linux-kernel - [PATCH 1/4] KVM: arm/arm64: vgic: Do not cond_resched

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Date:   Mon, 19 Nov 2018 17:07:56 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     linux-kernel@...r.kernel.org, kvmarm@...ts.cs.columbia.edu
Cc:     marc.zyngier@....com, Christoffer.Dall@....com,
        linux-arm-kernel@...ts.infradead.org,
        linux-rt-users@...r.kernel.org, tglx@...utronix.de,
        rostedt@...dmis.org, bigeasy@...utronix.de,
        Julien Thierry <julien.thierry@....com>,
        Christoffer Dall <christoffer.dall@....com>,
        stable@...r.kernel.org
Subject: [PATCH 1/4] KVM: arm/arm64: vgic: Do not cond_resched_lock() with IRQs disabled

To change the active state of an MMIO, halt is requested for all vcpus of
the affected guest before modifying the IRQ state. This is done by calling
cond_resched_lock() in vgic_mmio_change_active(). However interrupts are
disabled at this point and running a vcpu cannot get rescheduled.

Solve this by waiting for all vcpus to be halted after emmiting the halt
request.

Fixes commit 6c1b7521f4a07cc63bbe2dfe290efed47cdb780a ("KVM: arm/arm64:
Factor out functionality to get vgic mmio requester_vcpu")

Signed-off-by: Julien Thierry <julien.thierry@....com>
Suggested-by: Marc Zyngier <marc.zyngier@....com>
Cc: Christoffer Dall <christoffer.dall@....com>
Cc: Marc Zyngier <marc.zyngier@....com>
Cc: stable@...r.kernel.org
---
 virt/kvm/arm/vgic/vgic-mmio.c | 33 +++++++++++----------------------
 1 file changed, 11 insertions(+), 22 deletions(-)

diff --git a/virt/kvm/arm/vgic/vgic-mmio.c b/virt/kvm/arm/vgic/vgic-mmio.c
index f56ff1c..eefd877 100644
--- a/virt/kvm/arm/vgic/vgic-mmio.c
+++ b/virt/kvm/arm/vgic/vgic-mmio.c
@@ -313,27 +313,6 @@ static void vgic_mmio_change_active(struct kvm_vcpu *vcpu, struct vgic_irq *irq,

 	spin_lock_irqsave(&irq->irq_lock, flags);

-	/*
-	 * If this virtual IRQ was written into a list register, we
-	 * have to make sure the CPU that runs the VCPU thread has
-	 * synced back the LR state to the struct vgic_irq.
-	 *
-	 * As long as the conditions below are true, we know the VCPU thread
-	 * may be on its way back from the guest (we kicked the VCPU thread in
-	 * vgic_change_active_prepare)  and still has to sync back this IRQ,
-	 * so we release and re-acquire the spin_lock to let the other thread
-	 * sync back the IRQ.
-	 *
-	 * When accessing VGIC state from user space, requester_vcpu is
-	 * NULL, which is fine, because we guarantee that no VCPUs are running
-	 * when accessing VGIC state from user space so irq->vcpu->cpu is
-	 * always -1.
-	 */
-	while (irq->vcpu && /* IRQ may have state in an LR somewhere */
-	       irq->vcpu != requester_vcpu && /* Current thread is not the VCPU thread */
-	       irq->vcpu->cpu != -1) /* VCPU thread is running */
-		cond_resched_lock(&irq->irq_lock);
-
 	if (irq->hw) {
 		vgic_hw_irq_change_active(vcpu, irq, active, !requester_vcpu);
 	} else {
@@ -368,8 +347,18 @@ static void vgic_mmio_change_active(struct kvm_vcpu *vcpu, struct vgic_irq *irq,
  */
 static void vgic_change_active_prepare(struct kvm_vcpu *vcpu, u32 intid)
 {
-	if (intid > VGIC_NR_PRIVATE_IRQS)
+	if (intid > VGIC_NR_PRIVATE_IRQS) {
+		struct kvm_vcpu *tmp;
+		int i;
+
 		kvm_arm_halt_guest(vcpu->kvm);
+
+		/* Wait for each vcpu to be halted */
+		kvm_for_each_vcpu(i, tmp, vcpu->kvm) {
+			while (tmp->cpu != -1)
+				cond_resched();
+		}
+	}
 }

 /* See vgic_change_active_prepare */
--
1.9.1