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Date: Mon, 27 Mar 2023 17:56:53 +0200
From: Juergen Gross <jgross@...e.com>
To: Roger Pau Monné <roger.pau@...rix.com>
Cc: linux-kernel@...r.kernel.org,
Stefano Stabellini <sstabellini@...nel.org>,
xen-devel@...ts.xenproject.org,
"Rafael J. Wysocki" <rafael@...nel.org>,
Len Brown <lenb@...nel.org>,
Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
Alex Chiang <achiang@...com>, linux-acpi@...r.kernel.org
Subject: Re: [PATCH v5] ACPI: processor: Fix evaluating _PDC method when
running as Xen dom0
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@...rix.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
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