Re: [RFC PATCH v2 34/35] ACPI: Add _OSC bits to advertise OS support for toggling CPU present/enabled

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Wed, 13 Sep 2023 16:38:22 +0000
James Morse <james.morse@arm.com> wrote:

> Platform firmware can disabled a CPU, or make it not-present by making
> an eject-request notification, then waiting for the os to make it offline
> and call _EJx. After the firmware updates _STA with the new status.
> 
> Not all operating systems support this. For arm64 making CPUs not-present
> has never been supported. For all ACPI architectures, making CPUs disabled
> has recently been added. Firmware can't know what the OS has support for.
> 
> Add two new _OSC bits to advertise whether the OS supports the _STA enabled
> or present bits being toggled for CPUs. This will be important for arm64
> if systems that support physical CPU hotplug ever appear as arm64 linux
> doesn't currently support this, so firmware shouldn't try.

I'm not sure I like enabling this for all architectures though I guess
everyone will ignore it on those that have long supported 
changing the enabled bit. The hypervisors won't care if Linux claims
to support it or not.  I can see the argument for architectures that might
support it in the future.

I need to think a bit more about this, but maybe just having the online
capable bit OSC is safer in general. I guess it depends on whether there
are hypervisors out there implementing the x86 version of that even though
no one has yet posted patches for Linux.

Perhaps we just call these out as hints that we 'definitely' support them.
Otherwise we might for some architectures so poke it anyway.


OSC is late in boot, so what advantage is there in preventing it working?
We can't change any of the bring up / sizing etc as a result so might as
well let it through.

> 
> Advertising this support to firmware is useful for cloud orchestrators
> to know whether they can scale a particular VM by adding CPUs.
> 
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> I'm assuming ia64 with physical hotplug machines once existed, and
> that Loongarch machines with support for this don't.
> ---
>  arch/ia64/Kconfig             |  1 +
>  arch/x86/Kconfig              |  1 +
>  drivers/acpi/Kconfig          |  9 +++++++++
>  drivers/acpi/acpi_processor.c | 14 +++++++++++++-
>  drivers/acpi/bus.c            | 16 ++++++++++++++++
>  include/linux/acpi.h          |  4 ++++
>  6 files changed, 44 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/ia64/Kconfig b/arch/ia64/Kconfig
> index 54972f9fe804..13df676bad67 100644
> --- a/arch/ia64/Kconfig
> +++ b/arch/ia64/Kconfig
> @@ -17,6 +17,7 @@ config IA64
>  	select ARCH_MIGHT_HAVE_PC_SERIO
>  	select ACPI
>  	select ACPI_HOTPLUG_PRESENT_CPU if ACPI_PROCESSOR && HOTPLUG_CPU
> +	select ACPI_HOTPLUG_IGNORE_OSC  if ACPI
>  	select ACPI_NUMA if NUMA
>  	select ARCH_ENABLE_MEMORY_HOTPLUG
>  	select ARCH_ENABLE_MEMORY_HOTREMOVE
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 295a7a3debb6..5fea3ce9594e 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -61,6 +61,7 @@ config X86
>  	select ACPI_LEGACY_TABLES_LOOKUP	if ACPI
>  	select ACPI_SYSTEM_POWER_STATES_SUPPORT	if ACPI
>  	select ACPI_HOTPLUG_PRESENT_CPU		if ACPI_PROCESSOR && HOTPLUG_CPU
> +	select ACPI_HOTPLUG_IGNORE_OSC		if ACPI && HOTPLUG_CPU
>  	select ARCH_32BIT_OFF_T			if X86_32
>  	select ARCH_CLOCKSOURCE_INIT
>  	select ARCH_CORRECT_STACKTRACE_ON_KRETPROBE
> diff --git a/drivers/acpi/Kconfig b/drivers/acpi/Kconfig
> index 417f9f3077d2..c49978b4b11f 100644
> --- a/drivers/acpi/Kconfig
> +++ b/drivers/acpi/Kconfig
> @@ -310,6 +310,15 @@ config ACPI_HOTPLUG_PRESENT_CPU
>  	depends on ACPI_PROCESSOR && HOTPLUG_CPU
>  	select ACPI_CONTAINER
>  
> +config ACPI_HOTPLUG_IGNORE_OSC
> +	bool
> +	depends on ACPI_HOTPLUG_PRESENT_CPU
> +	help
> +	  Ignore whether firmware acknowledged support for toggling the CPU
> +	  present bit in _STA. Some architectures predate the _OSC bits, so
> +	  firmware doesn't know to do this.
> +
> +
>  config ACPI_PROCESSOR_AGGREGATOR
>  	tristate "Processor Aggregator"
>  	depends on ACPI_PROCESSOR
> diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c
> index b49859eab01a..87926f22c857 100644
> --- a/drivers/acpi/acpi_processor.c
> +++ b/drivers/acpi/acpi_processor.c
> @@ -181,6 +181,18 @@ static void __init acpi_pcc_cpufreq_init(void)
>  static void __init acpi_pcc_cpufreq_init(void) {}
>  #endif /* CONFIG_X86 */
>  
> +static bool acpi_processor_hotplug_present_supported(void)
> +{
> +	if (!IS_ENABLED(CONFIG_ACPI_HOTPLUG_PRESENT_CPU))
> +		return false;
> +
> +	/* x86 systems pre-date the _OSC bit */
> +	if (IS_ENABLED(CONFIG_ACPI_HOTPLUG_IGNORE_OSC))
> +		return true;
> +
> +	return osc_sb_hotplug_present_support_acked;
> +}
> +
>  /* Initialization */
>  static int acpi_processor_make_present(struct acpi_processor *pr)
>  {
> @@ -188,7 +200,7 @@ static int acpi_processor_make_present(struct acpi_processor *pr)
>  	acpi_status status;
>  	int ret;
>  
> -	if (!IS_ENABLED(CONFIG_ACPI_HOTPLUG_PRESENT_CPU)) {
> +	if (!acpi_processor_hotplug_present_supported()) {

