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SubjectRe: [PATCH v5] ACPI: processor: Fix evaluating _PDC method when running as Xen dom0
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
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