KVM: VMX: Handle PI descriptor updates during vcpu_put/load

Move the posted interrupt pre/post_block logic into vcpu_put/load
respectively, using the kvm_vcpu_is_blocking() to determining whether or
not the wakeup handler needs to be set (and unset).  This avoids updating
the PI descriptor if halt-polling is successful, reduces the number of
touchpoints for updating the descriptor, and eliminates the confusing
behavior of intentionally leaving a "stale" PI.NDST when a blocking vCPU
is scheduled back in after preemption.

The downside is that KVM will do the PID update twice if the vCPU is
preempted after prepare_to_rcuwait() but before schedule(), but that's a
rare case (and non-existent on !PREEMPT kernels).

The notable wart is the need to send a self-IPI on the wakeup vector if
an outstanding notification is pending after configuring the wakeup
vector.  Ideally, KVM would just do a kvm_vcpu_wake_up() in this case,
but the scheduler doesn't support waking a task from its preemption
notifier callback, i.e. while the task is right in the middle of
being scheduled out.

Note, setting the wakeup vector before halt-polling is not necessary:
once the pending IRQ will be recorded in the PIR, kvm_vcpu_has_events()
will detect this (via kvm_cpu_get_interrupt(), kvm_apic_get_interrupt(),
apic_has_interrupt_for_ppr() and finally vmx_sync_pir_to_irr()) and
terminate the polling.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20211208015236.1616697-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 64 insertions, 86 deletions

diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
index a82a15aa1982..63e2399f353a 100644
--- a/arch/x86/kvm/vmx/posted_intr.c
+++ b/arch/x86/kvm/vmx/posted_intr.c
@@ -52,6 +52,7 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
 {
 	struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
 	struct pi_desc old, new;
+	unsigned long flags;
 	unsigned int dest;
 
 	/*
@@ -62,23 +63,34 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
 	if (!enable_apicv || !lapic_in_kernel(vcpu))
 		return;
 
-	/* Nothing to do if PI.SN and PI.NDST both have the desired value. */
-	if (!pi_test_sn(pi_desc) && vcpu->cpu == cpu)
+	/*
+	 * If the vCPU wasn't on the wakeup list and wasn't migrated, then the
+	 * full update can be skipped as neither the vector nor the destination
+	 * needs to be changed.
+	 */
+	if (pi_desc->nv != POSTED_INTR_WAKEUP_VECTOR && vcpu->cpu == cpu) {
+		/*
+		 * Clear SN if it was set due to being preempted.  Again, do
+		 * this even if there is no assigned device for simplicity.
+		 */
+		if (pi_test_and_clear_sn(pi_desc))
+			goto after_clear_sn;
 		return;
+	}
+
+	local_irq_save(flags);
 
 	/*
-	 * If the 'nv' field is POSTED_INTR_WAKEUP_VECTOR, do not change
-	 * PI.NDST: pi_post_block is the one expected to change PID.NDST and the
-	 * wakeup handler expects the vCPU to be on the blocked_vcpu_list that
-	 * matches PI.NDST. Otherwise, a vcpu may not be able to be woken up
-	 * correctly.
+	 * If the vCPU was waiting for wakeup, remove the vCPU from the wakeup
+	 * list of the _previous_ pCPU, which will not be the same as the
+	 * current pCPU if the task was migrated.
 	 */
-	if (pi_desc->nv == POSTED_INTR_WAKEUP_VECTOR || vcpu->cpu == cpu) {
-		pi_clear_sn(pi_desc);
-		goto after_clear_sn;
+	if (pi_desc->nv == POSTED_INTR_WAKEUP_VECTOR) {
+		raw_spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->cpu));
+		list_del(&vcpu->blocked_vcpu_list);
+		raw_spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->cpu));
 	}
 
-	/* The full case.  Set the new destination and clear SN. */
 	dest = cpu_physical_id(cpu);
 	if (!x2apic_mode)
 		dest = (dest << 8) & 0xFF00;
@@ -86,10 +98,22 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
 	do {
 		old.control = new.control = READ_ONCE(pi_desc->control);
 
+		/*
+		 * Clear SN (as above) and refresh the destination APIC ID to
+		 * handle task migration (@cpu != vcpu->cpu).
+		 */
 		new.ndst = dest;
 		new.sn = 0;
+
+		/*
+		 * Restore the notification vector; in the blocking case, the
+		 * descriptor was modified on "put" to use the wakeup vector.
+		 */
+		new.nv = POSTED_INTR_VECTOR;
 	} while (pi_try_set_control(pi_desc, old.control, new.control));
 
+	local_irq_restore(flags);
+
 after_clear_sn:
 
 	/*
@@ -111,83 +135,24 @@ static bool vmx_can_use_vtd_pi(struct kvm *kvm)
 		irq_remapping_cap(IRQ_POSTING_CAP);
 }
 
