RE: [PATCH RFC v3 20/21] ACPI: Add _OSC bits to advertise OS support for toggling CPU present/enabled

[Jose Marinho <Jose.Marinho@arm.com>]

Hi Jonathan,

> -----Original Message-----
> From: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
> Sent: Friday, December 15, 2023 5:12 PM
> To: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
> Cc: linux-pm@vger.kernel.org; loongarch@lists.linux.dev; linux-
> acpi@vger.kernel.org; linux-arch@vger.kernel.org; linux-
> kernel@vger.kernel.org; linux-arm-kernel@lists.infradead.org; linux-
> riscv@lists.infradead.org; kvmarm@lists.linux.dev; x86@kernel.org; acpica-
> devel@lists.linuxfoundation.org; linux-csky@vger.kernel.org; linux-
> doc@vger.kernel.org; linux-ia64@vger.kernel.org; linux-
> parisc@vger.kernel.org; Salil Mehta <salil.mehta@huawei.com>; Jean-Philippe
> Brucker <jean-philippe@linaro.org>; Jianyong Wu <Jianyong.Wu@arm.com>;
> Justin He <Justin.He@arm.com>; James Morse <James.Morse@arm.com>;
> Jose Marinho <Jose.Marinho@arm.com>; Samer El-Haj-Mahmoud <Samer.El-
> Haj-Mahmoud@arm.com>
> Subject: Re: [PATCH RFC v3 20/21] ACPI: Add _OSC bits to advertise OS
> support for toggling CPU present/enabled
> 
> On Wed, 13 Dec 2023 12:50:54 +0000
> Russell King (Oracle) <rmk+kernel@armlinux.org.uk> wrote:
> 
> > From: James Morse <james.morse@arm.com>
> >
> > 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
> OS
> 
> > 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.
> >
> > 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>
> > Tested-by: Miguel Luis <miguel.luis@oracle.com>
> > Tested-by: Vishnu Pajjuri <vishnu@os.amperecomputing.com>
> > Tested-by: Jianyong Wu <jianyong.wu@arm.com>
> 
> I'm very much in favor of this _OSC but it hasn't been accepted yet I think...
> https://bugzilla.tianocore.org/show_bug.cgi?id=4481
> 
> Jose? Github suggests you are the proposer on this.

The addition of these _OSC bits was proposed by us on the forum in question.
The forum opted to pause the definition until additional practical information could be provided on the use-cases.

If anyone is interested in progressing the _OSC bit definition, you are invited to express that interest in the Bugzilla ticket.
Information that you should provide to increase the chances of the ticket being reopened:
- use-case for the new _OSC bits,
- what breaks (if anything) without the proposed _OSC bits.

We did receive additional comments:
- the proposed _OSC bits are not generic: the bits simply convey whether the guest OS understands CPU hot-plug, but it says nothing about the number of CPUs that the OS supports.
- There could be alternate schemes that do not rely on spec changes. E.g. there could be a hypervisor IMPDEF mechanism to describe if an OS image supports CPU hot-plug.

> 
> btw v4 looks ok but v5 in the tianocore github seems to have lost the actual
> OSC part.

Agree that, if we do progress with this spec change, v4 is the correct formulation we should adopt. 

Regards,
Jose

> 
> Jonathan
> 
> > ---
> > I'm assuming Loongarch machines do not support physical CPU hotplug.
> >
> > Changes since RFC v3:
> >  * Drop ia64 changes
> >  * Update James' comment below "---" to remove reference to ia64
> >
> > Outstanding comment:
> >  https://lore.kernel.org/r/20230914175021.000018fd@Huawei.com
> 
> 
> 
> > ---
> >  arch/x86/Kconfig              |  1 +
> >  drivers/acpi/Kconfig          |  9 +++++++++
> >  drivers/acpi/acpi_processor.c | 14 +++++++++++++-
> >  drivers/acpi/bus.c            | 16 ++++++++++++++++
> >  include/linux/acpi.h          |  4 ++++
> >  5 files changed, 43 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index
> > 64fc7c475ab0..33fc4dcd950c 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -60,6 +60,7 @@ 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
> > 9c5a43d0aff4..020e7c0ab985 100644
> > --- a/drivers/acpi/Kconfig
> > +++ b/drivers/acpi/Kconfig
> > @@ -311,6 +311,15 @@ 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 ea12e70dfd39..5bb207a7a1dd
> > 100644
> > --- a/drivers/acpi/acpi_processor.c
> > +++ b/drivers/acpi/acpi_processor.c
> > @@ -182,6 +182,18 @@ 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)  {
> > @@ -189,7 +201,7 @@ 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()) {
> >  		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
> > 72e64c0718c9..7122450739d6 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
> > 00be66683505..c572abac803c 100644
> > --- a/include/linux/acpi.h
> > +++ b/include/linux/acpi.h
> > @@ -559,12 +559,16 @@ 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