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[79.176.10.77]) by smtp.gmail.com with ESMTPSA id v4sm2280524qkj.28.2019.10.10.07.49.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Oct 2019 07:49:58 -0700 (PDT) Date: Thu, 10 Oct 2019 10:49:53 -0400 From: "Michael S. Tsirkin" To: Eduardo Habkost Subject: Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface Message-ID: <20191010104818-mutt-send-email-mst@kernel.org> References: <20191009132252.17860-1-imammedo@redhat.com> <20191010055356-mutt-send-email-mst@kernel.org> <20191010153815.4f7a3fc9@redhat.com> <20191010141652.GD21666@habkost.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191010141652.GD21666@habkost.net> 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: Laszlo Ersek , 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 Thu, Oct 10, 2019 at 11:16:52AM -0300, Eduardo Habkost 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. 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. > > > > > If answers to all of the above is yes, then I don't really like it: it > > > is better to keep all paravirt stuff in one place, namely in fw cfg. > > Lets discuss, what cpu hotplug fwcfg interface could look like in > > [PATCH 3/4] hw/i386: add facility to expose CPU topology over fw-cfg > > mail thread and clarify (dis)likes with concrete reasons. > > > > So far I managed to convince myself that we ought to reuse > > and extend current CPU hotplug interface with firmware features, > > to endup with consolidated cpu hotplug process without > > introducing duplicate ABIs, but I could be wrong so > > lets see if fwcfg will be the better approach. > > > > I was more inclined towards the approach in this patch, because I > see it as just a bug fix in the CPU hotplug interface (which > should have been using the hardware CPU identifier as the CPU > selector since the beginning). Well if ACPI is going to be using this, then that's different. Igor do you see any need for that? > Providing the missing information in fw_cfg isn't necessarily > bad, but please document it explicitly as a > hotplug_cpu_selector => cpu_hardware_id > mapping, so people won't use "CPU index" as a generic identifier > elsewhere. > > -- > Eduardo