Re: [PATCH v9 6/6] docs: Document CPU hotplug

[Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>]

Hi Mykyta,

Mykyta Poturai <Mykyta_Poturai@xxxxxxxx> writes:

> On 9/22/26 4:13 AM, Volodymyr Babchuk wrote:
>> Hi,
>> 
>> Mykyta Poturai <Mykyta_Poturai@xxxxxxxx> writes:
>> 
>> I have a couple of questions, please see below
>> 
>>> Signed-off-by: Mykyta Poturai <mykyta_poturai@xxxxxxxx>
>>> ---
>>> v8->v9:
>>> * fix typos
>>>
>>> v7->v8:
>>> * remove support status update
>>> * update config option name
>>>
>>> v6->v7:
>>> * add testing and limitations
>>>
>>> v5->v6:
>>> * no changes
>>>
>>> v4->v5:
>>> * s/supported/implemented/
>>> * update SUPPORT.md
>>>
>>> v3->v4:
>>> * update configuration section
>>>
>>> v2->v3:
>>> * patch introduced
>>> ---
>>>   docs/misc/cpu-hotplug.txt | 98 +++++++++++++++++++++++++++++++++++++++
>>>   1 file changed, 98 insertions(+)
>>>   create mode 100644 docs/misc/cpu-hotplug.txt
>>>
>>> diff --git a/docs/misc/cpu-hotplug.txt b/docs/misc/cpu-hotplug.txt
>>> new file mode 100644
>>> index 0000000000..4f6068db30
>>> --- /dev/null
>>> +++ b/docs/misc/cpu-hotplug.txt
>>> @@ -0,0 +1,98 @@
>>> +CPU Hotplug
>>> +===========
>>> +
>>> +CPU hotplug is a feature that allows pCPU cores to be added to or removed 
>>> from a
>>> +running system without requiring a reboot. It is implemented on x86 and 
>>> Arm64
>>> +architectures.
>>> +
>>> +Implementation Details
>>> +----------------------
>>> +
>>> +CPU hotplug is implemented through the `XEN_SYSCTL_CPU_HOTPLUG_*` sysctl 
>>> calls.
>>> +The specific calls are:
>>> +
>>> +- `XEN_SYSCTL_CPU_HOTPLUG_ONLINE`: Brings a pCPU online
>>> +- `XEN_SYSCTL_CPU_HOTPLUG_OFFLINE`: Takes a pCPU offline
>>> +- `XEN_SYSCTL_CPU_HOTPLUG_SMT_ENABLE`: Enables SMT threads (x86 only)
>>> +- `XEN_SYSCTL_CPU_HOTPLUG_SMT_DISABLE`: Disables SMT threads (x86 only)
>>> +
>>> +All cores can be disabled, assuming hardware support, except for the boot 
>>> core.
>>> +Sysctl calls are routed to the boot core before doing any actual up/down
>>> +operations on other cores.
>>> +
>>> +If there are Xen-bound interrupts pinned to the pCPU being offlined, they 
>>> will
>>> +be automatically migrated to other online pCPUs. Interrupts used by guest
>>> +domains are handled by the scheduler when it reschedules the vCPUs to a 
>>> new,
>>> +online, pCPU. When a pCPU is being onlined, some Xen-bound interrupts will 
>>> get
>>> +redistributed to the newly onlined pCPU to prevent imbalance.
>>> +
>>> +If pCPU being offlined has some vCPUs pinned to it, they will be 
>>> automatically
>>> +unpinned and migrated to other online pCPUs.
>>> +
>>> +Limitations
>>> +-----------
>>> +
>>> +On Arm64 cpu hotplug is currently not compatible with ITS, due to an issue 
>>> with
>>> +the redistributor assignment.
>> 
>> In this case you probably want to make sure that
>> CONFIG_CPU_ONLINE_OFFLINE is incompatible with CONFIG_HAS_ITS
>> 
>>> +
>>> +On Arm64 there can be problems with FFA if secure FW supports the 
>>> notification
>>> +ABI, and with TEE, as both use non-static IRQ actions.
>> 
>> Could you please elaborate on this? AFAIK, OP-TEE has own
>> cpu_up/cpu_down callbacks, so it should know which CPUs are online.
>
> It's more of a problem with the way interrupt handlers are set up in 
> Xen. Mykola already has fixes for it in a separate series, but I'm not 
> how to best sync up our efforts.
>

Ah, okay, got it. I think that in this case you also want to make sure
that both these features are not enabled in the same time.

-- 
WBR, Volodymyr