Re: KVM: PIC: enhance IPI avoidance

["David S. Ahern" <daahern@cisco.com>]

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 <mtosatti@redhat.com>
> 
> 
> 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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