Re: [RFC PATCH 3/3] arm64: Handle AArch32 tasks running on non AArch32 cpu

[Qais Yousef <qais.yousef@arm.com>]

On 10/09/20 10:33, Catalin Marinas wrote:
> On Fri, Oct 09, 2020 at 09:31:47AM +0100, Will Deacon wrote:
> > Honestly, I don't understand why we're trying to hide this asymmetry from
> > userspace by playing games with affinity masks in the kernel. Userspace
> > is likely to want to move things about _anyway_ because even amongst the
> > 32-bit capable cores, you may well have different clock frequencies to
> > contend with.
> > 
> > So I'd be *much* happier to let the schesduler do its thing, and if one
> > of these 32-bit tasks ends up on a core that can't deal with it, then
> > tough, it gets killed. Give userspace the information it needs to avoid
> > that happening in the first place, rather than implicitly limit the mask.
> > 
> > That way, the kernel support really boils down to two parts:
> > 
> >   1. Remove the sanity checks we have to prevent 32-bit applications running
> >      on asymmetric systems
> > 
> >   2. Tell userspace about the problem
> 
> This works for me as well as long as it is default off with a knob to
> turn it on. I'd prefer a sysctl (which can be driven from the command
> line in recent kernels IIRC) so that one can play with it a run-time.
> This way it's also a userspace choice and not an admin or whoever
> controls the cmdline (well, that's rather theoretical since the target
> is Android).

I like the cmdline option more. It implies a custom bootloader and user space
are required to enable this. Which in return implies they can write their own
custom driver to manage exporting this info to user-space. Reliefing us from
maintaining any ABI in mainline kernel.

Thanks

--
Qais Yousef