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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 58B88C47404 for ; Fri, 11 Oct 2019 08:03:25 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 27CD22089F for ; Fri, 11 Oct 2019 08:03:25 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 27CD22089F Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:46966 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iIptU-0004CR-B2 for qemu-devel@archiver.kernel.org; Fri, 11 Oct 2019 04:03:24 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:39273) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iIps7-0003UE-AH for qemu-devel@nongnu.org; Fri, 11 Oct 2019 04:02:00 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iIps1-00059O-IJ for qemu-devel@nongnu.org; Fri, 11 Oct 2019 04:01:58 -0400 Received: from mx1.redhat.com ([209.132.183.28]:38880) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iIps1-00058w-9g for qemu-devel@nongnu.org; Fri, 11 Oct 2019 04:01:53 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 42EEA3C919; Fri, 11 Oct 2019 08:01:52 +0000 (UTC) Received: from lacos-laptop-7.usersys.redhat.com (ovpn-120-177.rdu2.redhat.com [10.10.120.177]) by smtp.corp.redhat.com (Postfix) with ESMTP id 15437196B2; Fri, 11 Oct 2019 08:01:42 +0000 (UTC) Subject: Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface To: Eduardo Habkost , Igor Mammedov References: <20191009132252.17860-1-imammedo@redhat.com> <20191010055356-mutt-send-email-mst@kernel.org> <20191010153815.4f7a3fc9@redhat.com> <20191010095459-mutt-send-email-mst@kernel.org> <20191010175754.7c62cf8f@Igors-MacBook-Pro> <20191010192039.GE4084@habkost.net> From: Laszlo Ersek Message-ID: Date: Fri, 11 Oct 2019 10:01:42 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20191010192039.GE4084@habkost.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Fri, 11 Oct 2019 08:01:52 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "Michael S. Tsirkin" , qemu-devel@nongnu.org, Gerd Hoffmann , Paolo Bonzini , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 10/10/19 21:20, Eduardo Habkost wrote: > On Thu, Oct 10, 2019 at 05:57:54PM +0200, Igor Mammedov wrote: >> On Thu, 10 Oct 2019 09:59:42 -0400 >> "Michael S. Tsirkin" wrote: >> >>> On Thu, Oct 10, 2019 at 03:39:12PM +0200, Igor Mammedov wrote: >>>> On Thu, 10 Oct 2019 05:56:55 -0400 >>>> "Michael S. Tsirkin" wrote: >>>> >>>>> On Wed, Oct 09, 2019 at 09:22:49AM -0400, Igor Mammedov wrote: >>>>>> As an alternative to passing to firmware topology info via new fwcfg files >>>>>> so it could recreate APIC IDs based on it and order CPUs are enumerated, >>>>>> >>>>>> extend CPU hotplug interface to return APIC ID as response to the new command >>>>>> CPHP_GET_CPU_ID_CMD. >>>>> >>>>> One big piece missing here is motivation: >>>> I thought the only willing reader was Laszlo (who is aware of context) >>>> so I skipped on details and confused others :/ >>>> >>>>> Who's going to use this interface? >>>> In current state it's for firmware, since ACPI tables can cheat >>>> by having APIC IDs statically built in. >>>> >>>> If we were creating CPU objects in ACPI dynamically >>>> we would be using this command as well. >>> >>> I'm not sure how it's even possible to create devices dynamically. Well >>> I guess it's possible with LoadTable. Is this what you had in >>> mind? >> >> Yep. I even played this shiny toy and I can say it's very tempting one. >> On the other side, even problem of legacy OSes not working with it aside, >> it's hard to debug and reproduce compared to static tables. >> So from maintaining pov I dislike it enough to be against it. >> >> >>>> It would save >>>> us quite a bit space in ACPI blob but it would be a pain >>>> to debug and diagnose problems in ACPI tables, so I'd rather >>>> stay with static CPU descriptions in ACPI tables for the sake >>>> of maintenance. >>>>> So far CPU hotplug was used by the ACPI, so we didn't >>>>> really commit to a fixed interface too strongly. >>>>> >>>>> Is this a replacement to Laszlo's fw cfg interface? >>>>> If yes is the idea that OVMF going to depend on CPU hotplug directly then? >>>>> It does not depend on it now, does it? >>>> It doesn't, but then it doesn't support cpu hotplug, >>>> OVMF(SMM) needs to cooperate with QEMU "and" ACPI tables to perform >>>> the task and using the same interface/code path between all involved >>>> parties makes the task easier with the least amount of duplicated >>>> interfaces and more robust. >>>> >>>> Re-implementing alternative interface for firmware (fwcfg or what not) >>>> would work as well, but it's only question of time when ACPI and >>>> this new interface disagree on how world works and process falls >>>> apart. >>> >>> Then we should consider switching acpi to use fw cfg. >>> Or build another interface that can scale. >> >> Could be an option, it would be a pain to write a driver in AML for fwcfg access though >> (I've looked at possibility to access fwcfg from AML about a year ago and gave up. >> I'm definitely not volunteering for the second attempt and can't even give an estimate >> it it's viable approach). >> >> But what scaling issue you are talking about, exactly? >> With current CPU hotplug interface we can handle upto UNIT32_MAX cpus, and extend >> interface without need to increase IO window we are using now. >> >> Granted IO access it not fastest compared to fwcfg in DMA mode, but we already >> doing stop machine when switching to SMM which is orders of magnitude slower. >> Consensus was to compromise on speed of CPU hotplug versus more complex and more >> problematic unicast SMM mode in OVMF (can't find a particular email but we have discussed >> it with Laszlo already, when I considered ways to optimize hotplug speed) > > If we were designing the interface from the ground up, I would > agree with Michael. But I don't see why we would reimplement > everything from scratch now, if just providing the > cpu_selector => cpu_hardware_id mapping to firmware is enough to > make the existing interface work. > > If somebody is really unhappy with the current interface and > wants to implement a new purely fw_cfg-based one (and write the > corresponding ACPI code), they would be welcome. Let me re-iterate the difficulties quickly: - DMA-based fw_cfg is troublesome in SEV guests (do you want to mess with page table entries in AML methods? or pre-allocate an always decrypted opregion? how large?) - IO port based fw_cfg does not support writes (and I reckon that, when the *OS* handles a hotplug event, it does have to talk back to QEMU) - the CPU hotplug AML would have to arbitrate with Linux's own fw_cfg driver (which exposes fw_cfg files to userspace, yay! /s) In the phys world, CPU hotplug takes dedicated RAS hardware. Shoehorning CPU hotplug into *firmware* config, when in two use cases [*], the firmware shouldn't even know about CPU hotplug, feels messy. [*] being (a) SeaBIOS, and (b) OVMF built without SMM > I just don't see why we should spend our time doing that now. I have to agree, we're already spread thin. ... I must admit: I didn't expect this, but now I've grown to *prefer* the CPU hotplug register block! Laszlo