Re: [PATCH for-5.0 v2 8/9] acpi: cpuhp: spec: add typical usecases

[Laszlo Ersek <lersek@redhat.com>]

On 12/09/19 14:09, Igor Mammedov wrote:
> Document work-flows for
>   * enabling/detecting modern CPU hotplug interface
>   * finding a CPU with pending 'insert/remove' event
>   * enumerating present and possible CPUs
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
> v2:
>  - fix indent of "other values" that's just above
>    being added "Typical usecases:" section
>  - unindent "Typical usecases" to put it into right scope
>    (Laszlo)
>  - squash in ammended (using CPHP_GET_NEXT_CPU_WITH_EVENT_CMD)
>    "acpi: cpuhp: spec: document procedure for  enabling modern CPU hotplug"
>    (Laszlo)
> ---
>  docs/specs/acpi_cpu_hotplug.txt | 51 ++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 48 insertions(+), 3 deletions(-)
> 
> diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
> index 9879f9e..cb99cf3 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:
>  -------------------------------------

(1) If you need to post a v3 for other reasons, please consider
adjusting the "===" and "---" lines that surround the new heading.

Otherwise, don't bother.

>  Register block base address:
>      ICH9-LPC IO port 0x0cd8
> @@ -67,6 +67,7 @@ write access:
>      [0x0-0x3] CPU selector: (DWORD access)
>                selects active CPU device. All following accesses to other
>                registers will read/store data from/to selected CPU.
> +              Valid values: [0 .. max_cpus)
>      [0x4] CPU device control fields: (1 byte access)
>          bits:
>              0: reserved, OSPM must clear it before writing to register.
> @@ -98,4 +99,48 @@ write access:
>                2: stores value into OST status register, triggers
>                   ACPI_DEVICE_OST QMP event from QEMU to external applications
>                   with current values of OST event and status registers.
> -            other values: reserved
> +              other values: reserved
> +
> +Typical usecases:
> +    - (x86) Detecting and enabling modern CPU hotplug interface.
> +      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
> +        3. Store 0x0 to the 'Command field' register,
> +        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
> +
> +    - Get a cpu with pending event
> +      1. Store 0x0 to the 'CPU selector' register.
> +      2. Store 0x0 to the 'Command field' register.
> +      3. Read the 'CPU device status fields' register.
> +      4. If both bit#1 and bit#2 are clear in the value read, there is no CPU
> +         with a pending event and selected CPU remains unchanged.
> +      5. Otherwise, read the 'Command data' register. The value read is the
> +         selector of the CPU with the pending event (which is already
> +         selected).
> +
> +    - Enumerate CPUs present/non present CPUs
> +      01. Set the present CPU count to 0.
> +      02. Set the iterator to 0.
> +      03. Store 0x0 to the 'CPU selector' register, to ensure that it's in
> +          a valid state and that access to other registers won't be ignored.
> +      04. Store 0x0 to the 'Command field' register to make 'Command data'
> +          register return 'CPU selector' value of selected CPU
> +      05. Read the 'CPU device status fields' register.
> +      06. If bit#0 is set, increment the present CPU count.
> +      07. Increment the iterator.
> +      08. Store the iterator to the 'CPU selector' register.
> +      09. Read the 'Command data' register.
> +      10. If the value read is not zero, goto 05.
> +      11. Otherwise store 0x0 to the 'CPU selector' register, to put it
> +          into a valid state and exit.
> +          The iterator at this point equals "max_cpus".
> 

Awesome!

Reviewed-by: Laszlo Ersek <lersek@redhat.com>