I don't see the advantage of blocking on basis of what the firmware said.
It was clearly lying or didn't understand the question ;)

>  		pr_err_once("Changing CPU present bit is not supported\n");
>  		return -ENODEV;
>  	}
> diff --git a/drivers/acpi/bus.c b/drivers/acpi/bus.c
> index f41dda2d3493..123c28c2eda3 100644
> --- a/drivers/acpi/bus.c
> +++ b/drivers/acpi/bus.c
> @@ -298,6 +298,13 @@ EXPORT_SYMBOL_GPL(osc_sb_native_usb4_support_confirmed);
>  
>  bool osc_sb_cppc2_support_acked;
>  
> +/*
> + * ACPI 6.? Proposed Operating System Capabilities for modifying CPU
> + * present/enable.
> + */
> +bool osc_sb_hotplug_enabled_support_acked;
> +bool osc_sb_hotplug_present_support_acked;
> +
>  static u8 sb_uuid_str[] = "0811B06E-4A27-44F9-8D60-3CBBC22E7B48";
>  static void acpi_bus_osc_negotiate_platform_control(void)
>  {
> @@ -346,6 +353,11 @@ static void acpi_bus_osc_negotiate_platform_control(void)
>  
>  	if (!ghes_disable)
>  		capbuf[OSC_SUPPORT_DWORD] |= OSC_SB_APEI_SUPPORT;
> +
> +	capbuf[OSC_SUPPORT_DWORD] |= OSC_SB_HOTPLUG_ENABLED_SUPPORT;
> +	if (IS_ENABLED(CONFIG_ACPI_HOTPLUG_PRESENT_CPU))
> +		capbuf[OSC_SUPPORT_DWORD] |= OSC_SB_HOTPLUG_PRESENT_SUPPORT;
> +
>  	if (ACPI_FAILURE(acpi_get_handle(NULL, "\\_SB", &handle)))
>  		return;
>  
> @@ -383,6 +395,10 @@ static void acpi_bus_osc_negotiate_platform_control(void)
>  			capbuf_ret[OSC_SUPPORT_DWORD] & OSC_SB_NATIVE_USB4_SUPPORT;
>  		osc_cpc_flexible_adr_space_confirmed =
>  			capbuf_ret[OSC_SUPPORT_DWORD] & OSC_SB_CPC_FLEXIBLE_ADR_SPACE;
> +		osc_sb_hotplug_enabled_support_acked =
> +			capbuf_ret[OSC_SUPPORT_DWORD] & OSC_SB_HOTPLUG_ENABLED_SUPPORT;
> +		osc_sb_hotplug_present_support_acked =
> +			capbuf_ret[OSC_SUPPORT_DWORD] & OSC_SB_HOTPLUG_PRESENT_SUPPORT;
>  	}
>  
>  	kfree(context.ret.pointer);
> diff --git a/include/linux/acpi.h b/include/linux/acpi.h
> index 92cb25349a18..2ba7e0b10bcf 100644
> --- a/include/linux/acpi.h
> +++ b/include/linux/acpi.h
> @@ -580,12 +580,16 @@ acpi_status acpi_run_osc(acpi_handle handle, struct acpi_osc_context *context);
>  #define OSC_SB_NATIVE_USB4_SUPPORT		0x00040000
>  #define OSC_SB_PRM_SUPPORT			0x00200000
>  #define OSC_SB_FFH_OPR_SUPPORT			0x00400000
> +#define OSC_SB_HOTPLUG_ENABLED_SUPPORT		0x00800000
> +#define OSC_SB_HOTPLUG_PRESENT_SUPPORT		0x01000000
>  
>  extern bool osc_sb_apei_support_acked;
>  extern bool osc_pc_lpi_support_confirmed;
>  extern bool osc_sb_native_usb4_support_confirmed;
>  extern bool osc_sb_cppc2_support_acked;
>  extern bool osc_cpc_flexible_adr_space_confirmed;
> +extern bool osc_sb_hotplug_enabled_support_acked;
> +extern bool osc_sb_hotplug_present_support_acked;
>  
>  /* USB4 Capabilities */
>  #define OSC_USB_USB3_TUNNELING			0x00000001