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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED 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 5FC01C43144 for ; Fri, 29 Jun 2018 15:26:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 22E3C28058 for ; Fri, 29 Jun 2018 15:26:03 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 22E3C28058 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 S936719AbeF2P0B (ORCPT ); Fri, 29 Jun 2018 11:26:01 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:59086 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S935115AbeF2PZ7 (ORCPT ); Fri, 29 Jun 2018 11:25:59 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 3360440201C7; Fri, 29 Jun 2018 15:25:59 +0000 (UTC) Received: from vitty.brq.redhat.com.redhat.com (unknown [10.43.2.155]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 562E02026D68; Fri, 29 Jun 2018 15:25:57 +0000 (UTC) From: Vitaly Kuznetsov To: Roman Kagan Cc: kvm@vger.kernel.org, x86@kernel.org, Paolo Bonzini , Radim =?utf-8?B?S3LEjW3DocWZ?= , "K. Y. Srinivasan" , Haiyang Zhang , Stephen Hemminger , "Michael Kelley \(EOSG\)" , Mohammed Gamal , Cathy Avery , Wanpeng Li , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 2/5] KVM: x86: hyperv: introduce vp_index_to_vcpu_idx mapping References: <20180628135313.17468-1-vkuznets@redhat.com> <20180628135313.17468-3-vkuznets@redhat.com> <20180629101134.GA15656@rkaganb.sw.ru> <87y3exdh2o.fsf@vitty.brq.redhat.com> <20180629111227.GB15656@rkaganb.sw.ru> <87tvplddrr.fsf@vitty.brq.redhat.com> <20180629125216.GC15656@rkaganb.sw.ru> <87h8lld9hl.fsf@vitty.brq.redhat.com> <20180629143212.GD15656@rkaganb.sw.ru> Date: Fri, 29 Jun 2018 17:25:56 +0200 In-Reply-To: <20180629143212.GD15656@rkaganb.sw.ru> (Roman Kagan's message of "Fri, 29 Jun 2018 17:32:13 +0300") Message-ID: <878t6xd37f.fsf@vitty.brq.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.78 on 10.11.54.4 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.6]); Fri, 29 Jun 2018 15:25:59 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.6]); Fri, 29 Jun 2018 15:25:59 +0000 (UTC) for IP:'10.11.54.4' DOMAIN:'int-mx04.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'vkuznets@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Roman Kagan writes: > On Fri, Jun 29, 2018 at 03:10:14PM +0200, Vitaly Kuznetsov wrote: >> Roman Kagan writes: >> >> > On Fri, Jun 29, 2018 at 01:37:44PM +0200, Vitaly Kuznetsov wrote: >> >> The problem we're trying to solve here is: with PV TLB flush and IPI we >> >> need to walk through the supplied list of VP_INDEXes and get VCPU >> >> ids. Usually they match. But in case they don't [...] >> > >> > Why wouldn't they *in practice*? Only if the userspace wanted to be >> > funny and assigned VP_INDEXes randomly? I'm not sure we need to >> > optimize for this case. >> >> Can someone please remind me why we allow userspace to change it in the >> first place? > > I can ;) > > We used not to, and reported KVM's vcpu index as the VP_INDEX. However, > later we realized that VP_INDEX needed to be persistent across > migrations and otherwise also known to userspace. Relying on the kernel > to always initialize its indices in the same order was unacceptable, and > we came up with no better way of synchronizing VP_INDEX between the > userspace and the kernel than to let the former to set it explicitly. > > However, this is basically a future-proofing feature; in practice, both > QEMU and KVM initialize their indices in the same order. Thanks! But in the theoretical case when these indices start to differ after migration, users will notice a slowdown which will be hard to explain, right? Does it justify the need for vp_idx_to_vcpu_idx? In any case I sent v3 with vp_idx_to_vcpu_idx dropped for now, hope Radim is OK with us de-coupling these discussions. -- Vitaly