-void vmx_vcpu_pi_put(struct kvm_vcpu *vcpu)
-{
-	struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
-
-	if (!vmx_can_use_vtd_pi(vcpu->kvm))
-		return;
-
-	/* Set SN when the vCPU is preempted */
-	if (vcpu->preempted)
-		pi_set_sn(pi_desc);
-}
-
-static void __pi_post_block(struct kvm_vcpu *vcpu)
-{
-	struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
-	struct pi_desc old, new;
-	unsigned int dest;
-
-	/*
-	 * Remove the vCPU from the wakeup list of the _previous_ pCPU, which
-	 * will not be the same as the current pCPU if the task was migrated.
-	 */
-	raw_spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
-	list_del(&vcpu->blocked_vcpu_list);
-	raw_spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
-
-	dest = cpu_physical_id(vcpu->cpu);
-	if (!x2apic_mode)
-		dest = (dest << 8) & 0xFF00;
-
-	WARN(pi_desc->nv != POSTED_INTR_WAKEUP_VECTOR,
-	     "Wakeup handler not enabled while the vCPU was blocking");
-
-	do {
-		old.control = new.control = READ_ONCE(pi_desc->control);
-
-		new.ndst = dest;
-
-		/* set 'NV' to 'notification vector' */
-		new.nv = POSTED_INTR_VECTOR;
-	} while (pi_try_set_control(pi_desc, old.control, new.control));
-
-	vcpu->pre_pcpu = -1;
-}
-
 /*
- * This routine does the following things for vCPU which is going
- * to be blocked if VT-d PI is enabled.
- * - Store the vCPU to the wakeup list, so when interrupts happen
- *   we can find the right vCPU to wake up.
- * - Change the Posted-interrupt descriptor as below:
- *      'NV' <-- POSTED_INTR_WAKEUP_VECTOR
- * - If 'ON' is set during this process, which means at least one
- *   interrupt is posted for this vCPU, we cannot block it, in
- *   this case, return 1, otherwise, return 0.
- *
+ * Put the vCPU on this pCPU's list of vCPUs that needs to be awakened and set
+ * WAKEUP as the notification vector in the PI descriptor.
  */
-int pi_pre_block(struct kvm_vcpu *vcpu)
+static void pi_enable_wakeup_handler(struct kvm_vcpu *vcpu)
 {
-	struct pi_desc old, new;
 	struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
+	struct pi_desc old, new;
 	unsigned long flags;
 
-	if (!vmx_can_use_vtd_pi(vcpu->kvm) ||
-	    vmx_interrupt_blocked(vcpu))
-		return 0;
-
 	local_irq_save(flags);
 
-	vcpu->pre_pcpu = vcpu->cpu;
 	raw_spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->cpu));
 	list_add_tail(&vcpu->blocked_vcpu_list,
 		      &per_cpu(blocked_vcpu_on_cpu, vcpu->cpu));
 	raw_spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->cpu));
 
-	WARN(pi_desc->sn == 1,
-	     "Posted Interrupt Suppress Notification set before blocking");
+	WARN(pi_desc->sn, "PI descriptor SN field set before blocking");
 
 	do {
 		old.control = new.control = READ_ONCE(pi_desc->control);
@@ -196,24 +161,37 @@ int pi_pre_block(struct kvm_vcpu *vcpu)
 		new.nv = POSTED_INTR_WAKEUP_VECTOR;
 	} while (pi_try_set_control(pi_desc, old.control, new.control));
 
-	/* We should not block the vCPU if an interrupt is posted for it.  */
-	if (pi_test_on(pi_desc))
-		__pi_post_block(vcpu);
+	/*
+	 * Send a wakeup IPI to this CPU if an interrupt may have been posted
+	 * before the notification vector was updated, in which case the IRQ
+	 * will arrive on the non-wakeup vector.  An IPI is needed as calling
+	 * try_to_wake_up() from ->sched_out() isn't allowed (IRQs are not
+	 * enabled until it is safe to call try_to_wake_up() on the task being
+	 * scheduled out).
+	 */
+	if (pi_test_on(&new))
+		apic->send_IPI_self(POSTED_INTR_WAKEUP_VECTOR);
 
 	local_irq_restore(flags);
-	return (vcpu->pre_pcpu == -1);
 }
 
-void pi_post_block(struct kvm_vcpu *vcpu)
+void vmx_vcpu_pi_put(struct kvm_vcpu *vcpu)
 {
-	unsigned long flags;
+	struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
 
-	if (vcpu->pre_pcpu == -1)
+	if (!vmx_can_use_vtd_pi(vcpu->kvm))
 		return;
 
-	local_irq_save(flags);
-	__pi_post_block(vcpu);
-	local_irq_restore(flags);
+	if (kvm_vcpu_is_blocking(vcpu) && !vmx_interrupt_blocked(vcpu))
+		pi_enable_wakeup_handler(vcpu);
+
+	/*
+	 * Set SN when the vCPU is preempted.  Note, the vCPU can both be seen
+	 * as blocking and preempted, e.g. if it's preempted between setting
+	 * its wait state and manually scheduling out.
+	 */
+	if (vcpu->preempted)
+		pi_set_sn(pi_desc);
 }
 
 /*
@@ -254,7 +232,7 @@ bool pi_has_pending_interrupt(struct kvm_vcpu *vcpu)
  * Bail out of the block loop if the VM has an assigned
  * device, but the blocking vCPU didn't reconfigure the
  * PI.NV to the wakeup vector, i.e. the assigned device
- * came along after the initial check in pi_pre_block().
+ * came along after the initial check in vmx_vcpu_pi_put().
  */
 void vmx_pi_start_assignment(struct kvm *kvm)
 {