[PATCH 2/2] KVM: SVM: Set target pCPU during IRTE update if target vCPU is running

[Sean Christopherson <seanjc@google.com>]

Update the target pCPU for IOMMU doorbells when updating IRTE routing if
KVM is actively running the associated vCPU.  KVM currently only updates
the pCPU when loading the vCPU (via avic_vcpu_load()), and so doorbell
events will be delivered to the wrong pCPU until the vCPU goes through a
put+load cycle (which might very well "never" happen for the lifetime of
the VM), ultimately resulting in lost IRQs in the guest.

To avoid inserting a stale pCPU, e.g. due to racing between updating IRTE
routing and vCPU load/put, get the pCPU information from the vCPU's
Physical APIC ID table entry (a.k.a. avic_physical_id_cache in KVM) and
update the IRTE while holding ir_list_lock.  Add comments with --verbose
enabled to explain exactly what is and isn't protected by ir_list_lock.

Fixes: 411b44ba80ab ("svm: Implements update_pi_irte hook to setup posted interrupt")
Reported-by: dengqiao.joey <dengqiao.joey@bytedance.com>
Cc: stable@vger.kernel.org
Cc: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Cc: Joao Martins <joao.m.martins@oracle.com>
Cc: Maxim Levitsky <mlevitsk@redhat.com>
Cc: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/avic.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 8e041b215ddb..2092db892d7d 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -791,6 +791,7 @@ static int svm_ir_list_add(struct vcpu_svm *svm, struct amd_iommu_pi_data *pi)
 	int ret = 0;
 	unsigned long flags;
 	struct amd_svm_iommu_ir *ir;
+	u64 entry;
 
 	/**
 	 * In some cases, the existing irte is updated and re-set,
@@ -824,6 +825,18 @@ static int svm_ir_list_add(struct vcpu_svm *svm, struct amd_iommu_pi_data *pi)
 	ir->data = pi->ir_data;
 
 	spin_lock_irqsave(&svm->ir_list_lock, flags);
+
+	/*
+	 * Update the target pCPU for IOMMU doorbells if the vCPU is running.
+	 * If the vCPU is NOT running, i.e. is blocking or scheduled out, KVM
+	 * will update the pCPU info when the vCPU awkened and/or scheduled in.
+	 * See also avic_vcpu_load().
+	 */
+	entry = READ_ONCE(*(svm->avic_physical_id_cache));
+	if (entry & AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK)
+		amd_iommu_update_ga(entry & AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK,
+				    true, pi->ir_data);
+
 	list_add(&ir->node, &svm->ir_list);
 	spin_unlock_irqrestore(&svm->ir_list_lock, flags);
 out:
@@ -1031,6 +1044,13 @@ void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	if (kvm_vcpu_is_blocking(vcpu))
 		return;
 
+	/*
+	 * Grab the per-vCPU interrupt remapping lock even if the VM doesn't
+	 * _currently_ have assigned devices, as that can change.  Holding
+	 * ir_list_lock ensures that either svm_ir_list_add() will consume
+	 * up-to-date entry information, or that this task will wait until
+	 * svm_ir_list_add() completes to set the new target pCPU.
+	 */
 	spin_lock_irqsave(&svm->ir_list_lock, flags);
 
 	entry = READ_ONCE(*(svm->avic_physical_id_cache));
@@ -1067,6 +1087,14 @@ void avic_vcpu_put(struct kvm_vcpu *vcpu)
 	if (!(entry & AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK))
 		return;
 
+	/*
+	 * Take and hold the per-vCPU interrupt remapping lock while updating
+	 * the Physical ID entry even though the lock doesn't protect against
+	 * multiple writers (see above).  Holding ir_list_lock ensures that
+	 * either svm_ir_list_add() will consume up-to-date entry information,
+	 * or that this task will wait until svm_ir_list_add() completes to
+	 * mark the vCPU as not running.
+	 */
 	spin_lock_irqsave(&svm->ir_list_lock, flags);
 
 	avic_update_iommu_vcpu_affinity(vcpu, -1, 0);
-- 
2.41.0.640.ga95def55d0-goog