Re: [PATCHv3 03/14] cpu/hotplug: Add support for declaring CPU offlining not supported

["Huang, Kai" <kai.huang@intel.com>]

On Wed, 2023-11-15 at 15:00 +0300, Kirill A. Shutemov wrote:
> The ACPI MADT mailbox wakeup method doesn't allow to offline CPU after
> it got woke up.
> 
> Currently offlining hotplug is prevented based on the confidential
> computing attribute which is set for Intel TDX. But TDX is not
> the only possible user of the wake up method. The MADT wakeup can be
> implemented outside of a confidential computing environment. Offline
> support is a property of the wakeup method, not the CoCo implementation.
> 
> Introduce cpu_hotplug_not_supported() that can be called to indicate
> that CPU offlining should be disabled.

cpu_hotplug_not_supported() -> cpu_hotplug_disable_offlining().

> 
> This function is going to replace CC_ATTR_HOTPLUG_DISABLED for ACPI
> MADT.

Is MADT too generic? MADT wakeup?

> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  include/linux/cpu.h |  2 ++
>  kernel/cpu.c        | 13 ++++++++++++-
>  2 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index fc8094419084..46f2e34a0c5e 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -134,6 +134,7 @@ extern void cpus_read_lock(void);
>  extern void cpus_read_unlock(void);
>  extern int  cpus_read_trylock(void);
>  extern void lockdep_assert_cpus_held(void);
> +extern void cpu_hotplug_disable_offlining(void);
>  extern void cpu_hotplug_disable(void);
>  extern void cpu_hotplug_enable(void);
>  void clear_tasks_mm_cpumask(int cpu);
> @@ -149,6 +150,7 @@ static inline void cpus_read_lock(void) { }
>  static inline void cpus_read_unlock(void) { }
>  static inline int  cpus_read_trylock(void) { return true; }
>  static inline void lockdep_assert_cpus_held(void) { }
> +static inline void cpu_hotplug_disable_offlining(void) { }
>  static inline void cpu_hotplug_disable(void) { }
>  static inline void cpu_hotplug_enable(void) { }
>  static inline int remove_cpu(unsigned int cpu) { return -EPERM; }
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index a86972a91991..af8034ccda8e 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -484,6 +484,8 @@ static int cpu_hotplug_disabled;
>  
>  DEFINE_STATIC_PERCPU_RWSEM(cpu_hotplug_lock);
>  
> +static bool cpu_hotplug_offline_disabled;
> +
>  void cpus_read_lock(void)
>  {
>  	percpu_down_read(&cpu_hotplug_lock);
> @@ -543,6 +545,14 @@ static void lockdep_release_cpus_lock(void)
>  	rwsem_release(&cpu_hotplug_lock.dep_map, _THIS_IP_);
>  }
>  
> +/* Declare CPU offlining not supported */
> +void cpu_hotplug_disable_offlining(void)
> +{
> +	cpu_maps_update_begin();
> +	cpu_hotplug_offline_disabled = true;
> +	cpu_maps_update_done();
> +}
> +
>  /*
>   * Wait for currently running CPU hotplug operations to complete (if any) and
>   * disable future CPU hotplug (from sysfs). The 'cpu_add_remove_lock' protects
> @@ -1522,7 +1532,8 @@ static int cpu_down_maps_locked(unsigned int cpu, enum cpuhp_state target)
>  	 * If the platform does not support hotplug, report it explicitly to
>  	 * differentiate it from a transient offlining failure.
>  	 */
> -	if (cc_platform_has(CC_ATTR_HOTPLUG_DISABLED))
> +	if (cc_platform_has(CC_ATTR_HOTPLUG_DISABLED) ||
> +	    cpu_hotplug_offline_disabled)
>  		return -EOPNOTSUPP;
>  	if (cpu_hotplug_disabled)
>  		return -EBUSY;