Re: [PATCH v2] acpi/processor: fix evaluating _PDC method when running as Xen dom0

[Juergen Gross]

On 15.03.23 13:10, 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.
>
> 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@xxxxxxxxxx>
> ---
> Changes since v1:
>   - Reword commit message.
> ---
>   arch/x86/include/asm/xen/hypervisor.h | 10 ++++++++++
>   arch/x86/xen/enlighten.c              | 27 +++++++++++++++++++++++++++
>   drivers/acpi/processor_pdc.c          | 11 +++++++++++
>   3 files changed, 48 insertions(+)
>
> diff --git a/arch/x86/include/asm/xen/hypervisor.h b/arch/x86/include/asm/xen/hypervisor.h
> index 5fc35f889cd1..f14e39bce2cb 100644
> --- a/arch/x86/include/asm/xen/hypervisor.h
> +++ b/arch/x86/include/asm/xen/hypervisor.h
> @@ -63,4 +63,14 @@ void __init xen_pvh_init(struct boot_params *boot_params);
>   void __init mem_map_via_hcall(struct boot_params *boot_params_p);
>   #endif
>   
> +#ifdef CONFIG_XEN_DOM0
> +bool __init xen_processor_present(uint32_t acpi_id);
> +#else
> +static inline bool xen_processor_present(uint32_t acpi_id)
> +{
> +	BUG();
> +	return false;
> +}
> +#endif
> +
>   #endif /* _ASM_X86_XEN_HYPERVISOR_H */
> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> index b8db2148c07d..d4c44361a26c 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -346,3 +346,30 @@ void xen_arch_unregister_cpu(int num)
>   }
>   EXPORT_SYMBOL(xen_arch_unregister_cpu);
>   #endif
> +
> +#ifdef CONFIG_XEN_DOM0
> +bool __init xen_processor_present(uint32_t acpi_id)
> +{
> +	unsigned int i, maxid;
> +	struct xen_platform_op op = {
> +		.cmd = XENPF_get_cpuinfo,
> +		.interface_version = XENPF_INTERFACE_VERSION,
> +	};
> +	int ret = HYPERVISOR_platform_op(&op);
> +
> +	if (ret)
> +		return false;
> +
> +	maxid = op.u.pcpu_info.max_present;
> +	for (i = 0; i <= maxid; i++) {
> +		op.u.pcpu_info.xen_cpuid = i;
> +		ret = HYPERVISOR_platform_op(&op);
> +		if (ret)
> +			continue;
> +		if (op.u.pcpu_info.acpi_id == acpi_id)
> +			return op.u.pcpu_info.flags & XEN_PCPU_FLAGS_ONLINE;
> +	}
> +
> +	return false;
> +}
> +#endif

Did you consider not to do the hypercall again and again, but to reuse the
pcpu list from drivers/xen/pcpu.c instead? You'd need to store the acpi_id
in this list, of course.


Juergen

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