Re: [RFC PATCH v2 4/4] arm64: Export id_aar64fpr0 via sysfs

Will Deacon
2020-10-21T20:28:33Z

On Wed, Oct 21, 2020 at 08:57:36PM +0100, Qais Yousef wrote:
> On 10/21/20 18:23, Will Deacon wrote:
> > On Wed, Oct 21, 2020 at 05:07:30PM +0100, Qais Yousef wrote:
> > > > > For example, the new sysctl_enable_asym_32bit could be a cpumask instead of
> > > > > a bool as it currently is. Or we can make it a cmdline parameter too.
> > > > > In both cases some admin (bootloader or init process) has to ensure to fill it
> > > > > correctly for the target platform. The bootloader should be able to read the
> > > > > registers to figure out the mask. So more weight to make it a cmdline param.
> > > >
> > > > I think this is adding complexity for the sake of it. I'm much more in
> > >
> > > I actually think it reduces complexity. No special ABI to generate the mask
> > > from the kernel. The same opt-in flag is the cpumask too.
> >
> > Maybe I'm misunderstanding your proposal but having a cpumask instead of
>
> What I meant is that if we change the requirement to opt-in from a boolean
> switch
>
> sysctl.enable_32bit_asym=1
>
> to require the bootloader/init scripts provide the mask of aarch32 capable cpus
>
> sysctl.asym_32bit_cpus=0xf0
>
> This will achieve multiple things at the same time:
>
> * Defer cpus specification to platform designers who want to
> enable this feature on their platform.

What do you mean by this? The kernel will obviously have to check that
the mask is correct (i.e. it doesn't contain 64-bit only CPUs), at which
point it's a boolean in disguise.

> * We don't need a separate API to export which cpus are 32bit capable.
> They can read it directly from /proc/sys/kernel/asym_32bit_cpus.
> When it's 0 it means the system is not asymmetric.

I don't see how this is better than a separate cpumask for this purpose.
It feels like you're trying to overload the control and the identification,
but that just makes things confusing and hard to use as I now need to know
which logical CPUs correspond to which physical CPUs in order to set the
command-line.

> * If/when we want to disable this support in the future. The sysctl
> handler will just have to return 0 all the time and ignore all
> writes.

It can do that as a boolean too, right?

Will