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Fri, 6 Dec 2019 12:02:31 +0000 (UTC) Received: from lacos-laptop-7.usersys.redhat.com (ovpn-116-235.ams2.redhat.com [10.36.116.235]) by smtp.corp.redhat.com (Postfix) with ESMTP id F2A8619C4F; Fri, 6 Dec 2019 12:02:25 +0000 (UTC) Subject: Re: [PATCH for-5.0 8/8] acpi: cpuhp: spec: document procedure for enabling modern CPU hotplug To: Igor Mammedov References: <1575479147-6641-1-git-send-email-imammedo@redhat.com> <1575479147-6641-9-git-send-email-imammedo@redhat.com> <20191206114034.68eacf25@redhat.com> From: Laszlo Ersek Message-ID: <22091869-b659-b869-5232-d658acb3ef15@redhat.com> Date: Fri, 6 Dec 2019 13:02:24 +0100 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: <20191206114034.68eacf25@redhat.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-MC-Unique: dgvRos_aOlSbWcU0Ub50VA-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 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: pbonzini@redhat.com, philmd@redhat.com, qemu-devel@nongnu.org, mst@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 12/06/19 11:40, Igor Mammedov wrote: > On Thu, 5 Dec 2019 15:07:53 +0100 > Laszlo Ersek wrote: > >> On 12/04/19 18:05, Igor Mammedov wrote: >>> Describe how to enable and detect modern CPU hotplug interface. >>> Detection part is based on new CPHP_GET_CPU_ID_CMD command, >>> introduced by "acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command" patch. >>> >>> Signed-off-by: Igor Mammedov >>> --- >>> docs/specs/acpi_cpu_hotplug.txt | 22 ++++++++++++++++++++-- >>> 1 file changed, 20 insertions(+), 2 deletions(-) >> >> Could we make this usecase / workflow independent of the new >> CPHP_GET_CPU_ID_CMD command please? >> >> I'd like to suggest the following: >> >>> diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt >>> index bb33144..667b264 100644 >>> --- a/docs/specs/acpi_cpu_hotplug.txt >>> +++ b/docs/specs/acpi_cpu_hotplug.txt >>> @@ -15,14 +15,14 @@ CPU present bitmap for: >>> PIIX-PM (IO port 0xaf00-0xaf1f, 1-byte access) >>> One bit per CPU. Bit position reflects corresponding CPU APIC ID. Read-only. >>> The first DWORD in bitmap is used in write mode to switch from legacy >>> - to new CPU hotplug interface, write 0 into it to do switch. >>> + to modern CPU hotplug interface, write 0 into it to do switch. >>> --------------------------------------------------------------- >>> QEMU sets corresponding CPU bit on hot-add event and issues SCI >>> with GPE.2 event set. CPU present map is read by ACPI BIOS GPE.2 handler >>> to notify OS about CPU hot-add events. CPU hot-remove isn't supported. >>> >>> ===================================== >>> -ACPI CPU hotplug interface registers: >>> +Modern ACPI CPU hotplug interface registers: >>> ------------------------------------- >>> Register block base address: >>> ICH9-LPC IO port 0x0cd8 >>> @@ -105,6 +105,24 @@ write access: >>> other values: reserved >>> >>> Typical usecases: >>> + - (x86) Detecting and enabling modern CPU hotplug interface. >> >> (1) I think we can drop the (x86) restriction. (Because, we don't need >> to depend on APIC ID specifics; see below.) >> >>> + QEMU starts with legacy CPU hotplug interface enabled. Detecting and >>> + switching to modern interface is based on the 2 legacy CPU hotplug features: >>> + 1. Writes into CPU bitmap are ignored. >>> + 2. CPU bitmap always has bit#0 set, corresponding to boot CPU. >>> + >>> + Use following steps to detect and enable modern CPU hotplug interface: >>> + 1. Store 0x0 to the 'CPU selector' register, >>> + attempting to switch to modern mode >>> + 2. Store 0x0 to the 'CPU selector' register, >>> + to ensure valid selector value >> >> OK thus far. >> >>> + 3. Store 0x3 to the 'Command field' register, >>> + sets the 'Command data 2' register into architecture specific >>> + CPU