KVM: VMX: Remove vCPU from PI wakeup list before updating PID.NV

Remove the vCPU from the wakeup list before updating the notification
vector in the posted interrupt post-block helper.  There is no need to
wake the current vCPU as it is by definition not blocking.  Practically
speaking this is a nop as it only shaves a few meager cycles in the
unlikely case that the vCPU was migrated and the previous pCPU gets a
wakeup IRQ right before PID.NV is updated.  The real motivation is to
allow for more readable code in the future, when post-block is merged
with vmx_vcpu_pi_load(), at which point removal from the list will be
conditional on the old notification vector.

Opportunistically add comments to document why KVM has a per-CPU spinlock
that, at first glance, appears to be taken only on the owning CPU.
Explicitly call out that the spinlock must be taken with IRQs disabled, a
detail that was "lost" when KVM switched from spin_lock_irqsave() to
spin_lock(), with IRQs disabled for the entirety of the relevant path.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-29-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/posted_intr.c

@@ -11,10 +11,22 @@
 #include "vmx.h"
 
 /*
- * We maintain a per-CPU linked-list of vCPU, so in wakeup_handler() we
- * can find which vCPU should be waken up.
+ * Maintain a per-CPU list of vCPUs that need to be awakened by wakeup_handler()
+ * when a WAKEUP_VECTOR interrupted is posted.  vCPUs are added to the list when
+ * the vCPU is scheduled out and is blocking (e.g. in HLT) with IRQs enabled.
+ * The vCPUs posted interrupt descriptor is updated at the same time to set its
+ * notification vector to WAKEUP_VECTOR, so that posted interrupt from devices
+ * wake the target vCPUs.  vCPUs are removed from the list and the notification
+ * vector is reset when the vCPU is scheduled in.
  */
 static DEFINE_PER_CPU(struct list_head, blocked_vcpu_on_cpu);
+/*
+ * Protect the per-CPU list with a per-CPU spinlock to handle task migration.
+ * When a blocking vCPU is awakened _and_ migrated to a different pCPU, the
+ * ->sched_in() path will need to take the vCPU off the list of the _previous_
+ * CPU.  IRQs must be disabled when taking this lock, otherwise deadlock will
+ * occur if a wakeup IRQ arrives and attempts to acquire the lock.
+ */
 static DEFINE_PER_CPU(spinlock_t, blocked_vcpu_on_cpu_lock);
 
 static inline struct pi_desc *vcpu_to_pi_desc(struct kvm_vcpu *vcpu)
@@ -104,6 +116,14 @@ static void __pi_post_block(struct kvm_vcpu *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;
@@ -120,9 +140,6 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
 	} while (cmpxchg64(&pi_desc->control, old.control,
 			   new.control) != old.control);
 
-	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));
 	vcpu->pre_pcpu = -1;
 }