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From: Gleb Natapov <gleb@redhat.com>
Subject: Re: [RFC PATCH 0/2] irq destination caching prototype
Date: Mon, 13 Aug 2012 14:29:31 +0300
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Cc: Avi Kivity <avi@redhat.com>, kvm@vger.kernel.org,
	Jan Kiszka <jan.kiszka@web.de>
To: "Michael S. Tsirkin" <mst@redhat.com>
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On Mon, Aug 13, 2012 at 02:22:14PM +0300, Michael S. Tsirkin wrote:
> On Mon, Aug 13, 2012 at 02:12:41PM +0300, Gleb Natapov wrote:
> > On Mon, Aug 13, 2012 at 02:03:51PM +0300, Avi Kivity wrote:
> > > On 08/13/2012 02:01 PM, Gleb Natapov wrote:
> > > >> 
> > > >> Actually this is overkill.  Suppose we do an apicid->vcpu translation
> > > >> cache?  Then we retain O(1) behaviour, no need for a huge cache.
> > > >> 
> > > > Not sure I follow.
> > > 
> > > Unicast MSIs and IPIs can be speeded up by looking up the vcpu using the
> > > apic id, using a static lookup table (only changed when the guest
> > > updates apicid or a vcpu is inserted).
> > > 
> > To check that MSI/IPI is unicast you need to check a lot of things: delivery
> > mode, shorthand, dest mode, vector. In short everything but level. This
> > is exactly what kvm_irq_delivery_to_apic() is doing. Caching apicid->vcpu
> > is not enough, caching (delivery mode, shorthand, dest mode,
> > vector)->vcpu is enough and this is exactly what the patch does for irq
> > routing entries.
> 
> At least for MSI I think it is simple. Here's the relevant code from
> my old patch:
> 
> +static bool kvm_msi_is_multicast(unsigned dest, int dest_mode)
> +{
> +       if (dest_mode == 0)
> +               /* Physical mode. */
> +               return dest == 0xff;
> +       else
> +               /* Logical mode. */
> +               return dest & (dest - 1);
> +}
> 
MSI does not have shorthand, so it is simpler but the code above does
work for APIC_DFR_CLUSTER as far as I can tell and it does not check
lowest prio, which is not multicast, but should bot be cached.
 
--
			Gleb.