From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: [PATCH v4] KVM: Introduce direct MSI message injection for in-kernel irqchips Date: Wed, 04 Apr 2012 12:48:16 +0200 Message-ID: <4F7C26F0.5090901@siemens.com> References: <4F734EB3.20500@siemens.com> <4F748AAD.2040103@siemens.com> <4F74B484.30607@siemens.com> <4F7B24EA.2070300@redhat.com> <4F7B29B5.6060703@siemens.com> <20120404083821.GB3003@redhat.com> <4F7C09E7.3020005@redhat.com> <20120404085359.GA3404@redhat.com> <4F7C12E3.3050702@siemens.com> <4F7C161D.3090909@redhat.com> <4F7C16A4.6070303@siemens.com> <4F7C1A7A.5020902@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: "Michael S. Tsirkin" , Marcelo Tosatti , kvm , Eric Northup To: Avi Kivity Return-path: Received: from thoth.sbs.de ([192.35.17.2]:27048 "EHLO thoth.sbs.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756000Ab2DDKsV (ORCPT ); Wed, 4 Apr 2012 06:48:21 -0400 In-Reply-To: <4F7C1A7A.5020902@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 2012-04-04 11:55, Avi Kivity wrote: > On 04/04/2012 12:38 PM, Jan Kiszka wrote: >> On 2012-04-04 11:36, Avi Kivity wrote: >>> On 04/04/2012 12:22 PM, Jan Kiszka wrote: >>>>> >>>>>>> Until we do have this fast path we can just fill this value with zeros, >>>>>>> so kernel patch (almost) does not need to change for this - >>>>>>> just the header. >>>>>> >>>>>> Partially implemented interfaces invite breakage. >>>>> >>>>> Hmm true. OK scrap this idea then, it's not clear >>>>> whether we are going to optimize this anyway. >>>>> >>>> >>>> Also, the problem is that keeping that ID in userspace requires an >>>> infrastructure like the MSIRoutingCache that I proposed originally. Not >>>> much won /wrt invasiveness there. >>> >>> Internal qemu refactorings are not a driver for kvm interface changes. >> >> No, but qemu demonstrates the applicability and handiness of the kernel >> interfaces. > > True. > >>> >>>> So we should really do the routing >>>> optimization in the kernel - one day. >>> >>> No, we need to make a choice: >>> >>> explicit handles: array lookup, more expensive setup >>> no handles: hash loopup, more expensive, but no setup, and no artificial >>> limits >> >> ...and I think we should head for option 2. > > I'm not so sure anymore. Sorry about the U turn, but remind me why? In > the long term it will be slower. Likely not measurably slower. If you look at a message through the arch glasses, you can usually spot the destination directly, specifically if a message targets a single processor - no need for hashing and table lookups in the common case. In contrast, the maintenance costs for the current explicit route based model are significant as we see now. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux