted_intr.c
+++ b/arch/x86/kvm/vmx/posted_intr.c
@@ -28,11 +28,14 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
 	unsigned int dest;
 
 	/*
-	 * In case of hot-plug or hot-unplug, we may have to undo
-	 * vmx_vcpu_pi_put even if there is no assigned device.  And we
-	 * always keep PI.NDST up to date for simplicity: it makes the
-	 * code easier, and CPU migration is not a fast path.
+	 * To simplify hot-plug and dynamic toggling of APICv, keep PI.NDST and
+	 * PI.SN up-to-date even if there is no assigned device or if APICv is
+	 * deactivated due to a dynamic inhibit bit, e.g. for Hyper-V's SyncIC.
 	 */
+	if (!enable_apicv || !lapic_in_kernel(vcpu))
+		return;
+
+	/* Nothing to do if PI.SN==0 and the vCPU isn't being migrated. */
 	if (!pi_test_sn(pi_desc) && vcpu->cpu == cpu)
 		return;
 
-- 
2.33.0.882.g93a45727a2-goog