From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gleb Natapov Subject: Re: [RFC PATCH 0/2] irq destination caching prototype Date: Mon, 13 Aug 2012 14:41:47 +0300 Message-ID: <20120813114147.GA3341@redhat.com> References: <20120813101608.GS3341@redhat.com> <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> <5028E569.9050807@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "Michael S. Tsirkin" , kvm@vger.kernel.org, Jan Kiszka To: Avi Kivity Return-path: Received: from mx1.redhat.com ([209.132.183.28]:56922 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751063Ab2HMLlt (ORCPT ); Mon, 13 Aug 2012 07:41:49 -0400 Content-Disposition: inline In-Reply-To: <5028E569.9050807@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Mon, Aug 13, 2012 at 02:30:49PM +0300, Avi Kivity wrote: > On 08/13/2012 02:12 PM, 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. > > > apicid is checked in a loop, the others aren't. Along with dest_id. > apicid is > unpredicatable; the others are. What do you mean "unpredicatable"? > > I think we should use apicid loopup exclusively. It doesn't accelerate > everything, but most things, and is common to all unicast interrupts > except PIC (and we can also precompute the target vcpu for PIC, too). > We can change kvm_irq_delivery_to_apic() to avoid the loop if interrupt is physical, non broadcast, non low prio. Do whatever it does now otherwise. You think we do not need cache in such case? -- Gleb.