From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.6 required=3.0 tests=FROM_EXCESS_BASE64, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3ADECC468C6 for ; Thu, 19 Jul 2018 17:22:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E747C20671 for ; Thu, 19 Jul 2018 17:22:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E747C20671 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732085AbeGSSGN (ORCPT ); Thu, 19 Jul 2018 14:06:13 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:51922 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1731442AbeGSSGN (ORCPT ); Thu, 19 Jul 2018 14:06:13 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 7AD7ECE1CD; Thu, 19 Jul 2018 17:22:05 +0000 (UTC) Received: from flask (unknown [10.43.2.80]) by smtp.corp.redhat.com (Postfix) with SMTP id 8DBE6111AF0C; Thu, 19 Jul 2018 17:22:03 +0000 (UTC) Received: by flask (sSMTP sendmail emulation); Thu, 19 Jul 2018 19:22:02 +0200 Date: Thu, 19 Jul 2018 19:22:02 +0200 From: Radim =?utf-8?B?S3LEjW3DocWZ?= To: Paolo Bonzini Cc: Wanpeng Li , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Vitaly Kuznetsov Subject: Re: [PATCH v3 2/6] KVM: X86: Implement PV IPIs in linux guest Message-ID: <20180719172202.GD11749@flask> References: <1530598891-21370-1-git-send-email-wanpengli@tencent.com> <1530598891-21370-3-git-send-email-wanpengli@tencent.com> <20180719162826.GB11749@flask> <1357a0b5-0cd8-8cb5-6c61-c9662219bed0@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1357a0b5-0cd8-8cb5-6c61-c9662219bed0@redhat.com> X-Scanned-By: MIMEDefang 2.78 on 10.11.54.3 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.2]); Thu, 19 Jul 2018 17:22:05 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.2]); Thu, 19 Jul 2018 17:22:05 +0000 (UTC) for IP:'10.11.54.3' DOMAIN:'int-mx03.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'rkrcmar@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2018-07-19 18:47+0200, Paolo Bonzini: > On 19/07/2018 18:28, Radim Krčmář wrote: > >> + > >> + kvm_hypercall3(KVM_HC_SEND_IPI, ipi_bitmap_low, ipi_bitmap_high, vector); > > and > > > > kvm_hypercall3(KVM_HC_SEND_IPI, ipi_bitmap[0], ipi_bitmap[1], vector); > > > > Still, the main problem is that we can only address 128 APICs. > > > > A simple improvement would reuse the vector field (as we need only 8 > > bits) and put a 'offset' in the rest. The offset would say which > > cluster of 128 are we addressing. 24 bits of offset results in 2^31 > > total addressable CPUs (we probably should even use that many bits). > > The downside of this is that we can only address 128 at a time. > > > > It's basically the same as x2apic cluster mode, only with 128 cluster > > size instead of 16, so the code should be a straightforward port. > > And because x2apic code doesn't seem to use any division by the cluster > > size, we could even try to use kvm_hypercall4, add ipi_bitmap[2], and > > make the cluster size 192. :) > > I did suggest an offset earlier in the discussion. > > The main problem is that consecutive CPU ids do not map to consecutive > APIC ids. But still, we could do an hypercall whenever the total range > exceeds 64. Something like Right, the cluster x2apic implementation came with a second mapping to make this in linear time and send as little IPIs as possible: · /* Collapse cpus in a cluster so a single IPI per cluster is sent */ · for_each_cpu(cpu, tmpmsk) { · · struct cluster_mask *cmsk = per_cpu(cluster_masks, cpu); · · dest = 0; · · for_each_cpu_and(clustercpu, tmpmsk, &cmsk->mask) · · · dest |= per_cpu(x86_cpu_to_logical_apicid, clustercpu); · · if (!dest) · · · continue; · · __x2apic_send_IPI_dest(dest, vector, apic->dest_logical); · · /* Remove cluster CPUs from tmpmask */ · · cpumask_andnot(tmpmsk, tmpmsk, &cmsk->mask); · } I think that the extra memory consumption would be excusable.