[PATCH v3 04/26] KVM: VMX: Handle PI wakeup shenanigans during vcpu_put/load

Sean Christopherson
2021-12-08T01:55:03Z

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 smack dab in the middle of
being scheduled out.

Note, setting the wakeup vector before halt-polling is not necessary as
the pending IRQ will be recorded in the PIR and detected as a blocking-
breaking condition by kvm_vcpu_has_events() -> vmx_sync_pir_to_irr().

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
---
arch/x86/kvm/vmx/posted_intr.c | 150 ++++++++++++++-------------------
arch/x86/kvm/vmx/posted_intr.h | 8 +-
arch/x86/kvm/vmx/vmx.c | 5 --
3 files changed, 70 insertions(+), 93 deletions(-)

diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
index 88c53c521094..a1776d186225 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) {
+ spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->cpu));
+ list_del(&vcpu->blocked_vcpu_list);
+ 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.
- */
- spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
- list_del(&vcpu->blocked_vcpu_list);
- 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;
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));
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)
{
diff --git a/arch/x86/kvm/vmx/posted_intr.h b/arch/x86/kvm/vmx/posted_intr.h
index 36ae035f14aa..eb14e76b84ef 100644
--- a/arch/x86/kvm/vmx/posted_intr.h
+++ b/arch/x86/kvm/vmx/posted_intr.h
@@ -40,6 +40,12 @@ static inline bool pi_test_and_clear_on(struct pi_desc *pi_desc)
(unsigned long *)&pi_desc->control);
}

+static inline bool pi_test_and_clear_sn(struct pi_desc *pi_desc)
+{
+ return test_and_clear_bit(POSTED_INTR_SN,
+ (unsigned long *)&pi_desc->control);
+}
+
static inline bool pi_test_and_set_pir(int vector, struct pi_desc *pi_desc)
{
return test_and_set_bit(vector, (unsigned long *)pi_desc->pir);
@@ -88,8 +94,6 @@ static inline bool pi_test_sn(struct pi_desc *pi_desc)

void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu);
void vmx_vcpu_pi_put(struct kvm_vcpu *vcpu);
-int pi_pre_block(struct kvm_vcpu *vcpu);
-void pi_post_block(struct kvm_vcpu *vcpu);
void pi_wakeup_handler(void);
void __init pi_init_cpu(int cpu);
bool pi_has_pending_interrupt(struct kvm_vcpu *vcpu);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index a94f0fb80fd4..0254d7f64698 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7442,9 +7442,6 @@ void vmx_update_cpu_dirty_logging(struct kvm_vcpu *vcpu)

static int vmx_pre_block(struct kvm_vcpu *vcpu)
{
- if (pi_pre_block(vcpu))
- return 1;
-
if (kvm_lapic_hv_timer_in_use(vcpu))
kvm_lapic_switch_to_sw_timer(vcpu);

@@ -7455,8 +7452,6 @@ static void vmx_post_block(struct kvm_vcpu *vcpu)
{
if (kvm_x86_ops.set_hv_timer)
kvm_lapic_switch_to_hv_timer(vcpu);
-
- pi_post_block(vcpu);
}

static void vmx_setup_mce(struct kvm_vcpu *vcpu)
--
2.34.1.400.ga245620fadb-goog