Re: [PATCH 5/6] arm64: Advertise CPUs capable of running 32-bit applcations in sysfs

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Oct 27, 2020 at 09:51:17PM +0000, Will Deacon wrote:
> Since 32-bit applications will be killed if they are caught trying to
> execute on a 64-bit-only CPU in a mismatched system, advertise the set
> of 32-bit capable CPUs to userspace in sysfs.
> 
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  .../ABI/testing/sysfs-devices-system-cpu      |  8 ++++++++
>  arch/arm64/kernel/cpufeature.c                | 19 +++++++++++++++++++
>  2 files changed, 27 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
> index b555df825447..19893fb8e870 100644
> --- a/Documentation/ABI/testing/sysfs-devices-system-cpu
> +++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
> @@ -472,6 +472,14 @@ Description:	AArch64 CPU registers
>  		'identification' directory exposes the CPU ID registers for
>  		 identifying model and revision of the CPU.
>  
> +What:		/sys/devices/system/cpu/aarch32_el0
> +Date:		October 2020
> +Contact:	Linux ARM Kernel Mailing list <linux-arm-kernel@lists.infradead.org>
> +Description:	Identifies the subset of CPUs in the system that can execute
> +		AArch32 (32-bit ARM) applications. If absent, then all or none
> +		of the CPUs can execute AArch32 applications and execve() will
> +		behave accordingly.

How is this value represented?  A hint here would be nice.

> +
>  What:		/sys/devices/system/cpu/cpu#/cpu_capacity
>  Date:		December 2016
>  Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 2e2219cbd54c..9f29d4d1ef7e 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -67,6 +67,7 @@
>  #include <linux/crash_dump.h>
>  #include <linux/sort.h>
>  #include <linux/stop_machine.h>
> +#include <linux/sysfs.h>
>  #include <linux/types.h>
>  #include <linux/mm.h>
>  #include <linux/cpu.h>
> @@ -1236,6 +1237,24 @@ bool system_has_mismatched_32bit_el0(void)
>  	return fld == ID_AA64PFR0_EL0_64BIT_ONLY;
>  }
>  
> +static ssize_t aarch32_el0_show(struct kobject *kobj,
> +				struct kobj_attribute *attr, char *buf)
> +{
> +	const struct cpumask *mask = system_32bit_el0_cpumask();
> +	return sprintf(buf, "%*pbl\n", cpumask_pr_args(mask));

sysfs_emit()?

And a blank line to make checkpatch.pl happy :)

> +}
> +static const struct kobj_attribute aarch32_el0_attr = __ATTR_RO(aarch32_el0);

DEVICE_ATTR_RO()?

> +
> +static int __init aarch32_el0_sysfs_init(void)
> +{
> +	if (!__allow_mismatched_32bit_el0)
> +		return 0;
> +
> +	return sysfs_create_file(&cpu_subsys.dev_root->kobj,
> +				 &aarch32_el0_attr.attr);

device_create_file() please, dev_root is a struct device, no need to
"thunk" down to a "raw" sysfs call.

thanks,

greg k-h