Dave Hansen <dave.hansen@xxxxxxxxx> writes: > On 11/06/2017 12:57 AM, Ram Pai wrote: >> Expose useful information for programs using memory protection keys. >> Provide implementation for powerpc and x86. >> >> On a powerpc system with pkeys support, here is what is shown: >> >> $ head /sys/kernel/mm/protection_keys/* >> ==> /sys/kernel/mm/protection_keys/disable_access_supported <== >> true > > This is cute, but I don't think it should be part of the ABI. Put it in > debugfs if you want it for cute tests. The stuff that this tells you > can and should come from pkey_alloc() for the ABI. Yeah I agree this is not sysfs material. In particular the total/usable numbers are completely useless vs other threads allocating pkeys out from under you. > http://man7.org/linux/man-pages/man7/pkeys.7.html > >> Any application wanting to use protection keys needs to be able to >> function without them. They might be unavailable because the >> hardware that the application runs on does not support them, the >> kernel code does not contain support, the kernel support has been >> disabled, or because the keys have all been allocated, perhaps by a >> library the application is using. It is recommended that >> applications wanting to use protection keys should simply call >> pkey_alloc(2) and test whether the call succeeds, instead of >> attempting to detect support for the feature in any other way. > > Do you really not have standard way on ppc to say whether hardware > features are supported by the kernel? For instance, how do you know if > a given set of registers are known to and are being context-switched by > the kernel? Yes we do, we emit feature bits in the AT_HWCAP entry of the aux vector, same as some other architectures. But I don't see the need to use a feature bit for pkeys. If they're not supported then pkey_alloc() will just always fail. Apps have to handle that anyway because keys are a finite resource. cheers
