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=-6.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 D988FC433DF for ; Mon, 27 Jul 2020 10:48:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C0BF3206E7 for ; Mon, 27 Jul 2020 10:48:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727014AbgG0KsL (ORCPT ); Mon, 27 Jul 2020 06:48:11 -0400 Received: from foss.arm.com ([217.140.110.172]:41614 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726298AbgG0KsL (ORCPT ); Mon, 27 Jul 2020 06:48:11 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 600E630E; Mon, 27 Jul 2020 03:48:10 -0700 (PDT) Received: from [192.168.1.84] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A66B93F66E; Mon, 27 Jul 2020 03:48:07 -0700 (PDT) Subject: Re: [PATCH 0/9] arm64: Stolen time support To: zhukeqian Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Catalin Marinas , linux-doc@vger.kernel.org, Russell King , linux-arm-kernel@lists.infradead.org, Marc Zyngier , Paolo Bonzini , Will Deacon , kvmarm@lists.cs.columbia.edu, xiexiangyou@huawei.com, yebiaoxiang@huawei.com, "wanghaibin.wang@huawei.com >> Wanghaibin (D)" References: <20190802145017.42543-1-steven.price@arm.com> <1611996b-1ec1-dee7-ed61-b3b9df23f138@huawei.com> From: Steven Price Message-ID: <25c7f2e2-e779-4e97-fdc5-0aba9fcf0fbc@arm.com> Date: Mon, 27 Jul 2020 11:48:05 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <1611996b-1ec1-dee7-ed61-b3b9df23f138@huawei.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 21/07/2020 04:26, zhukeqian wrote: > Hi Steven, Hi Keqian, > On 2019/8/2 22:50, Steven Price wrote: >> This series add support for paravirtualized time for arm64 guests and >> KVM hosts following the specification in Arm's document DEN 0057A: >> >> https://developer.arm.com/docs/den0057/a >> >> It implements support for stolen time, allowing the guest to >> identify time when it is forcibly not executing. >> >> It doesn't implement support for Live Physical Time (LPT) as there are >> some concerns about the overheads and approach in the above > Do you plan to pick up LPT support? As there is demand of cross-frequency migration > (from older platform to newer platform). I don't have any plans to pick up the LPT support at the moment - feel free to pick it up! ;) > I am not clear about the overheads and approach problem here, could you please > give some detail information? Maybe we can work together to solve these concerns. :-) Fundamentally the issue here is that LPT only solves one small part of migration between different hosts. To successfully migrate between hosts with different CPU implementations it is also necessary to be able to virtualise various ID registers (e.g. MIDR_EL1, REVIDR_EL1, AIDR_EL1) which we have no support for currently. The problem with just virtualising the registers is how you handle errata. The guest will currently use those (and other) ID registers to decide whether to enable specific errata workarounds. But what errata should be enabled for a guest which might migrate to another host? What we ideally need is a mechanism to communicate to the guest what workarounds are required to successfully run on any of the hosts that the guest may be migrated to. You may also have the situation where the workarounds required for two hosts are mutually incompatible - something needs to understand this and do the "right thing" (most likely just reject this situation, i.e. prevent the migration). There are various options here: e.g. a para-virtualised interface to describe the workarounds (but this is hard to do in an OS-agnostic way), or virtual-ID registers describing an idealised environment where no workarounds are required (and only hosts that have no errata affecting a guest would be able to provide this). Given the above complexity and the fact that Armv8.6-A standardises the frequency to 1GHz this didn't seem worth continuing with. So LPT was dropped from the spec and patches to avoid holding up the stolen time support. However, if you have a use case which doesn't require such a generic migration (e.g. perhaps old and new platforms are based on the same IP) then it might be worth looking at bring this back. But to make the problem solvable it either needs to be restricted to platforms which are substantially the same (so the errata list will be identical), or there's work to be done in preparation to deal with migrating a guest successfully between hosts with potentially different errata requirements. Can you share more details about the hosts that you are interested in migrating between? Thanks, Steve