Re: [PATCH RFC v3 03/21] ACPI: processor: Register CPUs that are online, but not described in the DSDT

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> On 23 Jan 2024, at 08:27, Jonathan Cameron <jonathan.cameron@xxxxxxxxxx> wrote:
> On Mon, 22 Jan 2024 17:30:05 +0000
> "Russell King (Oracle)" <linux@xxxxxxxxxxxxxxx> wrote:
>> On Mon, Jan 22, 2024 at 05:22:46PM +0100, Rafael J. Wysocki wrote:
>>> On Mon, Jan 22, 2024 at 5:02 PM Jonathan Cameron
>>> <Jonathan.Cameron@xxxxxxxxxx> wrote:  
>>>> On Mon, 15 Jan 2024 11:06:29 +0000
>>>> "Russell King (Oracle)" <linux@xxxxxxxxxxxxxxx> wrote:
>>>>> On Mon, Dec 18, 2023 at 09:22:03PM +0100, Rafael J. Wysocki wrote:  
>>>>>> On Wed, Dec 13, 2023 at 1:49 PM Russell King <rmk+kernel@xxxxxxxxxxxxxxx> wrote:  
>>>>>>> From: James Morse <james.morse@xxxxxxx>
>>>>>>> ACPI has two descriptions of CPUs, one in the MADT/APIC table, the other
>>>>>>> in the DSDT. Both are required. (ACPI 6.5's 8.4 "Declaring Processors"
>>>>>>> says "Each processor in the system must be declared in the ACPI
>>>>>>> namespace"). Having two descriptions allows firmware authors to get
>>>>>>> this wrong.
>>>>>>> If CPUs are described in the MADT/APIC, they will be brought online
>>>>>>> early during boot. Once the register_cpu() calls are moved to ACPI,
>>>>>>> they will be based on the DSDT description of the CPUs. When CPUs are
>>>>>>> missing from the DSDT description, they will end up online, but not
>>>>>>> registered.
>>>>>>> Add a helper that runs after acpi_init() has completed to register
>>>>>>> CPUs that are online, but weren't found in the DSDT. Any CPU that
>>>>>>> is registered by this code triggers a firmware-bug warning and kernel
>>>>>>> taint.
>>>>>>> Qemu TCG only describes the first CPU in the DSDT, unless cpu-hotplug
>>>>>>> is configured.  
>>>>>> So why is this a kernel problem?  
>>>>> So what are you proposing should be the behaviour here? What this
>>>>> statement seems to be saying is that QEMU as it exists today only
>>>>> describes the first CPU in DSDT.  
>>>> This confuses me somewhat, because I'm far from sure which machines this
>>>> is true for in QEMU.  I'm guessing it's a legacy thing with
>>>> some old distro version of QEMU - so we'll have to paper over it anyway
>>>> but for current QEMU I'm not sure it's true.
>>>> Helpfully there are a bunch of ACPI table tests so I've been checking
>>>> through all the multi CPU cases.
>>>> CPU hotplug not enabled.
>>>> pc/DSDT.dimmpxm  - 4x Processor entries.  -smp 4
>>>> pc/DSDT.acpihmat - 2x Processor entries.  -smp 2
>>>> q35/DSDT.acpihmat - 2x Processor entries. -smp 2
>>>> virt/DSDT.acpihmatvirt - 4x ACPI0007 entries -smp 4
>>>> q35/DSDT.acpihmat-noinitiator - 4 x Processor () entries -smp 4
>>>> virt/DSDT.topology - 8x ACPI0007 entries
>>>> I've also looked at the code and we have various types of
>>>> CPU hotplug on x86 but they all build appropriate numbers of
>>>> Processor() entries in DSDT.
>>>> Arm likewise seems to build the right number of ACPI0007 entries
>>>> (and doesn't yet have CPU HP support).
>>>> If anyone can add a reference on why this is needed that would be very
>>>> helpful.  
>>> Yes, it would.
>>> Personally, I would prefer to assume that it is not necessary until it
>>> turns out that (1) there is firmware with this issue actually in use
>>> and (2) updating the firmware in question to follow the specification
>>> is not practical.
>>> Otherwise, we'd make it easier to ship non-compliant firmware for no
>>> good reason.  
>> If Salil can't come up with a reason, then I'm in favour of dropping
>> the patch like already done for patch 2. If the code change serves no
>> useful purpose, there's no point in making the change.
> Salil's out today, but I've messaged him to follow up later in the week.
> It 'might' be the odd cold plug path where QEMU half comes up, then extra
> CPUs are added, then it boots. (used by some orchestration frameworks)
> I don't have a set up for that and I won't get to creating one today anyway
> (we all love start of the year planning workshops!)
> I've +CC'd a few people have run tests on the various iterations of this
> work in the past.  Maybe one of them can shed some light on this?

IIUC, this patch covers a scenario for non compliant firmware and in which my 
tests for AArch64 using RFC v2 have been unable to trigger its error message so 
far. This does not mean, however, this patch should not be taken forward though.

It seems benevolent enough detecting non compliant firmware and still proceed 
while having whoever uses that firmware to get to know that.

I'm not sure, however, whether the reference to a specific VMM should be in the 
commit message though. That might not be anything to do with the kernel so a 
more meaningful rewrite on this separation of concerns could be useful.


> Jonathan

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