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 11:22:43 +0200 Message-ID: <4F7C12E3.3050702@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> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Avi Kivity , Marcelo Tosatti , kvm , Eric Northup To: "Michael S. Tsirkin" Return-path: Received: from david.siemens.de ([192.35.17.14]:33043 "EHLO david.siemens.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753594Ab2DDJWt (ORCPT ); Wed, 4 Apr 2012 05:22:49 -0400 In-Reply-To: <20120404085359.GA3404@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 2012-04-04 10:53, Michael S. Tsirkin wrote: > On Wed, Apr 04, 2012 at 11:44:23AM +0300, Avi Kivity wrote: >> On 04/04/2012 11:38 AM, Michael S. Tsirkin wrote: >>>> >>>>> >>>>> A performance note: delivering an interrupt needs to search all vcpus >>>>> for an APIC ID match. The previous plan was to cache (or pre-calculate) >>>>> this lookup in the irq routing table. Now it looks like we'll need a >>>>> separate cache for this. >>>> >>>> As this is non-existent until today, we don't regress here. And it can >>>> still be added on top later on, transparently. >>> >>> I always worry about hash collisions and the cost of >>> calculating good hash functions. >>> >>> We could instead return an index in the cache on injection, maintain in >>> userspace and use it for fast path on the next injection. >> >> Ahem, that is almost the existing routing table to a T. >> >>> Will make it easy to use an array index instead of a hash here, >>> and fallback to a slower ID lookup on mismatch. >> >> Need a free ioctl so we can reuse IDs. > > No, it could be kernel controlled not userspace controlled. We get both > and address and an index: > > if (table[u.i].addr == u.addr && table[u.i].data == u.data) { > return table[u.i].id; > } > > u.i = find_lru_idx(&table); > table[u.i].addr = u.addr; > table[u.i].data = u.data; > table[u.i].id = find_id(u.addr, u.data); > return table[u.i].id; > > >>> 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. So we should really do the routing optimization in the kernel - one day. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux