linux-kernel - Re: [PATCH RFC 2/2] kvm: set affinity hint for assigned device msi

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Date:	Thu, 12 Jan 2012 16:09:17 +0200
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 2/2] kvm: set affinity hint for assigned device msi

On 10/11/2011 08:38 PM, Michael S. Tsirkin wrote:
> To forward an interrupt to a vcpu that runs on
> a host cpu different from the current one,
> we need an ipi which likely will cost us as much
> as delivering the interrupt directly to that cpu would.
>
> Set irq affinity hint to point there, irq balancer
> can then take this into accound and balance
> interrupts accordingly.
>
>  
> +static void kvm_vcpu_host_irq_hint(struct kvm_vcpu *vcpu, int host_irq)
> +{
> +	const struct cpumask *mask;
> +	/* raw_smp_processor_id() is ok here: if we get preempted we can get a
> +	 * wrong value but we don't mind much. */
> +	if (host_irq >= 0 && unlikely(vcpu->cpu != raw_smp_processor_id())) {
> +		mask = get_cpu_mask(vcpu->cpu);
> +		irq_set_affinity_hint(host_irq, mask);
> +	}
> +}
> +	
>  int kvm_irq_delivery_to_apic(struct kvm *kvm, struct kvm_lapic *src,
>  		struct kvm_lapic_irq *irq, int host_irq)
>  {
> @@ -102,6 +114,7 @@ int kvm_irq_delivery_to_apic(struct kvm *kvm, struct kvm_lapic *src,
>  			if (r < 0)
>  				r = 0;
>  			r += kvm_apic_set_irq(vcpu, irq);
> +			kvm_vcpu_host_irq_hint(vcpu, host_irq);

Doing this every time seems excessive.  How about doing it every N
interrupts?  We can even collect information about which vcpus were
targeted, and then use a mask instead of just one vcpu.

-- 
error compiling committee.c: too many arguments to function

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