Message-ID: <CAJZ5v0irPpqEZkCLPmdMU4CxR6ma_j11Z6Nxx8c5fd0aFq9dBw@mail.gmail.com>
Date: Tue, 27 Jan 2026 19:00:35 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Jonathan Cameron <jonathan.cameron@...wei.com>, Lifeng Zheng <zhenglifeng1@...wei.com>
Cc: catalin.marinas@....com, linux-acpi@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linuxarm@...wei.com, gshan@...hat.com, 
	miguel.luis@...cle.com, guohanjun@...wei.com, zhanjie9@...ilicon.com, 
	lihuisong@...wei.com, yubowen8@...wei.com, zhangpengjie2@...wei.com, 
	wangzhi12@...wei.com, linhongye@...artners.com, salil.mehta@...wei.com, 
	Viresh Kumar <viresh.kumar@...aro.org>, Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] ACPI: processor: Add acpi_processor_start() back to parse
 _CPC tables before CPU online

+linux-pm and Viresh

On Tue, Jan 27, 2026 at 5:58 PM Jonathan Cameron
<jonathan.cameron@...wei.com> wrote:
>
> On Tue, 27 Jan 2026 15:42:16 +0100
> "Rafael J. Wysocki" <rafael@...nel.org> wrote:
>
> > On Tue, Jan 20, 2026 at 12:33 PM Lifeng Zheng <zhenglifeng1@...wei.com> wrote:
> > >
> > > Currently, if boot with maxcpus less than NR_CPUS, the cppc_cpufreq driver
> > > will fail to register. Because it requires the domain information of all
> > > possible CPUs to construct shared_cpu_map, which shows the CPUs that share
> > > the same domain.
> > >
> > > Commit c1385c1f0ba3 ("ACPI: processor: Simplify initial onlining to use
> > > same path for cold and hotplug") removes probe() of acpi_processor_driver
> > > and makes acpi_cppc_processor_probe() only being called the first time CPU
> > > goes online. This means that CPUs that haven't yet gone online will not
> > > have pre-parsed _CPC objects and causes cppc_cpufreq driver register fail.
> > >
> > > Add acpi_processor_start() back as the probe() callback of
> > > acpi_processor_driver and call acpi_cppc_processor_probe() in it to make
> > > sure all _CPC tables will be parsed when acpi_processor_driver registered.
> > >
> > > Fixes: c1385c1f0ba3 ("ACPI: processor: Simplify initial onlining to use same path for cold and hotplug")
> > > Signed-off-by: Lifeng Zheng <zhenglifeng1@...wei.com>
> > > ---
> > >  drivers/acpi/processor_driver.c | 30 ++++++++++++++++++++++++++----
> > >  1 file changed, 26 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/acpi/processor_driver.c b/drivers/acpi/processor_driver.c
> > > index 65e779be64ff..c8b4daf580b0 100644
> > > --- a/drivers/acpi/processor_driver.c
> > > +++ b/drivers/acpi/processor_driver.c
> > > @@ -33,6 +33,7 @@ MODULE_AUTHOR("Paul Diefenbaugh");
> > >  MODULE_DESCRIPTION("ACPI Processor Driver");
> > >  MODULE_LICENSE("GPL");
> > >
> > > +static int acpi_processor_start(struct device *dev);
> > >  static int acpi_processor_stop(struct device *dev);
> > >
> > >  static const struct acpi_device_id processor_device_ids[] = {
> > > @@ -46,6 +47,7 @@ static struct device_driver acpi_processor_driver = {
> > >         .name = "processor",
> > >         .bus = &cpu_subsys,
> > >         .acpi_match_table = processor_device_ids,
> > > +       .probe = acpi_processor_start,
> > >         .remove = acpi_processor_stop,
> > >  };
> > >
> > > @@ -162,10 +164,6 @@ static int __acpi_processor_start(struct acpi_device *device)
> > >         if (!pr)
> > >                 return -ENODEV;
> > >
> > > -       result = acpi_cppc_processor_probe(pr);
> > > -       if (result && !IS_ENABLED(CONFIG_ACPI_CPU_FREQ_PSS))
> > > -               dev_dbg(&device->dev, "CPPC data invalid or not present\n");
> > > -
> > >         if (!cpuidle_get_driver() || cpuidle_get_driver() == &acpi_idle_driver)
> > >                 acpi_processor_power_init(pr);
> > >
> > > @@ -192,6 +190,30 @@ static int __acpi_processor_start(struct acpi_device *device)
> > >         return result;
> > >  }
> > >
> > > +static int acpi_processor_start(struct device *dev)
> > > +{
> > > +       struct acpi_device *device = ACPI_COMPANION(dev);
> > > +       struct acpi_processor *pr;
> > > +       int result;
> > > +
> > > +       if (!device)
> > > +               return -ENODEV;
> > > +
> > > +       pr = acpi_driver_data(device);
> > > +       if (!pr)
> > > +               return -ENODEV;
> > > +
> > > +       /* Protect against concurrent CPU hotplug operations */
> > > +       cpu_hotplug_disable();
> > > +       result = acpi_cppc_processor_probe(pr);
> > > +       cpu_hotplug_enable();
> >
> > This means that CPPC will be initialized for vCPUs that are not
> > enabled on ARM if I'm not mistaken.
>
> If we are just talking powered down at boot it used to do that
> so I assume it was fine. The corner case is ones we are explicitly
> saying are not onlineable yet but marked online capable and will
> turn up later.
>
> >
> > I'm not sure if it is valid to do so.
>
> The conclusion of the following is I think this is fine but I'm not
> entirely confident about it.
>
> I'm struggling to figure out the right answer to this and
> it's not easy to test. I vaguely recall having some nasty emulation
> hacks to poke some x86 related _CPC stuff a while back.
> I might be able to hack something up for this as well and try to
> create pathological corner cases.
>
> The short answer is CPPC + hotplug isn't a thing today in KVM + QEMU,
> but that's not to say it never will be if someone virtualizes CPC for
> a guest.  Let's consider that hypothetical virtualization / emulation.
>
> So the questions:
> 1) Does simply making this acpi_cppc_processor_probe() result in any
>    register accesses to the registers that might be found in _CPC or
>    used via other ACPI methods?
> 2) Can we rely on a a VMM not do something nasty if those are accessed
>    on CPUs that haven't been instantiated yet?  e.g. Bus error.
>    A related useful question is: Can we assume these registers are
>    accessible on offlined CPUs?  If they can be unsafe to access from
>    CPUs that are temporary powered down / offline then I think we are fine because
>    the CPPC code must guarantee not to access them. (I'm relying on this!)
>
> For the particular case Lifeng has run into, I think the code that matters
> (beyond instantiation of the infrastructure) is the creation of the
> domain info in acpi_get_psd(). I think _PSD can only be static data so
> shouldn't cause any register accesses to the powered down CPUs.
>
> So 'probably' fine + we'll not really know unless we get CPU HP and
> CPC.
>
> Alternative much more complex change would be to separate the grabbing of
> static data (done here) from setting up anything dynamic which would remain
> in the hotplug handler.  If those registers haven't been discovered we definitely
> can't access them from the cpu freq driver.

I'm thinking that maybe cppc_cpufreq should be updated instead.

I'm not really sure why it needs to collect information on offline
CPUs.  Surely, they don't matter until they are brought online.

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