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

[Will Deacon]

On Wed, Oct 28, 2020 at 03:14:43PM +0000, Catalin Marinas wrote:
> On Wed, Oct 28, 2020 at 12:27:59PM +0000, Will Deacon wrote:
> > On Wed, Oct 28, 2020 at 12:15:07PM +0000, Catalin Marinas wrote:
> > > On Tue, Oct 27, 2020 at 09:51:17PM +0000, Will Deacon wrote:
> > > > 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
> > > 
> > > Nitpick: should we call this aarch32_el0_present? It's not exactly
> > > present as we populate it as CPUs come online but it's closer to this
> > > mask than to the online one.
> > 
> > I don't think so, because a CPU could be set in this mask but not in the
> > present mask, which is hugely confusing it it has "present" in the name!
> 
> How can it end up here but not in the present mask? We populate present
> early if they have a corresponding DT entry.

I was under the impression that physical CPU hotplug with ACPI would clear
the entry in the present mask, but I can't say I have any machines that I
can test that with and it looks like it might only be implemented for x86
at the moment.

That said, it looks like cpu_die_early() also marks CPUs as not being
present and this can happen due to a late capability conflict.

Will