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

[Will Deacon]

On Wed, Oct 28, 2020 at 09:37:46AM +0100, Greg Kroah-Hartman wrote:
> 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 at 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 at 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.

It's in the same format as
/sys/devices/system/cpu/{online,offline,possible,present}, so I'll just say
that (although the text for those doesn't seem to specify it either...).

> >  What:		/sys/devices/system/cpu/cpu#/cpu_capacity
> >  Date:		December 2016
> >  Contact:	Linux kernel mailing list <linux-kernel at 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 :)

Hehe, yeah ok.

> > +}
> > +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.

Totally missed I had a struct device in my hand, so hopefully that will tidy
things up a little bit.

Cheers,

Will