identifier mode >> >> (2) Can we please store command 0 here >> (CPHP_GET_NEXT_CPU_WITH_EVENT_CMD)? > > that could work too, as far "Command data 2" is defined before > we use it here. > Point is to define "Command data 2" state with command 0 and leave our hands > free when it comes to reserved (so it won't get in a way in the future > if we need to 'unreserve' it in some context) > >> >> That might change the selector value, yes. (Even if that happens, the >> new selector will be *valid*.) >> >> But the main point is that, with 0 stored to the command register, one >> of the following *four* cases will hold, subsequently: >> >> (2a) if register block is totally missing: >> >> --> Offset#0 will read as all-bits-one (unassigned read) >> >> (2b) if register block is legacy-only: >> >> --> Offset#0 will read as nonzero, due to CPU#0 being always present >> >> (2c) if the modern register block is active, but the "Command data 2" >> register is *not* yet described in the spec file: >> >> --> Offset#0 will read as zero, because it is *reserved*: >> >>> read access: >>> offset: >>> [0x0-0x3] reserved <---- HERE >> >> (2d) if the modern register block is active, and the "Command data 2" >> register *is* described in the spec file: >> >> --> the "Command data 2" register (at offset#0) will read as zero, >> because: >> >>> read access: >>> offset: >>> [0x0-0x3] Command data 2: (DWORD access) >>> if last stored 'Command field' value: >>> 3: upper 32 bits of architecture specific identifying CPU value >>> (n x86 case: 0x0) >>> other values: reserved <------ HERE >> >> and then step#4 applies just the same: >> >> On 12/04/19 18:05, Igor Mammedov wrote: >>> + 4. Read the 'Command data 2' register. >>> + If read value is 0x0, the modern interface is enabled. >>> + Otherwise legacy or no CPU hotplug interface available >>> + >> >> because "read value is 0x0" corresponds to the *union* of cases (2c) and >> (2d) -- namely, "the modern register block is active". >> >> My proposal above is what I implemented for OVMF in October: >> >> [edk2-devel] [PATCH v2 3/3] >> OvmfPkg/PlatformPei: rewrite MaxCpuCountInitialization() for CPU hotplug >> >> http://mid.mail-archive.com/20191022221554.14963-4-lersek@redhat.com >> >> and it works very well. >> >> So the benefits would be: >> >> - I wouldn't have to rewrite that (complex!) patch :) , >> >> - the logic would not store the new CPHP_GET_CPU_ID_CMD command, only >> read offset#0, >> >> - the logic would not be x86 specific (it would not have to rely on the >> most significant 32 bits of the 64-bit arch-specific CPU identifier -- >> here: the APIC ID -- being zero). >> >> Furthermore, >> >> (3) In step#4, I suggest replacing 'Command data 2' with "offset 0", > Spec uses field names so I'd rather use 'Command data 2' here instead of > direct offset to be consistent with the rest of the spec. > > >> (4) finally, I'd suggest squashing this patch (updated as requested >> above) into patch "acpi: cpuhp: spec: add typical usecases". >> >> Using my suggestion (2), you can define the "modern interface >> enablement" workflow as well, without any dependency on >> CPHP_GET_CPU_ID_CMD. The only thing that's necessary is the small update >> from (3), and then you can describe all three important use cases in one >> go, in patch#6. > > I'd add extra patch that defines 'Command data 2' for command 0 > and after that it should be possible to squash detection usecase > into 6/8. Sounds good! Thanks! Laszlo > >> And then you can introduce CPHP_GET_CPU_ID_CMD in the last patch >> (patch#7), while staying compatible with the previously-documented >> workflows. >> >> Pretty please? :) >> >> Thanks! >> Laszlo >> >>> - Get a cpu with pending event >>> 1. Store 0x0 to the 'CPU selector' register. >>> 2. Store 0x0 to the 'Command field' register. >>> >