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 v5:
 - Include bug.h header for the inline dummy helper.
 - Constify pcpu local variable in xen_processor_present().

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            | 11 +++++++++++
 3 files changed, 42 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..b3e3d1bb37f3 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)
+{
+	const 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..0efeb652f9b8 100644
--- a/include/xen/xen.h
+++ b/include/xen/xen.h
@@ -71,4 +71,15 @@ 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
+#include <linux/bug.h>
+static inline bool xen_processor_present(uint32_t acpi_id)
+{
+	BUG();
+	return false;
+}
+#endif
+
 #endif	/* _XEN_XEN_H */
-- 
2.40.0

On Wed, Mar 22, 2023 at 12:13 PM Roger Pau Monne <roger.pau@citrix.com> 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 v5:
>  - Include bug.h header for the inline dummy helper.
>  - Constify pcpu local variable in xen_processor_present().
>
> 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            | 11 +++++++++++
>  3 files changed, 42 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..b3e3d1bb37f3 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)
> +{
> +       const 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..0efeb652f9b8 100644
> --- a/include/xen/xen.h
> +++ b/include/xen/xen.h
> @@ -71,4 +71,15 @@ 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
> +#include <linux/bug.h>
> +static inline bool xen_processor_present(uint32_t acpi_id)
> +{
> +       BUG();
> +       return false;
> +}
> +#endif
> +
>  #endif /* _XEN_XEN_H */
> --

Applied as 6.4 material.

I've added the R-by from Juergen to it, because the difference between
it and the v4 for which the tag was provided is merely a build fix.