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_HELO_NONE,SPF_PASS 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 1848BC47404 for ; Fri, 11 Oct 2019 10:49:29 +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 E16A9206CD for ; Fri, 11 Oct 2019 10:49:28 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E16A9206CD 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]:48386 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iIsUC-0006i7-2C for qemu-devel@archiver.kernel.org; Fri, 11 Oct 2019 06:49:28 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:38769) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iIsSZ-000675-4j for qemu-devel@nongnu.org; Fri, 11 Oct 2019 06:47:48 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iIsSW-0001Hp-IA for qemu-devel@nongnu.org; Fri, 11 Oct 2019 06:47:45 -0400 Received: from mx1.redhat.com ([209.132.183.28]:50994) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iIsSW-0001Gz-9X for qemu-devel@nongnu.org; Fri, 11 Oct 2019 06:47:44 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 0C7DB302C076; Fri, 11 Oct 2019 10:47:43 +0000 (UTC) Received: from localhost (unknown [10.43.2.182]) by smtp.corp.redhat.com (Postfix) with ESMTP id 5283960872; Fri, 11 Oct 2019 10:47:34 +0000 (UTC) Date: Fri, 11 Oct 2019 12:47:32 +0200 From: Igor Mammedov To: Eduardo Habkost Subject: Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface Message-ID: <20191011124732.6fdc1b3f@redhat.com> In-Reply-To: <20191010192039.GE4084@habkost.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.46]); Fri, 11 Oct 2019 10:47:43 +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" , Laszlo Ersek , qemu-devel@nongnu.org, Gerd Hoffmann , Paolo Bonzini , Philippe =?UTF-8?B?TWF0aGlldS1EYXVkw6k=?= , Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, 10 Oct 2019 16:20:39 -0300 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. I just don't > see why we should spend our time doing that now. Right, we can give fwcfg a shot next time we try to allocate new register block for a new PV interface, assuming it suits interface requirements.