Dave Hansen dave.hansen@intel.com 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.
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