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 EE87EC47404 for ; Fri, 11 Oct 2019 13:02:25 +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 C856021D7D for ; Fri, 11 Oct 2019 13:02:25 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C856021D7D 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]:49948 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iIuYq-0004cs-WE for qemu-devel@archiver.kernel.org; Fri, 11 Oct 2019 09:02:25 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:59322) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iIuWr-0003Sr-Fp for qemu-devel@nongnu.org; Fri, 11 Oct 2019 09:00:32 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iIuWp-0007yG-6E for qemu-devel@nongnu.org; Fri, 11 Oct 2019 09:00:20 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55056) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iIuWo-0007xi-U3 for qemu-devel@nongnu.org; Fri, 11 Oct 2019 09:00:19 -0400 Received: from mail-qt1-f200.google.com (mail-qt1-f200.google.com [209.85.160.200]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 7B7AD51F16 for ; Fri, 11 Oct 2019 13:00:17 +0000 (UTC) Received: by mail-qt1-f200.google.com with SMTP id k53so9337301qtk.0 for ; Fri, 11 Oct 2019 06:00:17 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=Z/aR1vLs+4n9Xu1c+z2vc+oIGUT9Z+Zv5pj3DkZNfNA=; b=OUU1rq7oThRUeO/owLj5srRCUsFUt8NUhWKy9xCAhfCmH31kkXKh1+bIkRSNQCYx/w URX0vCnsx+QRhvkv8cEQqzyosXSAE17ti7j/Sw7Vl70VoP6kyPhT5C75t+s7NYoaFOPx /w+ac5yUZoUbzPdipzHWzToaMEbpaGT279LGOvalYoBckTVtiVPY6C9VvTdlW63Elfd4 WT3szvlma1SfiDzLmeR2c7ydkQd4O9mPnvWpm0seXZTS+LAR5ZauSmld0JKzgcDOmt0E cAko/cW8f6XZEdOr8eAHC+TzHA4w1LJRKZC/pZcUJ+YNGNNXLHBzN1s6D3o60d6eoCHa QGHw== X-Gm-Message-State: APjAAAWBfI7B4gLwlwuMXRVQ3NzdHIqiiUn6l5r/SZGSWlUcFQo0GwO4 DkdtPH2xY1nNPpvuLvEXpGNgOC2a3jbYTE8ZsdMqcv8rY1Z+4xzw2KIP70m7JHjFn9sMtSuivRv ezrqQ4RX/05ozSjw= X-Received: by 2002:a37:9a0d:: with SMTP id c13mr15731002qke.93.1570798816654; Fri, 11 Oct 2019 06:00:16 -0700 (PDT) X-Google-Smtp-Source: APXvYqwfs5gQIgjM7GiT+ZRk5rFLGA7fbs5w5rZpbJPRURSox2wXuU/vRECCqjr/tUxr8NlBRsyKZg== X-Received: by 2002:a37:9a0d:: with SMTP id c13mr15730955qke.93.1570798816290; Fri, 11 Oct 2019 06:00:16 -0700 (PDT) Received: from redhat.com (bzq-79-176-10-77.red.bezeqint.net. [79.176.10.77]) by smtp.gmail.com with ESMTPSA id c14sm4532533qta.80.2019.10.11.06.00.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 11 Oct 2019 06:00:15 -0700 (PDT) Date: Fri, 11 Oct 2019 09:00:10 -0400 From: "Michael S. Tsirkin" To: Laszlo Ersek Subject: Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface Message-ID: <20191011085852-mutt-send-email-mst@kernel.org> 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-Disposition: inline In-Reply-To: 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: Eduardo Habkost , qemu-devel@nongnu.org, Gerd Hoffmann , Paolo Bonzini , Igor Mammedov , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Oct 11, 2019 at 10:01:42AM +0200, Laszlo Ersek wrote: > 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 agree. So ACPI should use a dedicated interface. > > 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 OK, send an ack then. -- MST