RFC: PIC: enhance IPI avoidance

Marcelo Tosatti
2008-07-11T21:09:17Z

The PIC code makes little effort to avoid kvm_vcpu_kick(), resulting in
unnecessary guest exits in some conditions.

For example, if the timer interrupt is routed through the IOAPIC, IRR
for IRQ 0 will get set but not cleared, since the APIC is handling the
acks.

This means that everytime an interrupt < 16 is triggered, the priority
logic will find IRQ0 pending and send an IPI to vcpu0 (in case IRQ0 is
not masked, which is Linux's case).

Introduce a new variable isr_ack to represent the IRQ's for which the
guest has been signalled / cleared the ISR. Use it to avoid more than
one IPI per trigger-ack cycle, in addition to the avoidance when ISR is
set in get_priority().

diff --git a/arch/x86/kvm/i8259.c b/arch/x86/kvm/i8259.c
index c31164e..b8665e3 100644
--- a/arch/x86/kvm/i8259.c
+++ b/arch/x86/kvm/i8259.c
@@ -191,6 +191,7 @@ void kvm_pic_reset(struct kvm_kpic_state *s)
s->irr = 0;
s->imr = 0;
s->isr = 0;
+ s->isr_ack = 0xff;
s->priority_add = 0;
s->irq_base = 0;
s->read_reg_select = 0;
@@ -244,6 +245,7 @@ static void pic_ioport_write(void *opaque, u32 addr, u32 val)
if (priority != 8) {
irq = (priority + s->priority_add) & 7;
s->isr &= ~(1 << irq);
+ s->isr_ack |= (1 << irq);
if (cmd == 5)
s->priority_add = (irq + 1) & 7;
pic_update_irq(s->pics_state);
@@ -252,6 +254,7 @@ static void pic_ioport_write(void *opaque, u32 addr, u32 val)
case 3:
irq = val & 7;
s->isr &= ~(1 << irq);
+ s->isr_ack |= (1 << irq);
pic_update_irq(s->pics_state);
break;
case 6:
@@ -261,6 +264,7 @@ static void pic_ioport_write(void *opaque, u32 addr, u32 val)
case 7:
irq = val & 7;
s->isr &= ~(1 << irq);
+ s->isr_ack |= (1 << irq);
s->priority_add = (irq + 1) & 7;
pic_update_irq(s->pics_state);
break;
@@ -300,10 +304,12 @@ static u32 pic_poll_read(struct kvm_kpic_state *s, u32 addr1)
if (ret >= 0) {
if (addr1 >> 7) {
s->pics_state->pics[0].isr &= ~(1 << 2);
+ s->pics_state->pics[0].isr_ack |= (1 << 2);
s->pics_state->pics[0].irr &= ~(1 << 2);
}
s->irr &= ~(1 << ret);
s->isr &= ~(1 << ret);
+ s->isr_ack |= (1 << ret);
if (addr1 >> 7 || ret != 2)
pic_update_irq(s->pics_state);
} else {
@@ -422,10 +428,14 @@ static void pic_irq_request(void *opaque, int level)
{
struct kvm *kvm = opaque;
struct kvm_vcpu *vcpu = kvm->vcpus[0];
+ struct kvm_pic *s = pic_irqchip(kvm);
+ int irq = pic_get_irq(&s->pics[0]);

- pic_irqchip(kvm)->output = level;
- if (vcpu)
+ s->output = level;
+ if (vcpu && level && (s->pics[0].isr_ack & (1 << irq))) {
+ s->pics[0].isr_ack &= ~(1 << irq);
kvm_vcpu_kick(vcpu);
+ }
}

struct kvm_pic *kvm_create_pic(struct kvm *kvm)
diff --git a/arch/x86/kvm/irq.h b/arch/x86/kvm/irq.h
index 07ff2ae..25922ce 100644
--- a/arch/x86/kvm/irq.h
+++ b/arch/x86/kvm/irq.h
@@ -42,6 +42,7 @@ struct kvm_kpic_state {
u8 irr; /* interrupt request register */
u8 imr; /* interrupt mask register */
u8 isr; /* interrupt service register */
+ u8 isr_ack;
u8 priority_add; /* highest irq priority */
u8 irq_base;
u8 read_reg_select;