On 27.03.23 17:49, Roger Pau Monné wrote:
> On Mon, Mar 27, 2023 at 03:58:26PM +0200, Juergen Gross wrote:
>> 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).
> 
> Currently CONFIG_ARCH_MIGHT_HAVE_ACPI_PDC is only selected for x86 and
> ia64, so I assumed if we ever needed this for Arm someone would have
> to write a proper handler for it for Xen.

Ah, okay, I didn't check that.

Sorry for the noise,


Juergen