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

[Qais Yousef]

On 10/09/20 13:40, Catalin Marinas wrote:
> On Fri, Oct 09, 2020 at 12:31:56PM +0100, Qais Yousef wrote:
> > 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.
> 
> Regardless of whether it's cmdline or sysctl, I'm strongly opposed to
> custom drivers for exposing this information to user. It leads to
> custom incompatible ABIs scattered around.

Yeah I see what you mean.

So for now I'll remove the Kconfig dependency and replace it with a sysctl
instead such that system_supports_asym_32bit_el0() only returns true if the
system is asym AND the sysctl is true.

Thanks

--
Qais Yousef

> Note that user can already check the MIDR_EL1 value if it knows which
> CPU type and revision has 32-bit support.
> 
> -- 
> Catalin