KVM: x86: Add support for local interrupt requests from userspace

In order to enable userspace PIC support, the userspace PIC needs to
be able to inject local interrupts even when the APICs are in the
kernel.

KVM_INTERRUPT now supports sending local interrupts to an APIC when
APICs are in the kernel.

The ready_for_interrupt_request flag is now only set when the CPU/APIC
will immediately accept and inject an interrupt (i.e. APIC has not
masked the PIC).

When the PIC wishes to initiate an INTA cycle with, say, CPU0, it
kicks CPU0 out of the guest, and renedezvous with CPU0 once it arrives
in userspace.

When the CPU/APIC unmasks the PIC, a KVM_EXIT_IRQ_WINDOW_OPEN is
triggered, so that userspace has a chance to inject a PIC interrupt
if it had been pending.

Overall, this design can lead to a small number of spurious userspace
renedezvous. In particular, whenever the PIC transistions from low to
high while it is masked and whenever the PIC becomes unmasked while
it is low.

Note: this does not buffer more than one local interrupt in the
kernel, so the VMM needs to enter the guest in order to complete
interrupt injection before injecting an additional interrupt.

Compiles for x86.

Can pass the KVM Unit Tests.

Signed-off-by: Steve Rutherford <srutherford@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 26 insertions, 6 deletions

diff --git a/arch/x86/kvm/irq.c b/arch/x86/kvm/irq.c
index b653ae202c8e..097060e33bd6 100644
--- a/arch/x86/kvm/irq.c
+++ b/arch/x86/kvm/irq.c
@@ -38,14 +38,27 @@ int kvm_cpu_has_pending_timer(struct kvm_vcpu *vcpu)
 EXPORT_SYMBOL(kvm_cpu_has_pending_timer);
 
 /*
+ * check if there is a pending userspace external interrupt
+ */
+static int pending_userspace_extint(struct kvm_vcpu *v)
+{
+	return v->arch.pending_external_vector != -1;
+}
+
+/*
  * check if there is pending interrupt from
  * non-APIC source without intack.
  */
 static int kvm_cpu_has_extint(struct kvm_vcpu *v)
 {
-	if (kvm_apic_accept_pic_intr(v))
-		return pic_irqchip(v->kvm)->output;	/* PIC */
-	else
+	u8 accept = kvm_apic_accept_pic_intr(v);
+
+	if (accept) {
+		if (irqchip_split(v->kvm))
+			return pending_userspace_extint(v);
+		else
+			return pic_irqchip(v->kvm)->output;
+	} else
 		return 0;
 }
 
@@ -91,9 +104,16 @@ EXPORT_SYMBOL_GPL(kvm_cpu_has_interrupt);
  */
 static int kvm_cpu_get_extint(struct kvm_vcpu *v)
 {
-	if (kvm_cpu_has_extint(v))
-		return kvm_pic_read_irq(v->kvm); /* PIC */
-	return -1;
+	if (kvm_cpu_has_extint(v)) {
+		if (irqchip_split(v->kvm)) {
+			int vector = v->arch.pending_external_vector;
+
+			v->arch.pending_external_vector = -1;
+			return vector;
+		} else
+			return kvm_pic_read_irq(v->kvm); /* PIC */
+	} else
+		return -1;
 }
 
 /*