On 21.03.23 15:19, Roger Pau Monne wrote:
> In ACPI systems, the OS can direct power management, as opposed to the
> firmware.  This OS-directed Power Management is called OSPM.  Part of
> telling the firmware that the OS going to direct power management is
> making ACPI "_PDC" (Processor Driver Capabilities) calls.  These _PDC
> methods must be evaluated for every processor object.  If these _PDC
> calls are not completed for every processor it can lead to
> inconsistency and later failures in things like the CPU frequency
> driver.
> 
> In a Xen system, the dom0 kernel is responsible for system-wide power
> management.  The dom0 kernel is in charge of OSPM.  However, the
> number of CPUs available to dom0 can be different than the number of
> CPUs physically present on the system.
> 
> This leads to a problem: the dom0 kernel needs to evaluate _PDC for
> all the processors, but it can't always see them.
> 
> In dom0 kernels, ignore the existing ACPI method for determining if a
> processor is physically present because it might not be accurate.
> Instead, ask the hypervisor for this information.
> 
> Fix this by introducing a custom function to use when running as Xen
> dom0 in order to check whether a processor object matches a CPU that's
> online.  Such checking is done using the existing information fetched
> by the Xen pCPU subsystem, extending it to also store the ACPI ID.
> 
> This ensures that _PDC method gets evaluated for all physically online
> CPUs, regardless of the number of CPUs made available to dom0.
> 
> Fixes: 5d554a7bb064 ('ACPI: processor: add internal processor_physically_present()')
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> ---
> Changes since v4:
>   - Move definition/declaration of xen_processor_present() to different
>     header.
>   - Fold subject edit.
> 
> Changes since v3:
>   - Protect xen_processor_present() definition with CONFIG_ACPI.
> 
> Changes since v2:
>   - Extend and use the existing pcpu functionality.
> 
> Changes since v1:
>   - Reword commit message.
> ---
>   drivers/acpi/processor_pdc.c | 11 +++++++++++
>   drivers/xen/pcpu.c           | 20 ++++++++++++++++++++
>   include/xen/xen.h            | 10 ++++++++++
>   3 files changed, 41 insertions(+)
> 
> diff --git a/drivers/acpi/processor_pdc.c b/drivers/acpi/processor_pdc.c
> index 8c3f82c9fff3..18fb04523f93 100644
> --- a/drivers/acpi/processor_pdc.c
> +++ b/drivers/acpi/processor_pdc.c
> @@ -14,6 +14,8 @@
>   #include <linux/acpi.h>
>   #include <acpi/processor.h>
>   
> +#include <xen/xen.h>
> +
>   #include "internal.h"
>   
>   static bool __init processor_physically_present(acpi_handle handle)
> @@ -47,6 +49,15 @@ static bool __init processor_physically_present(acpi_handle handle)
>   		return false;
>   	}
>   
> +	if (xen_initial_domain())
> +		/*
> +		 * When running as a Xen dom0 the number of processors Linux
> +		 * sees can be different from the real number of processors on
> +		 * the system, and we still need to execute _PDC for all of
> +		 * them.
> +		 */
> +		return xen_processor_present(acpi_id);
> +
>   	type = (acpi_type == ACPI_TYPE_DEVICE) ? 1 : 0;
>   	cpuid = acpi_get_cpuid(handle, type, acpi_id);
>   
> diff --git a/drivers/xen/pcpu.c b/drivers/xen/pcpu.c
> index fd3a644b0855..1814f8762f54 100644
> --- a/drivers/xen/pcpu.c
> +++ b/drivers/xen/pcpu.c
> @@ -58,6 +58,7 @@ struct pcpu {
>   	struct list_head list;
>   	struct device dev;
>   	uint32_t cpu_id;
> +	uint32_t acpi_id;
>   	uint32_t flags;
>   };
>   
> @@ -249,6 +250,7 @@ static struct pcpu *create_and_register_pcpu(struct xenpf_pcpuinfo *info)
>   
>   	INIT_LIST_HEAD(&pcpu->list);
>   	pcpu->cpu_id = info->xen_cpuid;
> +	pcpu->acpi_id = info->acpi_id;
>   	pcpu->flags = info->flags;
>   
>   	/* Need hold on xen_pcpu_lock before pcpu list manipulations */
> @@ -381,3 +383,21 @@ static int __init xen_pcpu_init(void)
>   	return ret;
>   }
>   arch_initcall(xen_pcpu_init);
> +
> +#ifdef CONFIG_ACPI
> +bool __init xen_processor_present(uint32_t acpi_id)
> +{
> +	struct pcpu *pcpu;
> +	bool online = false;
> +
> +	mutex_lock(&xen_pcpu_lock);
> +	list_for_each_entry(pcpu, &xen_pcpus, list)
> +		if (pcpu->acpi_id == acpi_id) {
> +			online = pcpu->flags & XEN_PCPU_FLAGS_ONLINE;
> +			break;
> +		}
> +	mutex_unlock(&xen_pcpu_lock);
> +
> +	return online;
> +}
> +#endif
> diff --git a/include/xen/xen.h b/include/xen/xen.h
> index 7adf59837c25..4410e74f3eb5 100644
> --- a/include/xen/xen.h
> +++ b/include/xen/xen.h
> @@ -71,4 +71,14 @@ static inline void xen_free_unpopulated_pages(unsigned int nr_pages,
>   }
>   #endif
>   
> +#if defined(CONFIG_XEN_DOM0) && defined(CONFIG_ACPI) && defined(CONFIG_X86)
> +bool __init xen_processor_present(uint32_t acpi_id);
> +#else
> +static inline bool xen_processor_present(uint32_t acpi_id)
> +{
> +	BUG();

Is this really a good idea?

Arm64 supports ACPI, too, as well as XEN_DOM0. I think you either need to
provide a stub for that case, too, or you need make this stub non-fatal
for callers (I guess returning false is fine, as currently there are no
hypercalls on Arm which would allow to control physical CPUs based on
ACPI-Id).

Stefano, can you confirm this?


Juergen