From mboxrd@z Thu Jan  1 00:00:00 1970
From: Gleb Natapov <gleb@redhat.com>
Subject: Re: [RFC PATCH 0/2] irq destination caching prototype
Date: Mon, 13 Aug 2012 14:43:13 +0300
Message-ID: <20120813114313.GB3341@redhat.com>
References: <5028D52D.9050104@redhat.com>
 <20120813102402.GT3341@redhat.com>
 <5028D788.3070006@redhat.com>
 <20120813103845.GA16300@redhat.com>
 <5028DDCD.1090205@redhat.com>
 <20120813110108.GX3341@redhat.com>
 <5028DF17.2050008@redhat.com>
 <20120813111241.GY3341@redhat.com>
 <20120813112214.GB16801@redhat.com>
 <20120813112931.GZ3341@redhat.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: Avi Kivity <avi@redhat.com>, kvm@vger.kernel.org,
	Jan Kiszka <jan.kiszka@web.de>
To: "Michael S. Tsirkin" <mst@redhat.com>
Return-path: <kvm-owner@vger.kernel.org>
Received: from mx1.redhat.com ([209.132.183.28]:4818 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1751338Ab2HMLnP (ORCPT <rfc822;kvm@vger.kernel.org>);
	Mon, 13 Aug 2012 07:43:15 -0400
Content-Disposition: inline
In-Reply-To: <20120813112931.GZ3341@redhat.com>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

On Mon, Aug 13, 2012 at 02:29:31PM +0300, Gleb Natapov wrote:
> 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.
>  
It also a little bit pessimistic for logical mode. Dest may have more
than one bit set, but be delivered to only one cpu.

--
			Gleb.