This patched worked very nicely for me -- about an 8% performance
improvement for my workload.
david
Marcelo Tosatti wrote:
> KVM: PIC: enhance IPI avoidance
>
> 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().
>
> Signed-off-by: Marcelo Tosatti <[EMAIL PROTECTED]>
>
>
> Index: kvm/arch/x86/kvm/i8259.c
> ===================================================================
> --- kvm.orig/arch/x86/kvm/i8259.c
> +++ kvm/arch/x86/kvm/i8259.c
> @@ -33,6 +33,7 @@
> static void pic_clear_isr(struct kvm_kpic_state *s, int irq)
> {
> s->isr &= ~(1 << irq);
> + s->isr_ack |= (1 << irq);
> }
>
> /*
> @@ -213,6 +214,7 @@ void kvm_pic_reset(struct kvm_kpic_state
> 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;
> @@ -444,10 +446,14 @@ static void pic_irq_request(void *opaque
> {
> 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)
> Index: kvm/arch/x86/kvm/irq.h
> ===================================================================
> --- kvm.orig/arch/x86/kvm/irq.h
> +++ kvm/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; /* interrupt ack detection */
> u8 priority_add; /* highest irq priority */
> u8 irq_base;
> u8 read_reg_select;
> --
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