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
